Update OpenZFS to 2.0.0-rc3-gfc5966

[FreeBSD/FreeBSD.git] / module / zfs / arc.c

/*
 * CDDL HEADER START
 *
 * The contents of this file are subject to the terms of the
 * Common Development and Distribution License (the "License").
 * You may not use this file except in compliance with the License.
 *
 * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
 * or http://www.opensolaris.org/os/licensing.
 * See the License for the specific language governing permissions
 * and limitations under the License.
 *
 * When distributing Covered Code, include this CDDL HEADER in each
 * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
 * If applicable, add the following below this CDDL HEADER, with the
 * fields enclosed by brackets "[]" replaced with your own identifying
 * information: Portions Copyright [yyyy] [name of copyright owner]
 *
 * CDDL HEADER END
 */
/*
 * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
 * Copyright (c) 2018, Joyent, Inc.
 * Copyright (c) 2011, 2020, Delphix. All rights reserved.
 * Copyright (c) 2014, Saso Kiselkov. All rights reserved.
 * Copyright (c) 2017, Nexenta Systems, Inc.  All rights reserved.
 * Copyright (c) 2019, loli10K <ezomori.nozomu@gmail.com>. All rights reserved.
 * Copyright (c) 2020, George Amanakis. All rights reserved.
 * Copyright (c) 2019, Klara Inc.
 * Copyright (c) 2019, Allan Jude
 * Copyright (c) 2020, The FreeBSD Foundation [1]
 *
 * [1] Portions of this software were developed by Allan Jude
 *     under sponsorship from the FreeBSD Foundation.
 */

/*
 * DVA-based Adjustable Replacement Cache
 *
 * While much of the theory of operation used here is
 * based on the self-tuning, low overhead replacement cache
 * presented by Megiddo and Modha at FAST 2003, there are some
 * significant differences:
 *
 * 1. The Megiddo and Modha model assumes any page is evictable.
 * Pages in its cache cannot be "locked" into memory.  This makes
 * the eviction algorithm simple: evict the last page in the list.
 * This also make the performance characteristics easy to reason
 * about.  Our cache is not so simple.  At any given moment, some
 * subset of the blocks in the cache are un-evictable because we
 * have handed out a reference to them.  Blocks are only evictable
 * when there are no external references active.  This makes
 * eviction far more problematic:  we choose to evict the evictable
 * blocks that are the "lowest" in the list.
 *
 * There are times when it is not possible to evict the requested
 * space.  In these circumstances we are unable to adjust the cache
 * size.  To prevent the cache growing unbounded at these times we
 * implement a "cache throttle" that slows the flow of new data
 * into the cache until we can make space available.
 *
 * 2. The Megiddo and Modha model assumes a fixed cache size.
 * Pages are evicted when the cache is full and there is a cache
 * miss.  Our model has a variable sized cache.  It grows with
 * high use, but also tries to react to memory pressure from the
 * operating system: decreasing its size when system memory is
 * tight.
 *
 * 3. The Megiddo and Modha model assumes a fixed page size. All
 * elements of the cache are therefore exactly the same size.  So
 * when adjusting the cache size following a cache miss, its simply
 * a matter of choosing a single page to evict.  In our model, we
 * have variable sized cache blocks (ranging from 512 bytes to
 * 128K bytes).  We therefore choose a set of blocks to evict to make
 * space for a cache miss that approximates as closely as possible
 * the space used by the new block.
 *
 * See also:  "ARC: A Self-Tuning, Low Overhead Replacement Cache"
 * by N. Megiddo & D. Modha, FAST 2003
 */

/*
 * The locking model:
 *
 * A new reference to a cache buffer can be obtained in two
 * ways: 1) via a hash table lookup using the DVA as a key,
 * or 2) via one of the ARC lists.  The arc_read() interface
 * uses method 1, while the internal ARC algorithms for
 * adjusting the cache use method 2.  We therefore provide two
 * types of locks: 1) the hash table lock array, and 2) the
 * ARC list locks.
 *
 * Buffers do not have their own mutexes, rather they rely on the
 * hash table mutexes for the bulk of their protection (i.e. most
 * fields in the arc_buf_hdr_t are protected by these mutexes).
 *
 * buf_hash_find() returns the appropriate mutex (held) when it
 * locates the requested buffer in the hash table.  It returns
 * NULL for the mutex if the buffer was not in the table.
 *
 * buf_hash_remove() expects the appropriate hash mutex to be
 * already held before it is invoked.
 *
 * Each ARC state also has a mutex which is used to protect the
 * buffer list associated with the state.  When attempting to
 * obtain a hash table lock while holding an ARC list lock you
 * must use: mutex_tryenter() to avoid deadlock.  Also note that
 * the active state mutex must be held before the ghost state mutex.
 *
 * It as also possible to register a callback which is run when the
 * arc_meta_limit is reached and no buffers can be safely evicted.  In
 * this case the arc user should drop a reference on some arc buffers so
 * they can be reclaimed and the arc_meta_limit honored.  For example,
 * when using the ZPL each dentry holds a references on a znode.  These
 * dentries must be pruned before the arc buffer holding the znode can
 * be safely evicted.
 *
 * Note that the majority of the performance stats are manipulated
 * with atomic operations.
 *
 * The L2ARC uses the l2ad_mtx on each vdev for the following:
 *
 *      - L2ARC buflist creation
 *      - L2ARC buflist eviction
 *      - L2ARC write completion, which walks L2ARC buflists
 *      - ARC header destruction, as it removes from L2ARC buflists
 *      - ARC header release, as it removes from L2ARC buflists
 */

/*
 * ARC operation:
 *
 * Every block that is in the ARC is tracked by an arc_buf_hdr_t structure.
 * This structure can point either to a block that is still in the cache or to
 * one that is only accessible in an L2 ARC device, or it can provide
 * information about a block that was recently evicted. If a block is
 * only accessible in the L2ARC, then the arc_buf_hdr_t only has enough
 * information to retrieve it from the L2ARC device. This information is
 * stored in the l2arc_buf_hdr_t sub-structure of the arc_buf_hdr_t. A block
 * that is in this state cannot access the data directly.
 *
 * Blocks that are actively being referenced or have not been evicted
 * are cached in the L1ARC. The L1ARC (l1arc_buf_hdr_t) is a structure within
 * the arc_buf_hdr_t that will point to the data block in memory. A block can
 * only be read by a consumer if it has an l1arc_buf_hdr_t. The L1ARC
 * caches data in two ways -- in a list of ARC buffers (arc_buf_t) and
 * also in the arc_buf_hdr_t's private physical data block pointer (b_pabd).
 *
 * The L1ARC's data pointer may or may not be uncompressed. The ARC has the
 * ability to store the physical data (b_pabd) associated with the DVA of the
 * arc_buf_hdr_t. Since the b_pabd is a copy of the on-disk physical block,
 * it will match its on-disk compression characteristics. This behavior can be
 * disabled by setting 'zfs_compressed_arc_enabled' to B_FALSE. When the
 * compressed ARC functionality is disabled, the b_pabd will point to an
 * uncompressed version of the on-disk data.
 *
 * Data in the L1ARC is not accessed by consumers of the ARC directly. Each
 * arc_buf_hdr_t can have multiple ARC buffers (arc_buf_t) which reference it.
 * Each ARC buffer (arc_buf_t) is being actively accessed by a specific ARC
 * consumer. The ARC will provide references to this data and will keep it
 * cached until it is no longer in use. The ARC caches only the L1ARC's physical
 * data block and will evict any arc_buf_t that is no longer referenced. The
 * amount of memory consumed by the arc_buf_ts' data buffers can be seen via the
 * "overhead_size" kstat.
 *
 * Depending on the consumer, an arc_buf_t can be requested in uncompressed or
 * compressed form. The typical case is that consumers will want uncompressed
 * data, and when that happens a new data buffer is allocated where the data is
 * decompressed for them to use. Currently the only consumer who wants
 * compressed arc_buf_t's is "zfs send", when it streams data exactly as it
 * exists on disk. When this happens, the arc_buf_t's data buffer is shared
 * with the arc_buf_hdr_t.
 *
 * Here is a diagram showing an arc_buf_hdr_t referenced by two arc_buf_t's. The
 * first one is owned by a compressed send consumer (and therefore references
 * the same compressed data buffer as the arc_buf_hdr_t) and the second could be
 * used by any other consumer (and has its own uncompressed copy of the data
 * buffer).
 *
 *   arc_buf_hdr_t
 *   +-----------+
 *   | fields    |
 *   | common to |
 *   | L1- and   |
 *   | L2ARC     |
 *   +-----------+
 *   | l2arc_buf_hdr_t
 *   |           |
 *   +-----------+
 *   | l1arc_buf_hdr_t
 *   |           |              arc_buf_t
 *   | b_buf     +------------>+-----------+      arc_buf_t
 *   | b_pabd    +-+           |b_next     +---->+-----------+
 *   +-----------+ |           |-----------|     |b_next     +-->NULL
 *                 |           |b_comp = T |     +-----------+
 *                 |           |b_data     +-+   |b_comp = F |
 *                 |           +-----------+ |   |b_data     +-+
 *                 +->+------+               |   +-----------+ |
 *        compressed  |      |               |                 |
 *           data     |      |<--------------+                 | uncompressed
 *                    +------+          compressed,            |     data
 *                                        shared               +-->+------+
 *                                         data                    |      |
 *                                                                 |      |
 *                                                                 +------+
 *
 * When a consumer reads a block, the ARC must first look to see if the
 * arc_buf_hdr_t is cached. If the hdr is cached then the ARC allocates a new
 * arc_buf_t and either copies uncompressed data into a new data buffer from an
 * existing uncompressed arc_buf_t, decompresses the hdr's b_pabd buffer into a
 * new data buffer, or shares the hdr's b_pabd buffer, depending on whether the
 * hdr is compressed and the desired compression characteristics of the
 * arc_buf_t consumer. If the arc_buf_t ends up sharing data with the
 * arc_buf_hdr_t and both of them are uncompressed then the arc_buf_t must be
 * the last buffer in the hdr's b_buf list, however a shared compressed buf can
 * be anywhere in the hdr's list.
 *
 * The diagram below shows an example of an uncompressed ARC hdr that is
 * sharing its data with an arc_buf_t (note that the shared uncompressed buf is
 * the last element in the buf list):
 *
 *                arc_buf_hdr_t
 *                +-----------+
 *                |           |
 *                |           |
 *                |           |
 *                +-----------+
 * l2arc_buf_hdr_t|           |
 *                |           |
 *                +-----------+
 * l1arc_buf_hdr_t|           |
 *                |           |                 arc_buf_t    (shared)
 *                |    b_buf  +------------>+---------+      arc_buf_t
 *                |           |             |b_next   +---->+---------+
 *                |  b_pabd   +-+           |---------|     |b_next   +-->NULL
 *                +-----------+ |           |         |     +---------+
 *                              |           |b_data   +-+   |         |
 *                              |           +---------+ |   |b_data   +-+
 *                              +->+------+             |   +---------+ |
 *                                 |      |             |               |
 *                   uncompressed  |      |             |               |
 *                        data     +------+             |               |
 *                                    ^                 +->+------+     |
 *                                    |       uncompressed |      |     |
 *                                    |           data     |      |     |
 *                                    |                    +------+     |
 *                                    +---------------------------------+
 *
 * Writing to the ARC requires that the ARC first discard the hdr's b_pabd
 * since the physical block is about to be rewritten. The new data contents
 * will be contained in the arc_buf_t. As the I/O pipeline performs the write,
 * it may compress the data before writing it to disk. The ARC will be called
 * with the transformed data and will bcopy the transformed on-disk block into
 * a newly allocated b_pabd. Writes are always done into buffers which have
 * either been loaned (and hence are new and don't have other readers) or
 * buffers which have been released (and hence have their own hdr, if there
 * were originally other readers of the buf's original hdr). This ensures that
 * the ARC only needs to update a single buf and its hdr after a write occurs.
 *
 * When the L2ARC is in use, it will also take advantage of the b_pabd. The
 * L2ARC will always write the contents of b_pabd to the L2ARC. This means
 * that when compressed ARC is enabled that the L2ARC blocks are identical
 * to the on-disk block in the main data pool. This provides a significant
 * advantage since the ARC can leverage the bp's checksum when reading from the
 * L2ARC to determine if the contents are valid. However, if the compressed
 * ARC is disabled, then the L2ARC's block must be transformed to look
 * like the physical block in the main data pool before comparing the
 * checksum and determining its validity.
 *
 * The L1ARC has a slightly different system for storing encrypted data.
 * Raw (encrypted + possibly compressed) data has a few subtle differences from
 * data that is just compressed. The biggest difference is that it is not
 * possible to decrypt encrypted data (or vice-versa) if the keys aren't loaded.
 * The other difference is that encryption cannot be treated as a suggestion.
 * If a caller would prefer compressed data, but they actually wind up with
 * uncompressed data the worst thing that could happen is there might be a
 * performance hit. If the caller requests encrypted data, however, we must be
 * sure they actually get it or else secret information could be leaked. Raw
 * data is stored in hdr->b_crypt_hdr.b_rabd. An encrypted header, therefore,
 * may have both an encrypted version and a decrypted version of its data at
 * once. When a caller needs a raw arc_buf_t, it is allocated and the data is
 * copied out of this header. To avoid complications with b_pabd, raw buffers
 * cannot be shared.
 */

#include <sys/spa.h>
#include <sys/zio.h>
#include <sys/spa_impl.h>
#include <sys/zio_compress.h>
#include <sys/zio_checksum.h>
#include <sys/zfs_context.h>
#include <sys/arc.h>
#include <sys/zfs_refcount.h>
#include <sys/vdev.h>
#include <sys/vdev_impl.h>
#include <sys/dsl_pool.h>
#include <sys/zio_checksum.h>
#include <sys/multilist.h>
#include <sys/abd.h>
#include <sys/zil.h>
#include <sys/fm/fs/zfs.h>
#include <sys/callb.h>
#include <sys/kstat.h>
#include <sys/zthr.h>
#include <zfs_fletcher.h>
#include <sys/arc_impl.h>
#include <sys/trace_zfs.h>
#include <sys/aggsum.h>
#include <cityhash.h>
#include <sys/vdev_trim.h>
#include <sys/zstd/zstd.h>

#ifndef _KERNEL
/* set with ZFS_DEBUG=watch, to enable watchpoints on frozen buffers */
boolean_t arc_watch = B_FALSE;
#endif

/*
 * This thread's job is to keep enough free memory in the system, by
 * calling arc_kmem_reap_soon() plus arc_reduce_target_size(), which improves
 * arc_available_memory().
 */
static zthr_t *arc_reap_zthr;

/*
 * This thread's job is to keep arc_size under arc_c, by calling
 * arc_evict(), which improves arc_is_overflowing().
 */
static zthr_t *arc_evict_zthr;

static kmutex_t arc_evict_lock;
static boolean_t arc_evict_needed = B_FALSE;

/*
 * Count of bytes evicted since boot.
 */
static uint64_t arc_evict_count;

/*
 * List of arc_evict_waiter_t's, representing threads waiting for the
 * arc_evict_count to reach specific values.
 */
static list_t arc_evict_waiters;

/*
 * When arc_is_overflowing(), arc_get_data_impl() waits for this percent of
 * the requested amount of data to be evicted.  For example, by default for
 * every 2KB that's evicted, 1KB of it may be "reused" by a new allocation.
 * Since this is above 100%, it ensures that progress is made towards getting
 * arc_size under arc_c.  Since this is finite, it ensures that allocations
 * can still happen, even during the potentially long time that arc_size is
 * more than arc_c.
 */
int zfs_arc_eviction_pct = 200;

/*
 * The number of headers to evict in arc_evict_state_impl() before
 * dropping the sublist lock and evicting from another sublist. A lower
 * value means we're more likely to evict the "correct" header (i.e. the
 * oldest header in the arc state), but comes with higher overhead
 * (i.e. more invocations of arc_evict_state_impl()).
 */
int zfs_arc_evict_batch_limit = 10;

/* number of seconds before growing cache again */
int arc_grow_retry = 5;

/*
 * Minimum time between calls to arc_kmem_reap_soon().
 */
int arc_kmem_cache_reap_retry_ms = 1000;

/* shift of arc_c for calculating overflow limit in arc_get_data_impl */
int zfs_arc_overflow_shift = 8;

/* shift of arc_c for calculating both min and max arc_p */
int arc_p_min_shift = 4;

/* log2(fraction of arc to reclaim) */
int arc_shrink_shift = 7;

/* percent of pagecache to reclaim arc to */
#ifdef _KERNEL
uint_t zfs_arc_pc_percent = 0;
#endif

/*
 * log2(fraction of ARC which must be free to allow growing).
 * I.e. If there is less than arc_c >> arc_no_grow_shift free memory,
 * when reading a new block into the ARC, we will evict an equal-sized block
 * from the ARC.
 *
 * This must be less than arc_shrink_shift, so that when we shrink the ARC,
 * we will still not allow it to grow.
 */
int                     arc_no_grow_shift = 5;


/*
 * minimum lifespan of a prefetch block in clock ticks
 * (initialized in arc_init())
 */
static int              arc_min_prefetch_ms;
static int              arc_min_prescient_prefetch_ms;

/*
 * If this percent of memory is free, don't throttle.
 */
int arc_lotsfree_percent = 10;

/*
 * The arc has filled available memory and has now warmed up.
 */
boolean_t arc_warm;

/*
 * These tunables are for performance analysis.
 */
unsigned long zfs_arc_max = 0;
unsigned long zfs_arc_min = 0;
unsigned long zfs_arc_meta_limit = 0;
unsigned long zfs_arc_meta_min = 0;
unsigned long zfs_arc_dnode_limit = 0;
unsigned long zfs_arc_dnode_reduce_percent = 10;
int zfs_arc_grow_retry = 0;
int zfs_arc_shrink_shift = 0;
int zfs_arc_p_min_shift = 0;
int zfs_arc_average_blocksize = 8 * 1024; /* 8KB */

/*
 * ARC dirty data constraints for arc_tempreserve_space() throttle.
 */
unsigned long zfs_arc_dirty_limit_percent = 50; /* total dirty data limit */
unsigned long zfs_arc_anon_limit_percent = 25;  /* anon block dirty limit */
unsigned long zfs_arc_pool_dirty_percent = 20;  /* each pool's anon allowance */

/*
 * Enable or disable compressed arc buffers.
 */
int zfs_compressed_arc_enabled = B_TRUE;

/*
 * ARC will evict meta buffers that exceed arc_meta_limit. This
 * tunable make arc_meta_limit adjustable for different workloads.
 */
unsigned long zfs_arc_meta_limit_percent = 75;

/*
 * Percentage that can be consumed by dnodes of ARC meta buffers.
 */
unsigned long zfs_arc_dnode_limit_percent = 10;

/*
 * These tunables are Linux specific
 */
unsigned long zfs_arc_sys_free = 0;
int zfs_arc_min_prefetch_ms = 0;
int zfs_arc_min_prescient_prefetch_ms = 0;
int zfs_arc_p_dampener_disable = 1;
int zfs_arc_meta_prune = 10000;
int zfs_arc_meta_strategy = ARC_STRATEGY_META_BALANCED;
int zfs_arc_meta_adjust_restarts = 4096;
int zfs_arc_lotsfree_percent = 10;

/* The 6 states: */
arc_state_t ARC_anon;
arc_state_t ARC_mru;
arc_state_t ARC_mru_ghost;
arc_state_t ARC_mfu;
arc_state_t ARC_mfu_ghost;
arc_state_t ARC_l2c_only;

arc_stats_t arc_stats = {
        { "hits",                       KSTAT_DATA_UINT64 },
        { "misses",                     KSTAT_DATA_UINT64 },
        { "demand_data_hits",           KSTAT_DATA_UINT64 },
        { "demand_data_misses",         KSTAT_DATA_UINT64 },
        { "demand_metadata_hits",       KSTAT_DATA_UINT64 },
        { "demand_metadata_misses",     KSTAT_DATA_UINT64 },
        { "prefetch_data_hits",         KSTAT_DATA_UINT64 },
        { "prefetch_data_misses",       KSTAT_DATA_UINT64 },
        { "prefetch_metadata_hits",     KSTAT_DATA_UINT64 },
        { "prefetch_metadata_misses",   KSTAT_DATA_UINT64 },
        { "mru_hits",                   KSTAT_DATA_UINT64 },
        { "mru_ghost_hits",             KSTAT_DATA_UINT64 },
        { "mfu_hits",                   KSTAT_DATA_UINT64 },
        { "mfu_ghost_hits",             KSTAT_DATA_UINT64 },
        { "deleted",                    KSTAT_DATA_UINT64 },
        { "mutex_miss",                 KSTAT_DATA_UINT64 },
        { "access_skip",                KSTAT_DATA_UINT64 },
        { "evict_skip",                 KSTAT_DATA_UINT64 },
        { "evict_not_enough",           KSTAT_DATA_UINT64 },
        { "evict_l2_cached",            KSTAT_DATA_UINT64 },
        { "evict_l2_eligible",          KSTAT_DATA_UINT64 },
        { "evict_l2_ineligible",        KSTAT_DATA_UINT64 },
        { "evict_l2_skip",              KSTAT_DATA_UINT64 },
        { "hash_elements",              KSTAT_DATA_UINT64 },
        { "hash_elements_max",          KSTAT_DATA_UINT64 },
        { "hash_collisions",            KSTAT_DATA_UINT64 },
        { "hash_chains",                KSTAT_DATA_UINT64 },
        { "hash_chain_max",             KSTAT_DATA_UINT64 },
        { "p",                          KSTAT_DATA_UINT64 },
        { "c",                          KSTAT_DATA_UINT64 },
        { "c_min",                      KSTAT_DATA_UINT64 },
        { "c_max",                      KSTAT_DATA_UINT64 },
        { "size",                       KSTAT_DATA_UINT64 },
        { "compressed_size",            KSTAT_DATA_UINT64 },
        { "uncompressed_size",          KSTAT_DATA_UINT64 },
        { "overhead_size",              KSTAT_DATA_UINT64 },
        { "hdr_size",                   KSTAT_DATA_UINT64 },
        { "data_size",                  KSTAT_DATA_UINT64 },
        { "metadata_size",              KSTAT_DATA_UINT64 },
        { "dbuf_size",                  KSTAT_DATA_UINT64 },
        { "dnode_size",                 KSTAT_DATA_UINT64 },
        { "bonus_size",                 KSTAT_DATA_UINT64 },
#if defined(COMPAT_FREEBSD11)
        { "other_size",                 KSTAT_DATA_UINT64 },
#endif
        { "anon_size",                  KSTAT_DATA_UINT64 },
        { "anon_evictable_data",        KSTAT_DATA_UINT64 },
        { "anon_evictable_metadata",    KSTAT_DATA_UINT64 },
        { "mru_size",                   KSTAT_DATA_UINT64 },
        { "mru_evictable_data",         KSTAT_DATA_UINT64 },
        { "mru_evictable_metadata",     KSTAT_DATA_UINT64 },
        { "mru_ghost_size",             KSTAT_DATA_UINT64 },
        { "mru_ghost_evictable_data",   KSTAT_DATA_UINT64 },
        { "mru_ghost_evictable_metadata", KSTAT_DATA_UINT64 },
        { "mfu_size",                   KSTAT_DATA_UINT64 },
        { "mfu_evictable_data",         KSTAT_DATA_UINT64 },
        { "mfu_evictable_metadata",     KSTAT_DATA_UINT64 },
        { "mfu_ghost_size",             KSTAT_DATA_UINT64 },
        { "mfu_ghost_evictable_data",   KSTAT_DATA_UINT64 },
        { "mfu_ghost_evictable_metadata", KSTAT_DATA_UINT64 },
        { "l2_hits",                    KSTAT_DATA_UINT64 },
        { "l2_misses",                  KSTAT_DATA_UINT64 },
        { "l2_feeds",                   KSTAT_DATA_UINT64 },
        { "l2_rw_clash",                KSTAT_DATA_UINT64 },
        { "l2_read_bytes",              KSTAT_DATA_UINT64 },
        { "l2_write_bytes",             KSTAT_DATA_UINT64 },
        { "l2_writes_sent",             KSTAT_DATA_UINT64 },
        { "l2_writes_done",             KSTAT_DATA_UINT64 },
        { "l2_writes_error",            KSTAT_DATA_UINT64 },
        { "l2_writes_lock_retry",       KSTAT_DATA_UINT64 },
        { "l2_evict_lock_retry",        KSTAT_DATA_UINT64 },
        { "l2_evict_reading",           KSTAT_DATA_UINT64 },
        { "l2_evict_l1cached",          KSTAT_DATA_UINT64 },
        { "l2_free_on_write",           KSTAT_DATA_UINT64 },
        { "l2_abort_lowmem",            KSTAT_DATA_UINT64 },
        { "l2_cksum_bad",               KSTAT_DATA_UINT64 },
        { "l2_io_error",                KSTAT_DATA_UINT64 },
        { "l2_size",                    KSTAT_DATA_UINT64 },
        { "l2_asize",                   KSTAT_DATA_UINT64 },
        { "l2_hdr_size",                KSTAT_DATA_UINT64 },
        { "l2_log_blk_writes",          KSTAT_DATA_UINT64 },
        { "l2_log_blk_avg_asize",       KSTAT_DATA_UINT64 },
        { "l2_log_blk_asize",           KSTAT_DATA_UINT64 },
        { "l2_log_blk_count",           KSTAT_DATA_UINT64 },
        { "l2_data_to_meta_ratio",      KSTAT_DATA_UINT64 },
        { "l2_rebuild_success",         KSTAT_DATA_UINT64 },
        { "l2_rebuild_unsupported",     KSTAT_DATA_UINT64 },
        { "l2_rebuild_io_errors",       KSTAT_DATA_UINT64 },
        { "l2_rebuild_dh_errors",       KSTAT_DATA_UINT64 },
        { "l2_rebuild_cksum_lb_errors", KSTAT_DATA_UINT64 },
        { "l2_rebuild_lowmem",          KSTAT_DATA_UINT64 },
        { "l2_rebuild_size",            KSTAT_DATA_UINT64 },
        { "l2_rebuild_asize",           KSTAT_DATA_UINT64 },
        { "l2_rebuild_bufs",            KSTAT_DATA_UINT64 },
        { "l2_rebuild_bufs_precached",  KSTAT_DATA_UINT64 },
        { "l2_rebuild_log_blks",        KSTAT_DATA_UINT64 },
        { "memory_throttle_count",      KSTAT_DATA_UINT64 },
        { "memory_direct_count",        KSTAT_DATA_UINT64 },
        { "memory_indirect_count",      KSTAT_DATA_UINT64 },
        { "memory_all_bytes",           KSTAT_DATA_UINT64 },
        { "memory_free_bytes",          KSTAT_DATA_UINT64 },
        { "memory_available_bytes",     KSTAT_DATA_INT64 },
        { "arc_no_grow",                KSTAT_DATA_UINT64 },
        { "arc_tempreserve",            KSTAT_DATA_UINT64 },
        { "arc_loaned_bytes",           KSTAT_DATA_UINT64 },
        { "arc_prune",                  KSTAT_DATA_UINT64 },
        { "arc_meta_used",              KSTAT_DATA_UINT64 },
        { "arc_meta_limit",             KSTAT_DATA_UINT64 },
        { "arc_dnode_limit",            KSTAT_DATA_UINT64 },
        { "arc_meta_max",               KSTAT_DATA_UINT64 },
        { "arc_meta_min",               KSTAT_DATA_UINT64 },
        { "async_upgrade_sync",         KSTAT_DATA_UINT64 },
        { "demand_hit_predictive_prefetch", KSTAT_DATA_UINT64 },
        { "demand_hit_prescient_prefetch", KSTAT_DATA_UINT64 },
        { "arc_need_free",              KSTAT_DATA_UINT64 },
        { "arc_sys_free",               KSTAT_DATA_UINT64 },
        { "arc_raw_size",               KSTAT_DATA_UINT64 },
        { "cached_only_in_progress",    KSTAT_DATA_UINT64 },
        { "abd_chunk_waste_size",       KSTAT_DATA_UINT64 },
};

#define ARCSTAT_MAX(stat, val) {                                        \
        uint64_t m;                                                     \
        while ((val) > (m = arc_stats.stat.value.ui64) &&               \
            (m != atomic_cas_64(&arc_stats.stat.value.ui64, m, (val)))) \
                continue;                                               \
}

#define ARCSTAT_MAXSTAT(stat) \
        ARCSTAT_MAX(stat##_max, arc_stats.stat.value.ui64)

/*
 * We define a macro to allow ARC hits/misses to be easily broken down by
 * two separate conditions, giving a total of four different subtypes for
 * each of hits and misses (so eight statistics total).
 */
#define ARCSTAT_CONDSTAT(cond1, stat1, notstat1, cond2, stat2, notstat2, stat) \
        if (cond1) {                                                    \
                if (cond2) {                                            \
                        ARCSTAT_BUMP(arcstat_##stat1##_##stat2##_##stat); \
                } else {                                                \
                        ARCSTAT_BUMP(arcstat_##stat1##_##notstat2##_##stat); \
                }                                                       \
        } else {                                                        \
                if (cond2) {                                            \
                        ARCSTAT_BUMP(arcstat_##notstat1##_##stat2##_##stat); \
                } else {                                                \
                        ARCSTAT_BUMP(arcstat_##notstat1##_##notstat2##_##stat);\
                }                                                       \
        }

/*
 * This macro allows us to use kstats as floating averages. Each time we
 * update this kstat, we first factor it and the update value by
 * ARCSTAT_AVG_FACTOR to shrink the new value's contribution to the overall
 * average. This macro assumes that integer loads and stores are atomic, but
 * is not safe for multiple writers updating the kstat in parallel (only the
 * last writer's update will remain).
 */
#define ARCSTAT_F_AVG_FACTOR    3
#define ARCSTAT_F_AVG(stat, value) \
        do { \
                uint64_t x = ARCSTAT(stat); \
                x = x - x / ARCSTAT_F_AVG_FACTOR + \
                    (value) / ARCSTAT_F_AVG_FACTOR; \
                ARCSTAT(stat) = x; \
                _NOTE(CONSTCOND) \
        } while (0)

kstat_t                 *arc_ksp;
static arc_state_t      *arc_anon;
static arc_state_t      *arc_mru_ghost;
static arc_state_t      *arc_mfu_ghost;
static arc_state_t      *arc_l2c_only;

arc_state_t     *arc_mru;
arc_state_t     *arc_mfu;

/*
 * There are several ARC variables that are critical to export as kstats --
 * but we don't want to have to grovel around in the kstat whenever we wish to
 * manipulate them.  For these variables, we therefore define them to be in
 * terms of the statistic variable.  This assures that we are not introducing
 * the possibility of inconsistency by having shadow copies of the variables,
 * while still allowing the code to be readable.
 */
#define arc_tempreserve ARCSTAT(arcstat_tempreserve)
#define arc_loaned_bytes        ARCSTAT(arcstat_loaned_bytes)
#define arc_meta_limit  ARCSTAT(arcstat_meta_limit) /* max size for metadata */
/* max size for dnodes */
#define arc_dnode_size_limit    ARCSTAT(arcstat_dnode_limit)
#define arc_meta_min    ARCSTAT(arcstat_meta_min) /* min size for metadata */
#define arc_meta_max    ARCSTAT(arcstat_meta_max) /* max size of metadata */
#define arc_need_free   ARCSTAT(arcstat_need_free) /* waiting to be evicted */

/* size of all b_rabd's in entire arc */
#define arc_raw_size    ARCSTAT(arcstat_raw_size)
/* compressed size of entire arc */
#define arc_compressed_size     ARCSTAT(arcstat_compressed_size)
/* uncompressed size of entire arc */
#define arc_uncompressed_size   ARCSTAT(arcstat_uncompressed_size)
/* number of bytes in the arc from arc_buf_t's */
#define arc_overhead_size       ARCSTAT(arcstat_overhead_size)

/*
 * There are also some ARC variables that we want to export, but that are
 * updated so often that having the canonical representation be the statistic
 * variable causes a performance bottleneck. We want to use aggsum_t's for these
 * instead, but still be able to export the kstat in the same way as before.
 * The solution is to always use the aggsum version, except in the kstat update
 * callback.
 */
aggsum_t arc_size;
aggsum_t arc_meta_used;
aggsum_t astat_data_size;
aggsum_t astat_metadata_size;
aggsum_t astat_dbuf_size;
aggsum_t astat_dnode_size;
aggsum_t astat_bonus_size;
aggsum_t astat_hdr_size;
aggsum_t astat_l2_hdr_size;
aggsum_t astat_abd_chunk_waste_size;

hrtime_t arc_growtime;
list_t arc_prune_list;
kmutex_t arc_prune_mtx;
taskq_t *arc_prune_taskq;

#define GHOST_STATE(state)      \
        ((state) == arc_mru_ghost || (state) == arc_mfu_ghost ||        \
        (state) == arc_l2c_only)

#define HDR_IN_HASH_TABLE(hdr)  ((hdr)->b_flags & ARC_FLAG_IN_HASH_TABLE)
#define HDR_IO_IN_PROGRESS(hdr) ((hdr)->b_flags & ARC_FLAG_IO_IN_PROGRESS)
#define HDR_IO_ERROR(hdr)       ((hdr)->b_flags & ARC_FLAG_IO_ERROR)
#define HDR_PREFETCH(hdr)       ((hdr)->b_flags & ARC_FLAG_PREFETCH)
#define HDR_PRESCIENT_PREFETCH(hdr)     \
        ((hdr)->b_flags & ARC_FLAG_PRESCIENT_PREFETCH)
#define HDR_COMPRESSION_ENABLED(hdr)    \
        ((hdr)->b_flags & ARC_FLAG_COMPRESSED_ARC)

#define HDR_L2CACHE(hdr)        ((hdr)->b_flags & ARC_FLAG_L2CACHE)
#define HDR_L2_READING(hdr)     \
        (((hdr)->b_flags & ARC_FLAG_IO_IN_PROGRESS) &&  \
        ((hdr)->b_flags & ARC_FLAG_HAS_L2HDR))
#define HDR_L2_WRITING(hdr)     ((hdr)->b_flags & ARC_FLAG_L2_WRITING)
#define HDR_L2_EVICTED(hdr)     ((hdr)->b_flags & ARC_FLAG_L2_EVICTED)
#define HDR_L2_WRITE_HEAD(hdr)  ((hdr)->b_flags & ARC_FLAG_L2_WRITE_HEAD)
#define HDR_PROTECTED(hdr)      ((hdr)->b_flags & ARC_FLAG_PROTECTED)
#define HDR_NOAUTH(hdr)         ((hdr)->b_flags & ARC_FLAG_NOAUTH)
#define HDR_SHARED_DATA(hdr)    ((hdr)->b_flags & ARC_FLAG_SHARED_DATA)

#define HDR_ISTYPE_METADATA(hdr)        \
        ((hdr)->b_flags & ARC_FLAG_BUFC_METADATA)
#define HDR_ISTYPE_DATA(hdr)    (!HDR_ISTYPE_METADATA(hdr))

#define HDR_HAS_L1HDR(hdr)      ((hdr)->b_flags & ARC_FLAG_HAS_L1HDR)
#define HDR_HAS_L2HDR(hdr)      ((hdr)->b_flags & ARC_FLAG_HAS_L2HDR)
#define HDR_HAS_RABD(hdr)       \
        (HDR_HAS_L1HDR(hdr) && HDR_PROTECTED(hdr) &&    \
        (hdr)->b_crypt_hdr.b_rabd != NULL)
#define HDR_ENCRYPTED(hdr)      \
        (HDR_PROTECTED(hdr) && DMU_OT_IS_ENCRYPTED((hdr)->b_crypt_hdr.b_ot))
#define HDR_AUTHENTICATED(hdr)  \
        (HDR_PROTECTED(hdr) && !DMU_OT_IS_ENCRYPTED((hdr)->b_crypt_hdr.b_ot))

/* For storing compression mode in b_flags */
#define HDR_COMPRESS_OFFSET     (highbit64(ARC_FLAG_COMPRESS_0) - 1)

#define HDR_GET_COMPRESS(hdr)   ((enum zio_compress)BF32_GET((hdr)->b_flags, \
        HDR_COMPRESS_OFFSET, SPA_COMPRESSBITS))
#define HDR_SET_COMPRESS(hdr, cmp) BF32_SET((hdr)->b_flags, \
        HDR_COMPRESS_OFFSET, SPA_COMPRESSBITS, (cmp));

#define ARC_BUF_LAST(buf)       ((buf)->b_next == NULL)
#define ARC_BUF_SHARED(buf)     ((buf)->b_flags & ARC_BUF_FLAG_SHARED)
#define ARC_BUF_COMPRESSED(buf) ((buf)->b_flags & ARC_BUF_FLAG_COMPRESSED)
#define ARC_BUF_ENCRYPTED(buf)  ((buf)->b_flags & ARC_BUF_FLAG_ENCRYPTED)

/*
 * Other sizes
 */

#define HDR_FULL_CRYPT_SIZE ((int64_t)sizeof (arc_buf_hdr_t))
#define HDR_FULL_SIZE ((int64_t)offsetof(arc_buf_hdr_t, b_crypt_hdr))
#define HDR_L2ONLY_SIZE ((int64_t)offsetof(arc_buf_hdr_t, b_l1hdr))

/*
 * Hash table routines
 */

#define HT_LOCK_ALIGN   64
#define HT_LOCK_PAD     (P2NPHASE(sizeof (kmutex_t), (HT_LOCK_ALIGN)))

struct ht_lock {
        kmutex_t        ht_lock;
#ifdef _KERNEL
        unsigned char   pad[HT_LOCK_PAD];
#endif
};

#define BUF_LOCKS 8192
typedef struct buf_hash_table {
        uint64_t ht_mask;
        arc_buf_hdr_t **ht_table;
        struct ht_lock ht_locks[BUF_LOCKS];
} buf_hash_table_t;

static buf_hash_table_t buf_hash_table;

#define BUF_HASH_INDEX(spa, dva, birth) \
        (buf_hash(spa, dva, birth) & buf_hash_table.ht_mask)
#define BUF_HASH_LOCK_NTRY(idx) (buf_hash_table.ht_locks[idx & (BUF_LOCKS-1)])
#define BUF_HASH_LOCK(idx)      (&(BUF_HASH_LOCK_NTRY(idx).ht_lock))
#define HDR_LOCK(hdr) \
        (BUF_HASH_LOCK(BUF_HASH_INDEX(hdr->b_spa, &hdr->b_dva, hdr->b_birth)))

uint64_t zfs_crc64_table[256];

/*
 * Level 2 ARC
 */

#define L2ARC_WRITE_SIZE        (8 * 1024 * 1024)       /* initial write max */
#define L2ARC_HEADROOM          2                       /* num of writes */

/*
 * If we discover during ARC scan any buffers to be compressed, we boost
 * our headroom for the next scanning cycle by this percentage multiple.
 */
#define L2ARC_HEADROOM_BOOST    200
#define L2ARC_FEED_SECS         1               /* caching interval secs */
#define L2ARC_FEED_MIN_MS       200             /* min caching interval ms */

/*
 * We can feed L2ARC from two states of ARC buffers, mru and mfu,
 * and each of the state has two types: data and metadata.
 */
#define L2ARC_FEED_TYPES        4

#define l2arc_writes_sent       ARCSTAT(arcstat_l2_writes_sent)
#define l2arc_writes_done       ARCSTAT(arcstat_l2_writes_done)

/* L2ARC Performance Tunables */
unsigned long l2arc_write_max = L2ARC_WRITE_SIZE;       /* def max write size */
unsigned long l2arc_write_boost = L2ARC_WRITE_SIZE;     /* extra warmup write */
unsigned long l2arc_headroom = L2ARC_HEADROOM;          /* # of dev writes */
unsigned long l2arc_headroom_boost = L2ARC_HEADROOM_BOOST;
unsigned long l2arc_feed_secs = L2ARC_FEED_SECS;        /* interval seconds */
unsigned long l2arc_feed_min_ms = L2ARC_FEED_MIN_MS;    /* min interval msecs */
int l2arc_noprefetch = B_TRUE;                  /* don't cache prefetch bufs */
int l2arc_feed_again = B_TRUE;                  /* turbo warmup */
int l2arc_norw = B_FALSE;                       /* no reads during writes */
int l2arc_meta_percent = 33;                    /* limit on headers size */

/*
 * L2ARC Internals
 */
static list_t L2ARC_dev_list;                   /* device list */
static list_t *l2arc_dev_list;                  /* device list pointer */
static kmutex_t l2arc_dev_mtx;                  /* device list mutex */
static l2arc_dev_t *l2arc_dev_last;             /* last device used */
static list_t L2ARC_free_on_write;              /* free after write buf list */
static list_t *l2arc_free_on_write;             /* free after write list ptr */
static kmutex_t l2arc_free_on_write_mtx;        /* mutex for list */
static uint64_t l2arc_ndev;                     /* number of devices */

typedef struct l2arc_read_callback {
        arc_buf_hdr_t           *l2rcb_hdr;             /* read header */
        blkptr_t                l2rcb_bp;               /* original blkptr */
        zbookmark_phys_t        l2rcb_zb;               /* original bookmark */
        int                     l2rcb_flags;            /* original flags */
        abd_t                   *l2rcb_abd;             /* temporary buffer */
} l2arc_read_callback_t;

typedef struct l2arc_data_free {
        /* protected by l2arc_free_on_write_mtx */
        abd_t           *l2df_abd;
        size_t          l2df_size;
        arc_buf_contents_t l2df_type;
        list_node_t     l2df_list_node;
} l2arc_data_free_t;

typedef enum arc_fill_flags {
        ARC_FILL_LOCKED         = 1 << 0, /* hdr lock is held */
        ARC_FILL_COMPRESSED     = 1 << 1, /* fill with compressed data */
        ARC_FILL_ENCRYPTED      = 1 << 2, /* fill with encrypted data */
        ARC_FILL_NOAUTH         = 1 << 3, /* don't attempt to authenticate */
        ARC_FILL_IN_PLACE       = 1 << 4  /* fill in place (special case) */
} arc_fill_flags_t;

static kmutex_t l2arc_feed_thr_lock;
static kcondvar_t l2arc_feed_thr_cv;
static uint8_t l2arc_thread_exit;

static kmutex_t l2arc_rebuild_thr_lock;
static kcondvar_t l2arc_rebuild_thr_cv;

enum arc_hdr_alloc_flags {
        ARC_HDR_ALLOC_RDATA = 0x1,
        ARC_HDR_DO_ADAPT = 0x2,
};


static abd_t *arc_get_data_abd(arc_buf_hdr_t *, uint64_t, void *, boolean_t);
static void *arc_get_data_buf(arc_buf_hdr_t *, uint64_t, void *);
static void arc_get_data_impl(arc_buf_hdr_t *, uint64_t, void *, boolean_t);
static void arc_free_data_abd(arc_buf_hdr_t *, abd_t *, uint64_t, void *);
static void arc_free_data_buf(arc_buf_hdr_t *, void *, uint64_t, void *);
static void arc_free_data_impl(arc_buf_hdr_t *hdr, uint64_t size, void *tag);
static void arc_hdr_free_abd(arc_buf_hdr_t *, boolean_t);
static void arc_hdr_alloc_abd(arc_buf_hdr_t *, int);
static void arc_access(arc_buf_hdr_t *, kmutex_t *);
static void arc_buf_watch(arc_buf_t *);

static arc_buf_contents_t arc_buf_type(arc_buf_hdr_t *);
static uint32_t arc_bufc_to_flags(arc_buf_contents_t);
static inline void arc_hdr_set_flags(arc_buf_hdr_t *hdr, arc_flags_t flags);
static inline void arc_hdr_clear_flags(arc_buf_hdr_t *hdr, arc_flags_t flags);

static boolean_t l2arc_write_eligible(uint64_t, arc_buf_hdr_t *);
static void l2arc_read_done(zio_t *);
static void l2arc_do_free_on_write(void);

/*
 * l2arc_mfuonly : A ZFS module parameter that controls whether only MFU
 *              metadata and data are cached from ARC into L2ARC.
 */
int l2arc_mfuonly = 0;

/*
 * L2ARC TRIM
 * l2arc_trim_ahead : A ZFS module parameter that controls how much ahead of
 *              the current write size (l2arc_write_max) we should TRIM if we
 *              have filled the device. It is defined as a percentage of the
 *              write size. If set to 100 we trim twice the space required to
 *              accommodate upcoming writes. A minimum of 64MB will be trimmed.
 *              It also enables TRIM of the whole L2ARC device upon creation or
 *              addition to an existing pool or if the header of the device is
 *              invalid upon importing a pool or onlining a cache device. The
 *              default is 0, which disables TRIM on L2ARC altogether as it can
 *              put significant stress on the underlying storage devices. This
 *              will vary depending of how well the specific device handles
 *              these commands.
 */
unsigned long l2arc_trim_ahead = 0;

/*
 * Performance tuning of L2ARC persistence:
 *
 * l2arc_rebuild_enabled : A ZFS module parameter that controls whether adding
 *              an L2ARC device (either at pool import or later) will attempt
 *              to rebuild L2ARC buffer contents.
 * l2arc_rebuild_blocks_min_l2size : A ZFS module parameter that controls
 *              whether log blocks are written to the L2ARC device. If the L2ARC
 *              device is less than 1GB, the amount of data l2arc_evict()
 *              evicts is significant compared to the amount of restored L2ARC
 *              data. In this case do not write log blocks in L2ARC in order
 *              not to waste space.
 */
int l2arc_rebuild_enabled = B_TRUE;
unsigned long l2arc_rebuild_blocks_min_l2size = 1024 * 1024 * 1024;

/* L2ARC persistence rebuild control routines. */
void l2arc_rebuild_vdev(vdev_t *vd, boolean_t reopen);
static void l2arc_dev_rebuild_thread(void *arg);
static int l2arc_rebuild(l2arc_dev_t *dev);

/* L2ARC persistence read I/O routines. */
static int l2arc_dev_hdr_read(l2arc_dev_t *dev);
static int l2arc_log_blk_read(l2arc_dev_t *dev,
    const l2arc_log_blkptr_t *this_lp, const l2arc_log_blkptr_t *next_lp,
    l2arc_log_blk_phys_t *this_lb, l2arc_log_blk_phys_t *next_lb,
    zio_t *this_io, zio_t **next_io);
static zio_t *l2arc_log_blk_fetch(vdev_t *vd,
    const l2arc_log_blkptr_t *lp, l2arc_log_blk_phys_t *lb);
static void l2arc_log_blk_fetch_abort(zio_t *zio);

/* L2ARC persistence block restoration routines. */
static void l2arc_log_blk_restore(l2arc_dev_t *dev,
    const l2arc_log_blk_phys_t *lb, uint64_t lb_asize, uint64_t lb_daddr);
static void l2arc_hdr_restore(const l2arc_log_ent_phys_t *le,
    l2arc_dev_t *dev);

/* L2ARC persistence write I/O routines. */
static void l2arc_log_blk_commit(l2arc_dev_t *dev, zio_t *pio,
    l2arc_write_callback_t *cb);

/* L2ARC persistence auxiliary routines. */
boolean_t l2arc_log_blkptr_valid(l2arc_dev_t *dev,
    const l2arc_log_blkptr_t *lbp);
static boolean_t l2arc_log_blk_insert(l2arc_dev_t *dev,
    const arc_buf_hdr_t *ab);
boolean_t l2arc_range_check_overlap(uint64_t bottom,
    uint64_t top, uint64_t check);
static void l2arc_blk_fetch_done(zio_t *zio);
static inline uint64_t
    l2arc_log_blk_overhead(uint64_t write_sz, l2arc_dev_t *dev);

/*
 * We use Cityhash for this. It's fast, and has good hash properties without
 * requiring any large static buffers.
 */
static uint64_t
buf_hash(uint64_t spa, const dva_t *dva, uint64_t birth)
{
        return (cityhash4(spa, dva->dva_word[0], dva->dva_word[1], birth));
}

#define HDR_EMPTY(hdr)                                          \
        ((hdr)->b_dva.dva_word[0] == 0 &&                       \
        (hdr)->b_dva.dva_word[1] == 0)

#define HDR_EMPTY_OR_LOCKED(hdr)                                \
        (HDR_EMPTY(hdr) || MUTEX_HELD(HDR_LOCK(hdr)))

#define HDR_EQUAL(spa, dva, birth, hdr)                         \
        ((hdr)->b_dva.dva_word[0] == (dva)->dva_word[0]) &&     \
        ((hdr)->b_dva.dva_word[1] == (dva)->dva_word[1]) &&     \
        ((hdr)->b_birth == birth) && ((hdr)->b_spa == spa)

static void
buf_discard_identity(arc_buf_hdr_t *hdr)
{
        hdr->b_dva.dva_word[0] = 0;
        hdr->b_dva.dva_word[1] = 0;
        hdr->b_birth = 0;
}

static arc_buf_hdr_t *
buf_hash_find(uint64_t spa, const blkptr_t *bp, kmutex_t **lockp)
{
        const dva_t *dva = BP_IDENTITY(bp);
        uint64_t birth = BP_PHYSICAL_BIRTH(bp);
        uint64_t idx = BUF_HASH_INDEX(spa, dva, birth);
        kmutex_t *hash_lock = BUF_HASH_LOCK(idx);
        arc_buf_hdr_t *hdr;

        mutex_enter(hash_lock);
        for (hdr = buf_hash_table.ht_table[idx]; hdr != NULL;
            hdr = hdr->b_hash_next) {
                if (HDR_EQUAL(spa, dva, birth, hdr)) {
                        *lockp = hash_lock;
                        return (hdr);
                }
        }
        mutex_exit(hash_lock);
        *lockp = NULL;
        return (NULL);
}

/*
 * Insert an entry into the hash table.  If there is already an element
 * equal to elem in the hash table, then the already existing element
 * will be returned and the new element will not be inserted.
 * Otherwise returns NULL.
 * If lockp == NULL, the caller is assumed to already hold the hash lock.
 */
static arc_buf_hdr_t *
buf_hash_insert(arc_buf_hdr_t *hdr, kmutex_t **lockp)
{
        uint64_t idx = BUF_HASH_INDEX(hdr->b_spa, &hdr->b_dva, hdr->b_birth);
        kmutex_t *hash_lock = BUF_HASH_LOCK(idx);
        arc_buf_hdr_t *fhdr;
        uint32_t i;

        ASSERT(!DVA_IS_EMPTY(&hdr->b_dva));
        ASSERT(hdr->b_birth != 0);
        ASSERT(!HDR_IN_HASH_TABLE(hdr));

        if (lockp != NULL) {
                *lockp = hash_lock;
                mutex_enter(hash_lock);
        } else {
                ASSERT(MUTEX_HELD(hash_lock));
        }

        for (fhdr = buf_hash_table.ht_table[idx], i = 0; fhdr != NULL;
            fhdr = fhdr->b_hash_next, i++) {
                if (HDR_EQUAL(hdr->b_spa, &hdr->b_dva, hdr->b_birth, fhdr))
                        return (fhdr);
        }

        hdr->b_hash_next = buf_hash_table.ht_table[idx];
        buf_hash_table.ht_table[idx] = hdr;
        arc_hdr_set_flags(hdr, ARC_FLAG_IN_HASH_TABLE);

        /* collect some hash table performance data */
        if (i > 0) {
                ARCSTAT_BUMP(arcstat_hash_collisions);
                if (i == 1)
                        ARCSTAT_BUMP(arcstat_hash_chains);

                ARCSTAT_MAX(arcstat_hash_chain_max, i);
        }

        ARCSTAT_BUMP(arcstat_hash_elements);
        ARCSTAT_MAXSTAT(arcstat_hash_elements);

        return (NULL);
}

static void
buf_hash_remove(arc_buf_hdr_t *hdr)
{
        arc_buf_hdr_t *fhdr, **hdrp;
        uint64_t idx = BUF_HASH_INDEX(hdr->b_spa, &hdr->b_dva, hdr->b_birth);

        ASSERT(MUTEX_HELD(BUF_HASH_LOCK(idx)));
        ASSERT(HDR_IN_HASH_TABLE(hdr));

        hdrp = &buf_hash_table.ht_table[idx];
        while ((fhdr = *hdrp) != hdr) {
                ASSERT3P(fhdr, !=, NULL);
                hdrp = &fhdr->b_hash_next;
        }
        *hdrp = hdr->b_hash_next;
        hdr->b_hash_next = NULL;
        arc_hdr_clear_flags(hdr, ARC_FLAG_IN_HASH_TABLE);

        /* collect some hash table performance data */
        ARCSTAT_BUMPDOWN(arcstat_hash_elements);

        if (buf_hash_table.ht_table[idx] &&
            buf_hash_table.ht_table[idx]->b_hash_next == NULL)
                ARCSTAT_BUMPDOWN(arcstat_hash_chains);
}

/*
 * Global data structures and functions for the buf kmem cache.
 */

static kmem_cache_t *hdr_full_cache;
static kmem_cache_t *hdr_full_crypt_cache;
static kmem_cache_t *hdr_l2only_cache;
static kmem_cache_t *buf_cache;

static void
buf_fini(void)
{
        int i;

#if defined(_KERNEL)
        /*
         * Large allocations which do not require contiguous pages
         * should be using vmem_free() in the linux kernel\
         */
        vmem_free(buf_hash_table.ht_table,
            (buf_hash_table.ht_mask + 1) * sizeof (void *));
#else
        kmem_free(buf_hash_table.ht_table,
            (buf_hash_table.ht_mask + 1) * sizeof (void *));
#endif
        for (i = 0; i < BUF_LOCKS; i++)
                mutex_destroy(&buf_hash_table.ht_locks[i].ht_lock);
        kmem_cache_destroy(hdr_full_cache);
        kmem_cache_destroy(hdr_full_crypt_cache);
        kmem_cache_destroy(hdr_l2only_cache);
        kmem_cache_destroy(buf_cache);
}

/*
 * Constructor callback - called when the cache is empty
 * and a new buf is requested.
 */
/* ARGSUSED */
static int
hdr_full_cons(void *vbuf, void *unused, int kmflag)
{
        arc_buf_hdr_t *hdr = vbuf;

        bzero(hdr, HDR_FULL_SIZE);
        hdr->b_l1hdr.b_byteswap = DMU_BSWAP_NUMFUNCS;
        cv_init(&hdr->b_l1hdr.b_cv, NULL, CV_DEFAULT, NULL);
        zfs_refcount_create(&hdr->b_l1hdr.b_refcnt);
        mutex_init(&hdr->b_l1hdr.b_freeze_lock, NULL, MUTEX_DEFAULT, NULL);
        list_link_init(&hdr->b_l1hdr.b_arc_node);
        list_link_init(&hdr->b_l2hdr.b_l2node);
        multilist_link_init(&hdr->b_l1hdr.b_arc_node);
        arc_space_consume(HDR_FULL_SIZE, ARC_SPACE_HDRS);

        return (0);
}

/* ARGSUSED */
static int
hdr_full_crypt_cons(void *vbuf, void *unused, int kmflag)
{
        arc_buf_hdr_t *hdr = vbuf;

        hdr_full_cons(vbuf, unused, kmflag);
        bzero(&hdr->b_crypt_hdr, sizeof (hdr->b_crypt_hdr));
        arc_space_consume(sizeof (hdr->b_crypt_hdr), ARC_SPACE_HDRS);

        return (0);
}

/* ARGSUSED */
static int
hdr_l2only_cons(void *vbuf, void *unused, int kmflag)
{
        arc_buf_hdr_t *hdr = vbuf;

        bzero(hdr, HDR_L2ONLY_SIZE);
        arc_space_consume(HDR_L2ONLY_SIZE, ARC_SPACE_L2HDRS);

        return (0);
}

/* ARGSUSED */
static int
buf_cons(void *vbuf, void *unused, int kmflag)
{
        arc_buf_t *buf = vbuf;

        bzero(buf, sizeof (arc_buf_t));
        mutex_init(&buf->b_evict_lock, NULL, MUTEX_DEFAULT, NULL);
        arc_space_consume(sizeof (arc_buf_t), ARC_SPACE_HDRS);

        return (0);
}

/*
 * Destructor callback - called when a cached buf is
 * no longer required.
 */
/* ARGSUSED */
static void
hdr_full_dest(void *vbuf, void *unused)
{
        arc_buf_hdr_t *hdr = vbuf;

        ASSERT(HDR_EMPTY(hdr));
        cv_destroy(&hdr->b_l1hdr.b_cv);
        zfs_refcount_destroy(&hdr->b_l1hdr.b_refcnt);
        mutex_destroy(&hdr->b_l1hdr.b_freeze_lock);
        ASSERT(!multilist_link_active(&hdr->b_l1hdr.b_arc_node));
        arc_space_return(HDR_FULL_SIZE, ARC_SPACE_HDRS);
}

/* ARGSUSED */
static void
hdr_full_crypt_dest(void *vbuf, void *unused)
{
        arc_buf_hdr_t *hdr = vbuf;

        hdr_full_dest(vbuf, unused);
        arc_space_return(sizeof (hdr->b_crypt_hdr), ARC_SPACE_HDRS);
}

/* ARGSUSED */
static void
hdr_l2only_dest(void *vbuf, void *unused)
{
        arc_buf_hdr_t *hdr __maybe_unused = vbuf;

        ASSERT(HDR_EMPTY(hdr));
        arc_space_return(HDR_L2ONLY_SIZE, ARC_SPACE_L2HDRS);
}

/* ARGSUSED */
static void
buf_dest(void *vbuf, void *unused)
{
        arc_buf_t *buf = vbuf;

        mutex_destroy(&buf->b_evict_lock);
        arc_space_return(sizeof (arc_buf_t), ARC_SPACE_HDRS);
}

static void
buf_init(void)
{
        uint64_t *ct = NULL;
        uint64_t hsize = 1ULL << 12;
        int i, j;

        /*
         * The hash table is big enough to fill all of physical memory
         * with an average block size of zfs_arc_average_blocksize (default 8K).
         * By default, the table will take up
         * totalmem * sizeof(void*) / 8K (1MB per GB with 8-byte pointers).
         */
        while (hsize * zfs_arc_average_blocksize < arc_all_memory())
                hsize <<= 1;
retry:
        buf_hash_table.ht_mask = hsize - 1;
#if defined(_KERNEL)
        /*
         * Large allocations which do not require contiguous pages
         * should be using vmem_alloc() in the linux kernel
         */
        buf_hash_table.ht_table =
            vmem_zalloc(hsize * sizeof (void*), KM_SLEEP);
#else
        buf_hash_table.ht_table =
            kmem_zalloc(hsize * sizeof (void*), KM_NOSLEEP);
#endif
        if (buf_hash_table.ht_table == NULL) {
                ASSERT(hsize > (1ULL << 8));
                hsize >>= 1;
                goto retry;
        }

        hdr_full_cache = kmem_cache_create("arc_buf_hdr_t_full", HDR_FULL_SIZE,
            0, hdr_full_cons, hdr_full_dest, NULL, NULL, NULL, 0);
        hdr_full_crypt_cache = kmem_cache_create("arc_buf_hdr_t_full_crypt",
            HDR_FULL_CRYPT_SIZE, 0, hdr_full_crypt_cons, hdr_full_crypt_dest,
            NULL, NULL, NULL, 0);
        hdr_l2only_cache = kmem_cache_create("arc_buf_hdr_t_l2only",
            HDR_L2ONLY_SIZE, 0, hdr_l2only_cons, hdr_l2only_dest, NULL,
            NULL, NULL, 0);
        buf_cache = kmem_cache_create("arc_buf_t", sizeof (arc_buf_t),
            0, buf_cons, buf_dest, NULL, NULL, NULL, 0);

        for (i = 0; i < 256; i++)
                for (ct = zfs_crc64_table + i, *ct = i, j = 8; j > 0; j--)
                        *ct = (*ct >> 1) ^ (-(*ct & 1) & ZFS_CRC64_POLY);

        for (i = 0; i < BUF_LOCKS; i++) {
                mutex_init(&buf_hash_table.ht_locks[i].ht_lock,
                    NULL, MUTEX_DEFAULT, NULL);
        }
}

#define ARC_MINTIME     (hz>>4) /* 62 ms */

/*
 * This is the size that the buf occupies in memory. If the buf is compressed,
 * it will correspond to the compressed size. You should use this method of
 * getting the buf size unless you explicitly need the logical size.
 */
uint64_t
arc_buf_size(arc_buf_t *buf)
{
        return (ARC_BUF_COMPRESSED(buf) ?
            HDR_GET_PSIZE(buf->b_hdr) : HDR_GET_LSIZE(buf->b_hdr));
}

uint64_t
arc_buf_lsize(arc_buf_t *buf)
{
        return (HDR_GET_LSIZE(buf->b_hdr));
}

/*
 * This function will return B_TRUE if the buffer is encrypted in memory.
 * This buffer can be decrypted by calling arc_untransform().
 */
boolean_t
arc_is_encrypted(arc_buf_t *buf)
{
        return (ARC_BUF_ENCRYPTED(buf) != 0);
}

/*
 * Returns B_TRUE if the buffer represents data that has not had its MAC
 * verified yet.
 */
boolean_t
arc_is_unauthenticated(arc_buf_t *buf)
{
        return (HDR_NOAUTH(buf->b_hdr) != 0);
}

void
arc_get_raw_params(arc_buf_t *buf, boolean_t *byteorder, uint8_t *salt,
    uint8_t *iv, uint8_t *mac)
{
        arc_buf_hdr_t *hdr = buf->b_hdr;

        ASSERT(HDR_PROTECTED(hdr));

        bcopy(hdr->b_crypt_hdr.b_salt, salt, ZIO_DATA_SALT_LEN);
        bcopy(hdr->b_crypt_hdr.b_iv, iv, ZIO_DATA_IV_LEN);
        bcopy(hdr->b_crypt_hdr.b_mac, mac, ZIO_DATA_MAC_LEN);
        *byteorder = (hdr->b_l1hdr.b_byteswap == DMU_BSWAP_NUMFUNCS) ?
            ZFS_HOST_BYTEORDER : !ZFS_HOST_BYTEORDER;
}

/*
 * Indicates how this buffer is compressed in memory. If it is not compressed
 * the value will be ZIO_COMPRESS_OFF. It can be made normally readable with
 * arc_untransform() as long as it is also unencrypted.
 */
enum zio_compress
arc_get_compression(arc_buf_t *buf)
{
        return (ARC_BUF_COMPRESSED(buf) ?
            HDR_GET_COMPRESS(buf->b_hdr) : ZIO_COMPRESS_OFF);
}

/*
 * Return the compression algorithm used to store this data in the ARC. If ARC
 * compression is enabled or this is an encrypted block, this will be the same
 * as what's used to store it on-disk. Otherwise, this will be ZIO_COMPRESS_OFF.
 */
static inline enum zio_compress
arc_hdr_get_compress(arc_buf_hdr_t *hdr)
{
        return (HDR_COMPRESSION_ENABLED(hdr) ?
            HDR_GET_COMPRESS(hdr) : ZIO_COMPRESS_OFF);
}

uint8_t
arc_get_complevel(arc_buf_t *buf)
{
        return (buf->b_hdr->b_complevel);
}

static inline boolean_t
arc_buf_is_shared(arc_buf_t *buf)
{
        boolean_t shared = (buf->b_data != NULL &&
            buf->b_hdr->b_l1hdr.b_pabd != NULL &&
            abd_is_linear(buf->b_hdr->b_l1hdr.b_pabd) &&
            buf->b_data == abd_to_buf(buf->b_hdr->b_l1hdr.b_pabd));
        IMPLY(shared, HDR_SHARED_DATA(buf->b_hdr));
        IMPLY(shared, ARC_BUF_SHARED(buf));
        IMPLY(shared, ARC_BUF_COMPRESSED(buf) || ARC_BUF_LAST(buf));

        /*
         * It would be nice to assert arc_can_share() too, but the "hdr isn't
         * already being shared" requirement prevents us from doing that.
         */

        return (shared);
}

/*
 * Free the checksum associated with this header. If there is no checksum, this
 * is a no-op.
 */
static inline void
arc_cksum_free(arc_buf_hdr_t *hdr)
{
        ASSERT(HDR_HAS_L1HDR(hdr));

        mutex_enter(&hdr->b_l1hdr.b_freeze_lock);
        if (hdr->b_l1hdr.b_freeze_cksum != NULL) {
                kmem_free(hdr->b_l1hdr.b_freeze_cksum, sizeof (zio_cksum_t));
                hdr->b_l1hdr.b_freeze_cksum = NULL;
        }
        mutex_exit(&hdr->b_l1hdr.b_freeze_lock);
}

/*
 * Return true iff at least one of the bufs on hdr is not compressed.
 * Encrypted buffers count as compressed.
 */
static boolean_t
arc_hdr_has_uncompressed_buf(arc_buf_hdr_t *hdr)
{
        ASSERT(hdr->b_l1hdr.b_state == arc_anon || HDR_EMPTY_OR_LOCKED(hdr));

        for (arc_buf_t *b = hdr->b_l1hdr.b_buf; b != NULL; b = b->b_next) {
                if (!ARC_BUF_COMPRESSED(b)) {
                        return (B_TRUE);
                }
        }
        return (B_FALSE);
}


/*
 * If we've turned on the ZFS_DEBUG_MODIFY flag, verify that the buf's data
 * matches the checksum that is stored in the hdr. If there is no checksum,
 * or if the buf is compressed, this is a no-op.
 */
static void
arc_cksum_verify(arc_buf_t *buf)
{
        arc_buf_hdr_t *hdr = buf->b_hdr;
        zio_cksum_t zc;

        if (!(zfs_flags & ZFS_DEBUG_MODIFY))
                return;

        if (ARC_BUF_COMPRESSED(buf))
                return;

        ASSERT(HDR_HAS_L1HDR(hdr));

        mutex_enter(&hdr->b_l1hdr.b_freeze_lock);

        if (hdr->b_l1hdr.b_freeze_cksum == NULL || HDR_IO_ERROR(hdr)) {
                mutex_exit(&hdr->b_l1hdr.b_freeze_lock);
                return;
        }

        fletcher_2_native(buf->b_data, arc_buf_size(buf), NULL, &zc);
        if (!ZIO_CHECKSUM_EQUAL(*hdr->b_l1hdr.b_freeze_cksum, zc))
                panic("buffer modified while frozen!");
        mutex_exit(&hdr->b_l1hdr.b_freeze_lock);
}

/*
 * This function makes the assumption that data stored in the L2ARC
 * will be transformed exactly as it is in the main pool. Because of
 * this we can verify the checksum against the reading process's bp.
 */
static boolean_t
arc_cksum_is_equal(arc_buf_hdr_t *hdr, zio_t *zio)
{
        ASSERT(!BP_IS_EMBEDDED(zio->io_bp));
        VERIFY3U(BP_GET_PSIZE(zio->io_bp), ==, HDR_GET_PSIZE(hdr));

        /*
         * Block pointers always store the checksum for the logical data.
         * If the block pointer has the gang bit set, then the checksum
         * it represents is for the reconstituted data and not for an
         * individual gang member. The zio pipeline, however, must be able to
         * determine the checksum of each of the gang constituents so it
         * treats the checksum comparison differently than what we need
         * for l2arc blocks. This prevents us from using the
         * zio_checksum_error() interface directly. Instead we must call the
         * zio_checksum_error_impl() so that we can ensure the checksum is
         * generated using the correct checksum algorithm and accounts for the
         * logical I/O size and not just a gang fragment.
         */
        return (zio_checksum_error_impl(zio->io_spa, zio->io_bp,
            BP_GET_CHECKSUM(zio->io_bp), zio->io_abd, zio->io_size,
            zio->io_offset, NULL) == 0);
}

/*
 * Given a buf full of data, if ZFS_DEBUG_MODIFY is enabled this computes a
 * checksum and attaches it to the buf's hdr so that we can ensure that the buf
 * isn't modified later on. If buf is compressed or there is already a checksum
 * on the hdr, this is a no-op (we only checksum uncompressed bufs).
 */
static void
arc_cksum_compute(arc_buf_t *buf)
{
        arc_buf_hdr_t *hdr = buf->b_hdr;

        if (!(zfs_flags & ZFS_DEBUG_MODIFY))
                return;

        ASSERT(HDR_HAS_L1HDR(hdr));

        mutex_enter(&buf->b_hdr->b_l1hdr.b_freeze_lock);
        if (hdr->b_l1hdr.b_freeze_cksum != NULL || ARC_BUF_COMPRESSED(buf)) {
                mutex_exit(&hdr->b_l1hdr.b_freeze_lock);
                return;
        }

        ASSERT(!ARC_BUF_ENCRYPTED(buf));
        ASSERT(!ARC_BUF_COMPRESSED(buf));
        hdr->b_l1hdr.b_freeze_cksum = kmem_alloc(sizeof (zio_cksum_t),
            KM_SLEEP);
        fletcher_2_native(buf->b_data, arc_buf_size(buf), NULL,
            hdr->b_l1hdr.b_freeze_cksum);
        mutex_exit(&hdr->b_l1hdr.b_freeze_lock);
        arc_buf_watch(buf);
}

#ifndef _KERNEL
void
arc_buf_sigsegv(int sig, siginfo_t *si, void *unused)
{
        panic("Got SIGSEGV at address: 0x%lx\n", (long)si->si_addr);
}
#endif

/* ARGSUSED */
static void
arc_buf_unwatch(arc_buf_t *buf)
{
#ifndef _KERNEL
        if (arc_watch) {
                ASSERT0(mprotect(buf->b_data, arc_buf_size(buf),
                    PROT_READ | PROT_WRITE));
        }
#endif
}

/* ARGSUSED */
static void
arc_buf_watch(arc_buf_t *buf)
{
#ifndef _KERNEL
        if (arc_watch)
                ASSERT0(mprotect(buf->b_data, arc_buf_size(buf),
                    PROT_READ));
#endif
}

static arc_buf_contents_t
arc_buf_type(arc_buf_hdr_t *hdr)
{
        arc_buf_contents_t type;
        if (HDR_ISTYPE_METADATA(hdr)) {
                type = ARC_BUFC_METADATA;
        } else {
                type = ARC_BUFC_DATA;
        }
        VERIFY3U(hdr->b_type, ==, type);
        return (type);
}

boolean_t
arc_is_metadata(arc_buf_t *buf)
{
        return (HDR_ISTYPE_METADATA(buf->b_hdr) != 0);
}

static uint32_t
arc_bufc_to_flags(arc_buf_contents_t type)
{
        switch (type) {
        case ARC_BUFC_DATA:
                /* metadata field is 0 if buffer contains normal data */
                return (0);
        case ARC_BUFC_METADATA:
                return (ARC_FLAG_BUFC_METADATA);
        default:
                break;
        }
        panic("undefined ARC buffer type!");
        return ((uint32_t)-1);
}

void
arc_buf_thaw(arc_buf_t *buf)
{
        arc_buf_hdr_t *hdr = buf->b_hdr;

        ASSERT3P(hdr->b_l1hdr.b_state, ==, arc_anon);
        ASSERT(!HDR_IO_IN_PROGRESS(hdr));

        arc_cksum_verify(buf);

        /*
         * Compressed buffers do not manipulate the b_freeze_cksum.
         */
        if (ARC_BUF_COMPRESSED(buf))
                return;

        ASSERT(HDR_HAS_L1HDR(hdr));
        arc_cksum_free(hdr);
        arc_buf_unwatch(buf);
}

void
arc_buf_freeze(arc_buf_t *buf)
{
        if (!(zfs_flags & ZFS_DEBUG_MODIFY))
                return;

        if (ARC_BUF_COMPRESSED(buf))
                return;

        ASSERT(HDR_HAS_L1HDR(buf->b_hdr));
        arc_cksum_compute(buf);
}

/*
 * The arc_buf_hdr_t's b_flags should never be modified directly. Instead,
 * the following functions should be used to ensure that the flags are
 * updated in a thread-safe way. When manipulating the flags either
 * the hash_lock must be held or the hdr must be undiscoverable. This
 * ensures that we're not racing with any other threads when updating
 * the flags.
 */
static inline void
arc_hdr_set_flags(arc_buf_hdr_t *hdr, arc_flags_t flags)
{
        ASSERT(HDR_EMPTY_OR_LOCKED(hdr));
        hdr->b_flags |= flags;
}

static inline void
arc_hdr_clear_flags(arc_buf_hdr_t *hdr, arc_flags_t flags)
{
        ASSERT(HDR_EMPTY_OR_LOCKED(hdr));
        hdr->b_flags &= ~flags;
}

/*
 * Setting the compression bits in the arc_buf_hdr_t's b_flags is
 * done in a special way since we have to clear and set bits
 * at the same time. Consumers that wish to set the compression bits
 * must use this function to ensure that the flags are updated in
 * thread-safe manner.
 */
static void
arc_hdr_set_compress(arc_buf_hdr_t *hdr, enum zio_compress cmp)
{
        ASSERT(HDR_EMPTY_OR_LOCKED(hdr));

        /*
         * Holes and embedded blocks will always have a psize = 0 so
         * we ignore the compression of the blkptr and set the
         * want to uncompress them. Mark them as uncompressed.
         */
        if (!zfs_compressed_arc_enabled || HDR_GET_PSIZE(hdr) == 0) {
                arc_hdr_clear_flags(hdr, ARC_FLAG_COMPRESSED_ARC);
                ASSERT(!HDR_COMPRESSION_ENABLED(hdr));
        } else {
                arc_hdr_set_flags(hdr, ARC_FLAG_COMPRESSED_ARC);
                ASSERT(HDR_COMPRESSION_ENABLED(hdr));
        }

        HDR_SET_COMPRESS(hdr, cmp);
        ASSERT3U(HDR_GET_COMPRESS(hdr), ==, cmp);
}

/*
 * Looks for another buf on the same hdr which has the data decompressed, copies
 * from it, and returns true. If no such buf exists, returns false.
 */
static boolean_t
arc_buf_try_copy_decompressed_data(arc_buf_t *buf)
{
        arc_buf_hdr_t *hdr = buf->b_hdr;
        boolean_t copied = B_FALSE;

        ASSERT(HDR_HAS_L1HDR(hdr));
        ASSERT3P(buf->b_data, !=, NULL);
        ASSERT(!ARC_BUF_COMPRESSED(buf));

        for (arc_buf_t *from = hdr->b_l1hdr.b_buf; from != NULL;
            from = from->b_next) {
                /* can't use our own data buffer */
                if (from == buf) {
                        continue;
                }

                if (!ARC_BUF_COMPRESSED(from)) {
                        bcopy(from->b_data, buf->b_data, arc_buf_size(buf));
                        copied = B_TRUE;
                        break;
                }
        }

        /*
         * There were no decompressed bufs, so there should not be a
         * checksum on the hdr either.
         */
        if (zfs_flags & ZFS_DEBUG_MODIFY)
                EQUIV(!copied, hdr->b_l1hdr.b_freeze_cksum == NULL);

        return (copied);
}

/*
 * Allocates an ARC buf header that's in an evicted & L2-cached state.
 * This is used during l2arc reconstruction to make empty ARC buffers
 * which circumvent the regular disk->arc->l2arc path and instead come
 * into being in the reverse order, i.e. l2arc->arc.
 */
static arc_buf_hdr_t *
arc_buf_alloc_l2only(size_t size, arc_buf_contents_t type, l2arc_dev_t *dev,
    dva_t dva, uint64_t daddr, int32_t psize, uint64_t birth,
    enum zio_compress compress, uint8_t complevel, boolean_t protected,
    boolean_t prefetch)
{
        arc_buf_hdr_t   *hdr;

        ASSERT(size != 0);
        hdr = kmem_cache_alloc(hdr_l2only_cache, KM_SLEEP);
        hdr->b_birth = birth;
        hdr->b_type = type;
        hdr->b_flags = 0;
        arc_hdr_set_flags(hdr, arc_bufc_to_flags(type) | ARC_FLAG_HAS_L2HDR);
        HDR_SET_LSIZE(hdr, size);
        HDR_SET_PSIZE(hdr, psize);
        arc_hdr_set_compress(hdr, compress);
        hdr->b_complevel = complevel;
        if (protected)
                arc_hdr_set_flags(hdr, ARC_FLAG_PROTECTED);
        if (prefetch)
                arc_hdr_set_flags(hdr, ARC_FLAG_PREFETCH);
        hdr->b_spa = spa_load_guid(dev->l2ad_vdev->vdev_spa);

        hdr->b_dva = dva;

        hdr->b_l2hdr.b_dev = dev;
        hdr->b_l2hdr.b_daddr = daddr;

        return (hdr);
}

/*
 * Return the size of the block, b_pabd, that is stored in the arc_buf_hdr_t.
 */
static uint64_t
arc_hdr_size(arc_buf_hdr_t *hdr)
{
        uint64_t size;

        if (arc_hdr_get_compress(hdr) != ZIO_COMPRESS_OFF &&
            HDR_GET_PSIZE(hdr) > 0) {
                size = HDR_GET_PSIZE(hdr);
        } else {
                ASSERT3U(HDR_GET_LSIZE(hdr), !=, 0);
                size = HDR_GET_LSIZE(hdr);
        }
        return (size);
}

static int
arc_hdr_authenticate(arc_buf_hdr_t *hdr, spa_t *spa, uint64_t dsobj)
{
        int ret;
        uint64_t csize;
        uint64_t lsize = HDR_GET_LSIZE(hdr);
        uint64_t psize = HDR_GET_PSIZE(hdr);
        void *tmpbuf = NULL;
        abd_t *abd = hdr->b_l1hdr.b_pabd;

        ASSERT(HDR_EMPTY_OR_LOCKED(hdr));
        ASSERT(HDR_AUTHENTICATED(hdr));
        ASSERT3P(hdr->b_l1hdr.b_pabd, !=, NULL);

        /*
         * The MAC is calculated on the compressed data that is stored on disk.
         * However, if compressed arc is disabled we will only have the
         * decompressed data available to us now. Compress it into a temporary
         * abd so we can verify the MAC. The performance overhead of this will
         * be relatively low, since most objects in an encrypted objset will
         * be encrypted (instead of authenticated) anyway.
         */
        if (HDR_GET_COMPRESS(hdr) != ZIO_COMPRESS_OFF &&
            !HDR_COMPRESSION_ENABLED(hdr)) {
                tmpbuf = zio_buf_alloc(lsize);
                abd = abd_get_from_buf(tmpbuf, lsize);
                abd_take_ownership_of_buf(abd, B_TRUE);
                csize = zio_compress_data(HDR_GET_COMPRESS(hdr),
                    hdr->b_l1hdr.b_pabd, tmpbuf, lsize, hdr->b_complevel);
                ASSERT3U(csize, <=, psize);
                abd_zero_off(abd, csize, psize - csize);
        }

        /*
         * Authentication is best effort. We authenticate whenever the key is
         * available. If we succeed we clear ARC_FLAG_NOAUTH.
         */
        if (hdr->b_crypt_hdr.b_ot == DMU_OT_OBJSET) {
                ASSERT3U(HDR_GET_COMPRESS(hdr), ==, ZIO_COMPRESS_OFF);
                ASSERT3U(lsize, ==, psize);
                ret = spa_do_crypt_objset_mac_abd(B_FALSE, spa, dsobj, abd,
                    psize, hdr->b_l1hdr.b_byteswap != DMU_BSWAP_NUMFUNCS);
        } else {
                ret = spa_do_crypt_mac_abd(B_FALSE, spa, dsobj, abd, psize,
                    hdr->b_crypt_hdr.b_mac);
        }

        if (ret == 0)
                arc_hdr_clear_flags(hdr, ARC_FLAG_NOAUTH);
        else if (ret != ENOENT)
                goto error;

        if (tmpbuf != NULL)
                abd_free(abd);

        return (0);

error:
        if (tmpbuf != NULL)
                abd_free(abd);

        return (ret);
}

/*
 * This function will take a header that only has raw encrypted data in
 * b_crypt_hdr.b_rabd and decrypt it into a new buffer which is stored in
 * b_l1hdr.b_pabd. If designated in the header flags, this function will
 * also decompress the data.
 */
static int
arc_hdr_decrypt(arc_buf_hdr_t *hdr, spa_t *spa, const zbookmark_phys_t *zb)
{
        int ret;
        abd_t *cabd = NULL;
        void *tmp = NULL;
        boolean_t no_crypt = B_FALSE;
        boolean_t bswap = (hdr->b_l1hdr.b_byteswap != DMU_BSWAP_NUMFUNCS);

        ASSERT(HDR_EMPTY_OR_LOCKED(hdr));
        ASSERT(HDR_ENCRYPTED(hdr));

        arc_hdr_alloc_abd(hdr, ARC_HDR_DO_ADAPT);

        ret = spa_do_crypt_abd(B_FALSE, spa, zb, hdr->b_crypt_hdr.b_ot,
            B_FALSE, bswap, hdr->b_crypt_hdr.b_salt, hdr->b_crypt_hdr.b_iv,
            hdr->b_crypt_hdr.b_mac, HDR_GET_PSIZE(hdr), hdr->b_l1hdr.b_pabd,
            hdr->b_crypt_hdr.b_rabd, &no_crypt);
        if (ret != 0)
                goto error;

        if (no_crypt) {
                abd_copy(hdr->b_l1hdr.b_pabd, hdr->b_crypt_hdr.b_rabd,
                    HDR_GET_PSIZE(hdr));
        }

        /*
         * If this header has disabled arc compression but the b_pabd is
         * compressed after decrypting it, we need to decompress the newly
         * decrypted data.
         */
        if (HDR_GET_COMPRESS(hdr) != ZIO_COMPRESS_OFF &&
            !HDR_COMPRESSION_ENABLED(hdr)) {
                /*
                 * We want to make sure that we are correctly honoring the
                 * zfs_abd_scatter_enabled setting, so we allocate an abd here
                 * and then loan a buffer from it, rather than allocating a
                 * linear buffer and wrapping it in an abd later.
                 */
                cabd = arc_get_data_abd(hdr, arc_hdr_size(hdr), hdr, B_TRUE);
                tmp = abd_borrow_buf(cabd, arc_hdr_size(hdr));

                ret = zio_decompress_data(HDR_GET_COMPRESS(hdr),
                    hdr->b_l1hdr.b_pabd, tmp, HDR_GET_PSIZE(hdr),
                    HDR_GET_LSIZE(hdr), &hdr->b_complevel);
                if (ret != 0) {
                        abd_return_buf(cabd, tmp, arc_hdr_size(hdr));
                        goto error;
                }

                abd_return_buf_copy(cabd, tmp, arc_hdr_size(hdr));
                arc_free_data_abd(hdr, hdr->b_l1hdr.b_pabd,
                    arc_hdr_size(hdr), hdr);
                hdr->b_l1hdr.b_pabd = cabd;
        }

        return (0);

error:
        arc_hdr_free_abd(hdr, B_FALSE);
        if (cabd != NULL)
                arc_free_data_buf(hdr, cabd, arc_hdr_size(hdr), hdr);

        return (ret);
}

/*
 * This function is called during arc_buf_fill() to prepare the header's
 * abd plaintext pointer for use. This involves authenticated protected
 * data and decrypting encrypted data into the plaintext abd.
 */
static int
arc_fill_hdr_crypt(arc_buf_hdr_t *hdr, kmutex_t *hash_lock, spa_t *spa,
    const zbookmark_phys_t *zb, boolean_t noauth)
{
        int ret;

        ASSERT(HDR_PROTECTED(hdr));

        if (hash_lock != NULL)
                mutex_enter(hash_lock);

        if (HDR_NOAUTH(hdr) && !noauth) {
                /*
                 * The caller requested authenticated data but our data has
                 * not been authenticated yet. Verify the MAC now if we can.
                 */
                ret = arc_hdr_authenticate(hdr, spa, zb->zb_objset);
                if (ret != 0)
                        goto error;
        } else if (HDR_HAS_RABD(hdr) && hdr->b_l1hdr.b_pabd == NULL) {
                /*
                 * If we only have the encrypted version of the data, but the
                 * unencrypted version was requested we take this opportunity
                 * to store the decrypted version in the header for future use.
                 */
                ret = arc_hdr_decrypt(hdr, spa, zb);
                if (ret != 0)
                        goto error;
        }

        ASSERT3P(hdr->b_l1hdr.b_pabd, !=, NULL);

        if (hash_lock != NULL)
                mutex_exit(hash_lock);

        return (0);

error:
        if (hash_lock != NULL)
                mutex_exit(hash_lock);

        return (ret);
}

/*
 * This function is used by the dbuf code to decrypt bonus buffers in place.
 * The dbuf code itself doesn't have any locking for decrypting a shared dnode
 * block, so we use the hash lock here to protect against concurrent calls to
 * arc_buf_fill().
 */
static void
arc_buf_untransform_in_place(arc_buf_t *buf, kmutex_t *hash_lock)
{
        arc_buf_hdr_t *hdr = buf->b_hdr;

        ASSERT(HDR_ENCRYPTED(hdr));
        ASSERT3U(hdr->b_crypt_hdr.b_ot, ==, DMU_OT_DNODE);
        ASSERT(HDR_EMPTY_OR_LOCKED(hdr));
        ASSERT3P(hdr->b_l1hdr.b_pabd, !=, NULL);

        zio_crypt_copy_dnode_bonus(hdr->b_l1hdr.b_pabd, buf->b_data,
            arc_buf_size(buf));
        buf->b_flags &= ~ARC_BUF_FLAG_ENCRYPTED;
        buf->b_flags &= ~ARC_BUF_FLAG_COMPRESSED;
        hdr->b_crypt_hdr.b_ebufcnt -= 1;
}

/*
 * Given a buf that has a data buffer attached to it, this function will
 * efficiently fill the buf with data of the specified compression setting from
 * the hdr and update the hdr's b_freeze_cksum if necessary. If the buf and hdr
 * are already sharing a data buf, no copy is performed.
 *
 * If the buf is marked as compressed but uncompressed data was requested, this
 * will allocate a new data buffer for the buf, remove that flag, and fill the
 * buf with uncompressed data. You can't request a compressed buf on a hdr with
 * uncompressed data, and (since we haven't added support for it yet) if you
 * want compressed data your buf must already be marked as compressed and have
 * the correct-sized data buffer.
 */
static int
arc_buf_fill(arc_buf_t *buf, spa_t *spa, const zbookmark_phys_t *zb,
    arc_fill_flags_t flags)
{
        int error = 0;
        arc_buf_hdr_t *hdr = buf->b_hdr;
        boolean_t hdr_compressed =
            (arc_hdr_get_compress(hdr) != ZIO_COMPRESS_OFF);
        boolean_t compressed = (flags & ARC_FILL_COMPRESSED) != 0;
        boolean_t encrypted = (flags & ARC_FILL_ENCRYPTED) != 0;
        dmu_object_byteswap_t bswap = hdr->b_l1hdr.b_byteswap;
        kmutex_t *hash_lock = (flags & ARC_FILL_LOCKED) ? NULL : HDR_LOCK(hdr);

        ASSERT3P(buf->b_data, !=, NULL);
        IMPLY(compressed, hdr_compressed || ARC_BUF_ENCRYPTED(buf));
        IMPLY(compressed, ARC_BUF_COMPRESSED(buf));
        IMPLY(encrypted, HDR_ENCRYPTED(hdr));
        IMPLY(encrypted, ARC_BUF_ENCRYPTED(buf));
        IMPLY(encrypted, ARC_BUF_COMPRESSED(buf));
        IMPLY(encrypted, !ARC_BUF_SHARED(buf));

        /*
         * If the caller wanted encrypted data we just need to copy it from
         * b_rabd and potentially byteswap it. We won't be able to do any
         * further transforms on it.
         */
        if (encrypted) {
                ASSERT(HDR_HAS_RABD(hdr));
                abd_copy_to_buf(buf->b_data, hdr->b_crypt_hdr.b_rabd,
                    HDR_GET_PSIZE(hdr));
                goto byteswap;
        }

        /*
         * Adjust encrypted and authenticated headers to accommodate
         * the request if needed. Dnode blocks (ARC_FILL_IN_PLACE) are
         * allowed to fail decryption due to keys not being loaded
         * without being marked as an IO error.
         */
        if (HDR_PROTECTED(hdr)) {
                error = arc_fill_hdr_crypt(hdr, hash_lock, spa,
                    zb, !!(flags & ARC_FILL_NOAUTH));
                if (error == EACCES && (flags & ARC_FILL_IN_PLACE) != 0) {
                        return (error);
                } else if (error != 0) {
                        if (hash_lock != NULL)
                                mutex_enter(hash_lock);
                        arc_hdr_set_flags(hdr, ARC_FLAG_IO_ERROR);
                        if (hash_lock != NULL)
                                mutex_exit(hash_lock);
                        return (error);
                }
        }

        /*
         * There is a special case here for dnode blocks which are
         * decrypting their bonus buffers. These blocks may request to
         * be decrypted in-place. This is necessary because there may
         * be many dnodes pointing into this buffer and there is
         * currently no method to synchronize replacing the backing
         * b_data buffer and updating all of the pointers. Here we use
         * the hash lock to ensure there are no races. If the need
         * arises for other types to be decrypted in-place, they must
         * add handling here as well.
         */
        if ((flags & ARC_FILL_IN_PLACE) != 0) {
                ASSERT(!hdr_compressed);
                ASSERT(!compressed);
                ASSERT(!encrypted);

                if (HDR_ENCRYPTED(hdr) && ARC_BUF_ENCRYPTED(buf)) {
                        ASSERT3U(hdr->b_crypt_hdr.b_ot, ==, DMU_OT_DNODE);

                        if (hash_lock != NULL)
                                mutex_enter(hash_lock);
                        arc_buf_untransform_in_place(buf, hash_lock);
                        if (hash_lock != NULL)
                                mutex_exit(hash_lock);

                        /* Compute the hdr's checksum if necessary */
                        arc_cksum_compute(buf);
                }

                return (0);
        }

        if (hdr_compressed == compressed) {
                if (!arc_buf_is_shared(buf)) {
                        abd_copy_to_buf(buf->b_data, hdr->b_l1hdr.b_pabd,
                            arc_buf_size(buf));
                }
        } else {
                ASSERT(hdr_compressed);
                ASSERT(!compressed);
                ASSERT3U(HDR_GET_LSIZE(hdr), !=, HDR_GET_PSIZE(hdr));

                /*
                 * If the buf is sharing its data with the hdr, unlink it and
                 * allocate a new data buffer for the buf.
                 */
                if (arc_buf_is_shared(buf)) {
                        ASSERT(ARC_BUF_COMPRESSED(buf));

                        /* We need to give the buf its own b_data */
                        buf->b_flags &= ~ARC_BUF_FLAG_SHARED;
                        buf->b_data =
                            arc_get_data_buf(hdr, HDR_GET_LSIZE(hdr), buf);
                        arc_hdr_clear_flags(hdr, ARC_FLAG_SHARED_DATA);

                        /* Previously overhead was 0; just add new overhead */
                        ARCSTAT_INCR(arcstat_overhead_size, HDR_GET_LSIZE(hdr));
                } else if (ARC_BUF_COMPRESSED(buf)) {
                        /* We need to reallocate the buf's b_data */
                        arc_free_data_buf(hdr, buf->b_data, HDR_GET_PSIZE(hdr),
                            buf);
                        buf->b_data =
                            arc_get_data_buf(hdr, HDR_GET_LSIZE(hdr), buf);

                        /* We increased the size of b_data; update overhead */
                        ARCSTAT_INCR(arcstat_overhead_size,
                            HDR_GET_LSIZE(hdr) - HDR_GET_PSIZE(hdr));
                }

                /*
                 * Regardless of the buf's previous compression settings, it
                 * should not be compressed at the end of this function.
                 */
                buf->b_flags &= ~ARC_BUF_FLAG_COMPRESSED;

                /*
                 * Try copying the data from another buf which already has a
                 * decompressed version. If that's not possible, it's time to
                 * bite the bullet and decompress the data from the hdr.
                 */
                if (arc_buf_try_copy_decompressed_data(buf)) {
                        /* Skip byteswapping and checksumming (already done) */
                        return (0);
                } else {
                        error = zio_decompress_data(HDR_GET_COMPRESS(hdr),
                            hdr->b_l1hdr.b_pabd, buf->b_data,
                            HDR_GET_PSIZE(hdr), HDR_GET_LSIZE(hdr),
                            &hdr->b_complevel);

                        /*
                         * Absent hardware errors or software bugs, this should
                         * be impossible, but log it anyway so we can debug it.
                         */
                        if (error != 0) {
                                zfs_dbgmsg(
                                    "hdr %px, compress %d, psize %d, lsize %d",
                                    hdr, arc_hdr_get_compress(hdr),
                                    HDR_GET_PSIZE(hdr), HDR_GET_LSIZE(hdr));
                                if (hash_lock != NULL)
                                        mutex_enter(hash_lock);
                                arc_hdr_set_flags(hdr, ARC_FLAG_IO_ERROR);
                                if (hash_lock != NULL)
                                        mutex_exit(hash_lock);
                                return (SET_ERROR(EIO));
                        }
                }
        }

byteswap:
        /* Byteswap the buf's data if necessary */
        if (bswap != DMU_BSWAP_NUMFUNCS) {
                ASSERT(!HDR_SHARED_DATA(hdr));
                ASSERT3U(bswap, <, DMU_BSWAP_NUMFUNCS);
                dmu_ot_byteswap[bswap].ob_func(buf->b_data, HDR_GET_LSIZE(hdr));
        }

        /* Compute the hdr's checksum if necessary */
        arc_cksum_compute(buf);

        return (0);
}

/*
 * If this function is being called to decrypt an encrypted buffer or verify an
 * authenticated one, the key must be loaded and a mapping must be made
 * available in the keystore via spa_keystore_create_mapping() or one of its
 * callers.
 */
int
arc_untransform(arc_buf_t *buf, spa_t *spa, const zbookmark_phys_t *zb,
    boolean_t in_place)
{
        int ret;
        arc_fill_flags_t flags = 0;

        if (in_place)
                flags |= ARC_FILL_IN_PLACE;

        ret = arc_buf_fill(buf, spa, zb, flags);
        if (ret == ECKSUM) {
                /*
                 * Convert authentication and decryption errors to EIO
                 * (and generate an ereport) before leaving the ARC.
                 */
                ret = SET_ERROR(EIO);
                spa_log_error(spa, zb);
                (void) zfs_ereport_post(FM_EREPORT_ZFS_AUTHENTICATION,
                    spa, NULL, zb, NULL, 0);
        }

        return (ret);
}

/*
 * Increment the amount of evictable space in the arc_state_t's refcount.
 * We account for the space used by the hdr and the arc buf individually
 * so that we can add and remove them from the refcount individually.
 */
static void
arc_evictable_space_increment(arc_buf_hdr_t *hdr, arc_state_t *state)
{
        arc_buf_contents_t type = arc_buf_type(hdr);

        ASSERT(HDR_HAS_L1HDR(hdr));

        if (GHOST_STATE(state)) {
                ASSERT0(hdr->b_l1hdr.b_bufcnt);
                ASSERT3P(hdr->b_l1hdr.b_buf, ==, NULL);
                ASSERT3P(hdr->b_l1hdr.b_pabd, ==, NULL);
                ASSERT(!HDR_HAS_RABD(hdr));
                (void) zfs_refcount_add_many(&state->arcs_esize[type],
                    HDR_GET_LSIZE(hdr), hdr);
                return;
        }

        ASSERT(!GHOST_STATE(state));
        if (hdr->b_l1hdr.b_pabd != NULL) {
                (void) zfs_refcount_add_many(&state->arcs_esize[type],
                    arc_hdr_size(hdr), hdr);
        }
        if (HDR_HAS_RABD(hdr)) {
                (void) zfs_refcount_add_many(&state->arcs_esize[type],
                    HDR_GET_PSIZE(hdr), hdr);
        }

        for (arc_buf_t *buf = hdr->b_l1hdr.b_buf; buf != NULL;
            buf = buf->b_next) {
                if (arc_buf_is_shared(buf))
                        continue;
                (void) zfs_refcount_add_many(&state->arcs_esize[type],
                    arc_buf_size(buf), buf);
        }
}

/*
 * Decrement the amount of evictable space in the arc_state_t's refcount.
 * We account for the space used by the hdr and the arc buf individually
 * so that we can add and remove them from the refcount individually.
 */
static void
arc_evictable_space_decrement(arc_buf_hdr_t *hdr, arc_state_t *state)
{
        arc_buf_contents_t type = arc_buf_type(hdr);

        ASSERT(HDR_HAS_L1HDR(hdr));

        if (GHOST_STATE(state)) {
                ASSERT0(hdr->b_l1hdr.b_bufcnt);
                ASSERT3P(hdr->b_l1hdr.b_buf, ==, NULL);
                ASSERT3P(hdr->b_l1hdr.b_pabd, ==, NULL);
                ASSERT(!HDR_HAS_RABD(hdr));
                (void) zfs_refcount_remove_many(&state->arcs_esize[type],
                    HDR_GET_LSIZE(hdr), hdr);
                return;
        }

        ASSERT(!GHOST_STATE(state));
        if (hdr->b_l1hdr.b_pabd != NULL) {
                (void) zfs_refcount_remove_many(&state->arcs_esize[type],
                    arc_hdr_size(hdr), hdr);
        }
        if (HDR_HAS_RABD(hdr)) {
                (void) zfs_refcount_remove_many(&state->arcs_esize[type],
                    HDR_GET_PSIZE(hdr), hdr);
        }

        for (arc_buf_t *buf = hdr->b_l1hdr.b_buf; buf != NULL;
            buf = buf->b_next) {
                if (arc_buf_is_shared(buf))
                        continue;
                (void) zfs_refcount_remove_many(&state->arcs_esize[type],
                    arc_buf_size(buf), buf);
        }
}

/*
 * Add a reference to this hdr indicating that someone is actively
 * referencing that memory. When the refcount transitions from 0 to 1,
 * we remove it from the respective arc_state_t list to indicate that
 * it is not evictable.
 */
static void
add_reference(arc_buf_hdr_t *hdr, void *tag)
{
        arc_state_t *state;

        ASSERT(HDR_HAS_L1HDR(hdr));
        if (!HDR_EMPTY(hdr) && !MUTEX_HELD(HDR_LOCK(hdr))) {
                ASSERT(hdr->b_l1hdr.b_state == arc_anon);
                ASSERT(zfs_refcount_is_zero(&hdr->b_l1hdr.b_refcnt));
                ASSERT3P(hdr->b_l1hdr.b_buf, ==, NULL);
        }

        state = hdr->b_l1hdr.b_state;

        if ((zfs_refcount_add(&hdr->b_l1hdr.b_refcnt, tag) == 1) &&
            (state != arc_anon)) {
                /* We don't use the L2-only state list. */
                if (state != arc_l2c_only) {
                        multilist_remove(state->arcs_list[arc_buf_type(hdr)],
                            hdr);
                        arc_evictable_space_decrement(hdr, state);
                }
                /* remove the prefetch flag if we get a reference */
                arc_hdr_clear_flags(hdr, ARC_FLAG_PREFETCH);
        }
}

/*
 * Remove a reference from this hdr. When the reference transitions from
 * 1 to 0 and we're not anonymous, then we add this hdr to the arc_state_t's
 * list making it eligible for eviction.
 */
static int
remove_reference(arc_buf_hdr_t *hdr, kmutex_t *hash_lock, void *tag)
{
        int cnt;
        arc_state_t *state = hdr->b_l1hdr.b_state;

        ASSERT(HDR_HAS_L1HDR(hdr));
        ASSERT(state == arc_anon || MUTEX_HELD(hash_lock));
        ASSERT(!GHOST_STATE(state));

        /*
         * arc_l2c_only counts as a ghost state so we don't need to explicitly
         * check to prevent usage of the arc_l2c_only list.
         */
        if (((cnt = zfs_refcount_remove(&hdr->b_l1hdr.b_refcnt, tag)) == 0) &&
            (state != arc_anon)) {
                multilist_insert(state->arcs_list[arc_buf_type(hdr)], hdr);
                ASSERT3U(hdr->b_l1hdr.b_bufcnt, >, 0);
                arc_evictable_space_increment(hdr, state);
        }
        return (cnt);
}

/*
 * Returns detailed information about a specific arc buffer.  When the
 * state_index argument is set the function will calculate the arc header
 * list position for its arc state.  Since this requires a linear traversal
 * callers are strongly encourage not to do this.  However, it can be helpful
 * for targeted analysis so the functionality is provided.
 */
void
arc_buf_info(arc_buf_t *ab, arc_buf_info_t *abi, int state_index)
{
        arc_buf_hdr_t *hdr = ab->b_hdr;
        l1arc_buf_hdr_t *l1hdr = NULL;
        l2arc_buf_hdr_t *l2hdr = NULL;
        arc_state_t *state = NULL;

        memset(abi, 0, sizeof (arc_buf_info_t));

        if (hdr == NULL)
                return;

        abi->abi_flags = hdr->b_flags;

        if (HDR_HAS_L1HDR(hdr)) {
                l1hdr = &hdr->b_l1hdr;
                state = l1hdr->b_state;
        }
        if (HDR_HAS_L2HDR(hdr))
                l2hdr = &hdr->b_l2hdr;

        if (l1hdr) {
                abi->abi_bufcnt = l1hdr->b_bufcnt;
                abi->abi_access = l1hdr->b_arc_access;
                abi->abi_mru_hits = l1hdr->b_mru_hits;
                abi->abi_mru_ghost_hits = l1hdr->b_mru_ghost_hits;
                abi->abi_mfu_hits = l1hdr->b_mfu_hits;
                abi->abi_mfu_ghost_hits = l1hdr->b_mfu_ghost_hits;
                abi->abi_holds = zfs_refcount_count(&l1hdr->b_refcnt);
        }

        if (l2hdr) {
                abi->abi_l2arc_dattr = l2hdr->b_daddr;
                abi->abi_l2arc_hits = l2hdr->b_hits;
        }

        abi->abi_state_type = state ? state->arcs_state : ARC_STATE_ANON;
        abi->abi_state_contents = arc_buf_type(hdr);
        abi->abi_size = arc_hdr_size(hdr);
}

/*
 * Move the supplied buffer to the indicated state. The hash lock
 * for the buffer must be held by the caller.
 */
static void
arc_change_state(arc_state_t *new_state, arc_buf_hdr_t *hdr,
    kmutex_t *hash_lock)
{
        arc_state_t *old_state;
        int64_t refcnt;
        uint32_t bufcnt;
        boolean_t update_old, update_new;
        arc_buf_contents_t buftype = arc_buf_type(hdr);

        /*
         * We almost always have an L1 hdr here, since we call arc_hdr_realloc()
         * in arc_read() when bringing a buffer out of the L2ARC.  However, the
         * L1 hdr doesn't always exist when we change state to arc_anon before
         * destroying a header, in which case reallocating to add the L1 hdr is
         * pointless.
         */
        if (HDR_HAS_L1HDR(hdr)) {
                old_state = hdr->b_l1hdr.b_state;
                refcnt = zfs_refcount_count(&hdr->b_l1hdr.b_refcnt);
                bufcnt = hdr->b_l1hdr.b_bufcnt;
                update_old = (bufcnt > 0 || hdr->b_l1hdr.b_pabd != NULL ||
                    HDR_HAS_RABD(hdr));
        } else {
                old_state = arc_l2c_only;
                refcnt = 0;
                bufcnt = 0;
                update_old = B_FALSE;
        }
        update_new = update_old;

        ASSERT(MUTEX_HELD(hash_lock));
        ASSERT3P(new_state, !=, old_state);
        ASSERT(!GHOST_STATE(new_state) || bufcnt == 0);
        ASSERT(old_state != arc_anon || bufcnt <= 1);

        /*
         * If this buffer is evictable, transfer it from the
         * old state list to the new state list.
         */
        if (refcnt == 0) {
                if (old_state != arc_anon && old_state != arc_l2c_only) {
                        ASSERT(HDR_HAS_L1HDR(hdr));
                        multilist_remove(old_state->arcs_list[buftype], hdr);

                        if (GHOST_STATE(old_state)) {
                                ASSERT0(bufcnt);
                                ASSERT3P(hdr->b_l1hdr.b_buf, ==, NULL);
                                update_old = B_TRUE;
                        }
                        arc_evictable_space_decrement(hdr, old_state);
                }
                if (new_state != arc_anon && new_state != arc_l2c_only) {
                        /*
                         * An L1 header always exists here, since if we're
                         * moving to some L1-cached state (i.e. not l2c_only or
                         * anonymous), we realloc the header to add an L1hdr
                         * beforehand.
                         */
                        ASSERT(HDR_HAS_L1HDR(hdr));
                        multilist_insert(new_state->arcs_list[buftype], hdr);

                        if (GHOST_STATE(new_state)) {
                                ASSERT0(bufcnt);
                                ASSERT3P(hdr->b_l1hdr.b_buf, ==, NULL);
                                update_new = B_TRUE;
                        }
                        arc_evictable_space_increment(hdr, new_state);
                }
        }

        ASSERT(!HDR_EMPTY(hdr));
        if (new_state == arc_anon && HDR_IN_HASH_TABLE(hdr))
                buf_hash_remove(hdr);

        /* adjust state sizes (ignore arc_l2c_only) */

        if (update_new && new_state != arc_l2c_only) {
                ASSERT(HDR_HAS_L1HDR(hdr));
                if (GHOST_STATE(new_state)) {
                        ASSERT0(bufcnt);

                        /*
                         * When moving a header to a ghost state, we first
                         * remove all arc buffers. Thus, we'll have a
                         * bufcnt of zero, and no arc buffer to use for
                         * the reference. As a result, we use the arc
                         * header pointer for the reference.
                         */
                        (void) zfs_refcount_add_many(&new_state->arcs_size,
                            HDR_GET_LSIZE(hdr), hdr);
                        ASSERT3P(hdr->b_l1hdr.b_pabd, ==, NULL);
                        ASSERT(!HDR_HAS_RABD(hdr));
                } else {
                        uint32_t buffers = 0;

                        /*
                         * Each individual buffer holds a unique reference,
                         * thus we must remove each of these references one
                         * at a time.
                         */
                        for (arc_buf_t *buf = hdr->b_l1hdr.b_buf; buf != NULL;
                            buf = buf->b_next) {
                                ASSERT3U(bufcnt, !=, 0);
                                buffers++;

                                /*
                                 * When the arc_buf_t is sharing the data
                                 * block with the hdr, the owner of the
                                 * reference belongs to the hdr. Only
                                 * add to the refcount if the arc_buf_t is
                                 * not shared.
                                 */
                                if (arc_buf_is_shared(buf))
                                        continue;

                                (void) zfs_refcount_add_many(
                                    &new_state->arcs_size,
                                    arc_buf_size(buf), buf);
                        }
                        ASSERT3U(bufcnt, ==, buffers);

                        if (hdr->b_l1hdr.b_pabd != NULL) {
                                (void) zfs_refcount_add_many(
                                    &new_state->arcs_size,
                                    arc_hdr_size(hdr), hdr);
                        }

                        if (HDR_HAS_RABD(hdr)) {
                                (void) zfs_refcount_add_many(
                                    &new_state->arcs_size,
                                    HDR_GET_PSIZE(hdr), hdr);
                        }
                }
        }

        if (update_old && old_state != arc_l2c_only) {
                ASSERT(HDR_HAS_L1HDR(hdr));
                if (GHOST_STATE(old_state)) {
                        ASSERT0(bufcnt);
                        ASSERT3P(hdr->b_l1hdr.b_pabd, ==, NULL);
                        ASSERT(!HDR_HAS_RABD(hdr));

                        /*
                         * When moving a header off of a ghost state,
                         * the header will not contain any arc buffers.
                         * We use the arc header pointer for the reference
                         * which is exactly what we did when we put the
                         * header on the ghost state.
                         */

                        (void) zfs_refcount_remove_many(&old_state->arcs_size,
                            HDR_GET_LSIZE(hdr), hdr);
                } else {
                        uint32_t buffers = 0;

                        /*
                         * Each individual buffer holds a unique reference,
                         * thus we must remove each of these references one
                         * at a time.
                         */
                        for (arc_buf_t *buf = hdr->b_l1hdr.b_buf; buf != NULL;
                            buf = buf->b_next) {
                                ASSERT3U(bufcnt, !=, 0);
                                buffers++;

                                /*
                                 * When the arc_buf_t is sharing the data
                                 * block with the hdr, the owner of the
                                 * reference belongs to the hdr. Only
                                 * add to the refcount if the arc_buf_t is
                                 * not shared.
                                 */
                                if (arc_buf_is_shared(buf))
                                        continue;

                                (void) zfs_refcount_remove_many(
                                    &old_state->arcs_size, arc_buf_size(buf),
                                    buf);
                        }
                        ASSERT3U(bufcnt, ==, buffers);
                        ASSERT(hdr->b_l1hdr.b_pabd != NULL ||
                            HDR_HAS_RABD(hdr));

                        if (hdr->b_l1hdr.b_pabd != NULL) {
                                (void) zfs_refcount_remove_many(
                                    &old_state->arcs_size, arc_hdr_size(hdr),
                                    hdr);
                        }

                        if (HDR_HAS_RABD(hdr)) {
                                (void) zfs_refcount_remove_many(
                                    &old_state->arcs_size, HDR_GET_PSIZE(hdr),
                                    hdr);
                        }
                }
        }

        if (HDR_HAS_L1HDR(hdr))
                hdr->b_l1hdr.b_state = new_state;

        /*
         * L2 headers should never be on the L2 state list since they don't
         * have L1 headers allocated.
         */
        ASSERT(multilist_is_empty(arc_l2c_only->arcs_list[ARC_BUFC_DATA]) &&
            multilist_is_empty(arc_l2c_only->arcs_list[ARC_BUFC_METADATA]));
}

void
arc_space_consume(uint64_t space, arc_space_type_t type)
{
        ASSERT(type >= 0 && type < ARC_SPACE_NUMTYPES);

        switch (type) {
        default:
                break;
        case ARC_SPACE_DATA:
                aggsum_add(&astat_data_size, space);
                break;
        case ARC_SPACE_META:
                aggsum_add(&astat_metadata_size, space);
                break;
        case ARC_SPACE_BONUS:
                aggsum_add(&astat_bonus_size, space);
                break;
        case ARC_SPACE_DNODE:
                aggsum_add(&astat_dnode_size, space);
                break;
        case ARC_SPACE_DBUF:
                aggsum_add(&astat_dbuf_size, space);
                break;
        case ARC_SPACE_HDRS:
                aggsum_add(&astat_hdr_size, space);
                break;
        case ARC_SPACE_L2HDRS:
                aggsum_add(&astat_l2_hdr_size, space);
                break;
        case ARC_SPACE_ABD_CHUNK_WASTE:
                /*
                 * Note: this includes space wasted by all scatter ABD's, not
                 * just those allocated by the ARC.  But the vast majority of
                 * scatter ABD's come from the ARC, because other users are
                 * very short-lived.
                 */
                aggsum_add(&astat_abd_chunk_waste_size, space);
                break;
        }

        if (type != ARC_SPACE_DATA && type != ARC_SPACE_ABD_CHUNK_WASTE)
                aggsum_add(&arc_meta_used, space);

        aggsum_add(&arc_size, space);
}

void
arc_space_return(uint64_t space, arc_space_type_t type)
{
        ASSERT(type >= 0 && type < ARC_SPACE_NUMTYPES);

        switch (type) {
        default:
                break;
        case ARC_SPACE_DATA:
                aggsum_add(&astat_data_size, -space);
                break;
        case ARC_SPACE_META:
                aggsum_add(&astat_metadata_size, -space);
                break;
        case ARC_SPACE_BONUS:
                aggsum_add(&astat_bonus_size, -space);
                break;
        case ARC_SPACE_DNODE:
                aggsum_add(&astat_dnode_size, -space);
                break;
        case ARC_SPACE_DBUF:
                aggsum_add(&astat_dbuf_size, -space);
                break;
        case ARC_SPACE_HDRS:
                aggsum_add(&astat_hdr_size, -space);
                break;
        case ARC_SPACE_L2HDRS:
                aggsum_add(&astat_l2_hdr_size, -space);
                break;
        case ARC_SPACE_ABD_CHUNK_WASTE:
                aggsum_add(&astat_abd_chunk_waste_size, -space);
                break;
        }

        if (type != ARC_SPACE_DATA && type != ARC_SPACE_ABD_CHUNK_WASTE) {
                ASSERT(aggsum_compare(&arc_meta_used, space) >= 0);
                /*
                 * We use the upper bound here rather than the precise value
                 * because the arc_meta_max value doesn't need to be
                 * precise. It's only consumed by humans via arcstats.
                 */
                if (arc_meta_max < aggsum_upper_bound(&arc_meta_used))
                        arc_meta_max = aggsum_upper_bound(&arc_meta_used);
                aggsum_add(&arc_meta_used, -space);
        }

        ASSERT(aggsum_compare(&arc_size, space) >= 0);
        aggsum_add(&arc_size, -space);
}

/*
 * Given a hdr and a buf, returns whether that buf can share its b_data buffer
 * with the hdr's b_pabd.
 */
static boolean_t
arc_can_share(arc_buf_hdr_t *hdr, arc_buf_t *buf)
{
        /*
         * The criteria for sharing a hdr's data are:
         * 1. the buffer is not encrypted
         * 2. the hdr's compression matches the buf's compression
         * 3. the hdr doesn't need to be byteswapped
         * 4. the hdr isn't already being shared
         * 5. the buf is either compressed or it is the last buf in the hdr list
         *
         * Criterion #5 maintains the invariant that shared uncompressed
         * bufs must be the final buf in the hdr's b_buf list. Reading this, you
         * might ask, "if a compressed buf is allocated first, won't that be the
         * last thing in the list?", but in that case it's impossible to create
         * a shared uncompressed buf anyway (because the hdr must be compressed
         * to have the compressed buf). You might also think that #3 is
         * sufficient to make this guarantee, however it's possible
         * (specifically in the rare L2ARC write race mentioned in
         * arc_buf_alloc_impl()) there will be an existing uncompressed buf that
         * is shareable, but wasn't at the time of its allocation. Rather than
         * allow a new shared uncompressed buf to be created and then shuffle
         * the list around to make it the last element, this simply disallows
         * sharing if the new buf isn't the first to be added.
         */
        ASSERT3P(buf->b_hdr, ==, hdr);
        boolean_t hdr_compressed =
            arc_hdr_get_compress(hdr) != ZIO_COMPRESS_OFF;
        boolean_t buf_compressed = ARC_BUF_COMPRESSED(buf) != 0;
        return (!ARC_BUF_ENCRYPTED(buf) &&
            buf_compressed == hdr_compressed &&
            hdr->b_l1hdr.b_byteswap == DMU_BSWAP_NUMFUNCS &&
            !HDR_SHARED_DATA(hdr) &&
            (ARC_BUF_LAST(buf) || ARC_BUF_COMPRESSED(buf)));
}

/*
 * Allocate a buf for this hdr. If you care about the data that's in the hdr,
 * or if you want a compressed buffer, pass those flags in. Returns 0 if the
 * copy was made successfully, or an error code otherwise.
 */
static int
arc_buf_alloc_impl(arc_buf_hdr_t *hdr, spa_t *spa, const zbookmark_phys_t *zb,
    void *tag, boolean_t encrypted, boolean_t compressed, boolean_t noauth,
    boolean_t fill, arc_buf_t **ret)
{
        arc_buf_t *buf;
        arc_fill_flags_t flags = ARC_FILL_LOCKED;

        ASSERT(HDR_HAS_L1HDR(hdr));
        ASSERT3U(HDR_GET_LSIZE(hdr), >, 0);
        VERIFY(hdr->b_type == ARC_BUFC_DATA ||
            hdr->b_type == ARC_BUFC_METADATA);
        ASSERT3P(ret, !=, NULL);
        ASSERT3P(*ret, ==, NULL);
        IMPLY(encrypted, compressed);

        hdr->b_l1hdr.b_mru_hits = 0;
        hdr->b_l1hdr.b_mru_ghost_hits = 0;
        hdr->b_l1hdr.b_mfu_hits = 0;
        hdr->b_l1hdr.b_mfu_ghost_hits = 0;
        hdr->b_l1hdr.b_l2_hits = 0;

        buf = *ret = kmem_cache_alloc(buf_cache, KM_PUSHPAGE);
        buf->b_hdr = hdr;
        buf->b_data = NULL;
        buf->b_next = hdr->b_l1hdr.b_buf;
        buf->b_flags = 0;

        add_reference(hdr, tag);

        /*
         * We're about to change the hdr's b_flags. We must either
         * hold the hash_lock or be undiscoverable.
         */
        ASSERT(HDR_EMPTY_OR_LOCKED(hdr));

        /*
         * Only honor requests for compressed bufs if the hdr is actually
         * compressed. This must be overridden if the buffer is encrypted since
         * encrypted buffers cannot be decompressed.
         */
        if (encrypted) {
                buf->b_flags |= ARC_BUF_FLAG_COMPRESSED;
                buf->b_flags |= ARC_BUF_FLAG_ENCRYPTED;
                flags |= ARC_FILL_COMPRESSED | ARC_FILL_ENCRYPTED;
        } else if (compressed &&
            arc_hdr_get_compress(hdr) != ZIO_COMPRESS_OFF) {
                buf->b_flags |= ARC_BUF_FLAG_COMPRESSED;
                flags |= ARC_FILL_COMPRESSED;
        }

        if (noauth) {
                ASSERT0(encrypted);
                flags |= ARC_FILL_NOAUTH;
        }

        /*
         * If the hdr's data can be shared then we share the data buffer and
         * set the appropriate bit in the hdr's b_flags to indicate the hdr is
         * sharing it's b_pabd with the arc_buf_t. Otherwise, we allocate a new
         * buffer to store the buf's data.
         *
         * There are two additional restrictions here because we're sharing
         * hdr -> buf instead of the usual buf -> hdr. First, the hdr can't be
         * actively involved in an L2ARC write, because if this buf is used by
         * an arc_write() then the hdr's data buffer will be released when the
         * write completes, even though the L2ARC write might still be using it.
         * Second, the hdr's ABD must be linear so that the buf's user doesn't
         * need to be ABD-aware.  It must be allocated via
         * zio_[data_]buf_alloc(), not as a page, because we need to be able
         * to abd_release_ownership_of_buf(), which isn't allowed on "linear
         * page" buffers because the ABD code needs to handle freeing them
         * specially.
         */
        boolean_t can_share = arc_can_share(hdr, buf) &&
            !HDR_L2_WRITING(hdr) &&
            hdr->b_l1hdr.b_pabd != NULL &&
            abd_is_linear(hdr->b_l1hdr.b_pabd) &&
            !abd_is_linear_page(hdr->b_l1hdr.b_pabd);

        /* Set up b_data and sharing */
        if (can_share) {
                buf->b_data = abd_to_buf(hdr->b_l1hdr.b_pabd);
                buf->b_flags |= ARC_BUF_FLAG_SHARED;
                arc_hdr_set_flags(hdr, ARC_FLAG_SHARED_DATA);
        } else {
                buf->b_data =
                    arc_get_data_buf(hdr, arc_buf_size(buf), buf);
                ARCSTAT_INCR(arcstat_overhead_size, arc_buf_size(buf));
        }
        VERIFY3P(buf->b_data, !=, NULL);

        hdr->b_l1hdr.b_buf = buf;
        hdr->b_l1hdr.b_bufcnt += 1;
        if (encrypted)
                hdr->b_crypt_hdr.b_ebufcnt += 1;

        /*
         * If the user wants the data from the hdr, we need to either copy or
         * decompress the data.
         */
        if (fill) {
                ASSERT3P(zb, !=, NULL);
                return (arc_buf_fill(buf, spa, zb, flags));
        }

        return (0);
}

static char *arc_onloan_tag = "onloan";

static inline void
arc_loaned_bytes_update(int64_t delta)
{
        atomic_add_64(&arc_loaned_bytes, delta);

        /* assert that it did not wrap around */
        ASSERT3S(atomic_add_64_nv(&arc_loaned_bytes, 0), >=, 0);
}

/*
 * Loan out an anonymous arc buffer. Loaned buffers are not counted as in
 * flight data by arc_tempreserve_space() until they are "returned". Loaned
 * buffers must be returned to the arc before they can be used by the DMU or
 * freed.
 */
arc_buf_t *
arc_loan_buf(spa_t *spa, boolean_t is_metadata, int size)
{
        arc_buf_t *buf = arc_alloc_buf(spa, arc_onloan_tag,
            is_metadata ? ARC_BUFC_METADATA : ARC_BUFC_DATA, size);

        arc_loaned_bytes_update(arc_buf_size(buf));

        return (buf);
}

arc_buf_t *
arc_loan_compressed_buf(spa_t *spa, uint64_t psize, uint64_t lsize,
    enum zio_compress compression_type, uint8_t complevel)
{
        arc_buf_t *buf = arc_alloc_compressed_buf(spa, arc_onloan_tag,
            psize, lsize, compression_type, complevel);

        arc_loaned_bytes_update(arc_buf_size(buf));

        return (buf);
}

arc_buf_t *
arc_loan_raw_buf(spa_t *spa, uint64_t dsobj, boolean_t byteorder,
    const uint8_t *salt, const uint8_t *iv, const uint8_t *mac,
    dmu_object_type_t ot, uint64_t psize, uint64_t lsize,
    enum zio_compress compression_type, uint8_t complevel)
{
        arc_buf_t *buf = arc_alloc_raw_buf(spa, arc_onloan_tag, dsobj,
            byteorder, salt, iv, mac, ot, psize, lsize, compression_type,
            complevel);

        atomic_add_64(&arc_loaned_bytes, psize);
        return (buf);
}


/*
 * Return a loaned arc buffer to the arc.
 */
void
arc_return_buf(arc_buf_t *buf, void *tag)
{
        arc_buf_hdr_t *hdr = buf->b_hdr;

        ASSERT3P(buf->b_data, !=, NULL);
        ASSERT(HDR_HAS_L1HDR(hdr));
        (void) zfs_refcount_add(&hdr->b_l1hdr.b_refcnt, tag);
        (void) zfs_refcount_remove(&hdr->b_l1hdr.b_refcnt, arc_onloan_tag);

        arc_loaned_bytes_update(-arc_buf_size(buf));
}

/* Detach an arc_buf from a dbuf (tag) */
void
arc_loan_inuse_buf(arc_buf_t *buf, void *tag)
{
        arc_buf_hdr_t *hdr = buf->b_hdr;

        ASSERT3P(buf->b_data, !=, NULL);
        ASSERT(HDR_HAS_L1HDR(hdr));
        (void) zfs_refcount_add(&hdr->b_l1hdr.b_refcnt, arc_onloan_tag);
        (void) zfs_refcount_remove(&hdr->b_l1hdr.b_refcnt, tag);

        arc_loaned_bytes_update(arc_buf_size(buf));
}

static void
l2arc_free_abd_on_write(abd_t *abd, size_t size, arc_buf_contents_t type)
{
        l2arc_data_free_t *df = kmem_alloc(sizeof (*df), KM_SLEEP);

        df->l2df_abd = abd;
        df->l2df_size = size;
        df->l2df_type = type;
        mutex_enter(&l2arc_free_on_write_mtx);
        list_insert_head(l2arc_free_on_write, df);
        mutex_exit(&l2arc_free_on_write_mtx);
}

static void
arc_hdr_free_on_write(arc_buf_hdr_t *hdr, boolean_t free_rdata)
{
        arc_state_t *state = hdr->b_l1hdr.b_state;
        arc_buf_contents_t type = arc_buf_type(hdr);
        uint64_t size = (free_rdata) ? HDR_GET_PSIZE(hdr) : arc_hdr_size(hdr);

        /* protected by hash lock, if in the hash table */
        if (multilist_link_active(&hdr->b_l1hdr.b_arc_node)) {
                ASSERT(zfs_refcount_is_zero(&hdr->b_l1hdr.b_refcnt));
                ASSERT(state != arc_anon && state != arc_l2c_only);

                (void) zfs_refcount_remove_many(&state->arcs_esize[type],
                    size, hdr);
        }
        (void) zfs_refcount_remove_many(&state->arcs_size, size, hdr);
        if (type == ARC_BUFC_METADATA) {
                arc_space_return(size, ARC_SPACE_META);
        } else {
                ASSERT(type == ARC_BUFC_DATA);
                arc_space_return(size, ARC_SPACE_DATA);
        }

        if (free_rdata) {
                l2arc_free_abd_on_write(hdr->b_crypt_hdr.b_rabd, size, type);
        } else {
                l2arc_free_abd_on_write(hdr->b_l1hdr.b_pabd, size, type);
        }
}

/*
 * Share the arc_buf_t's data with the hdr. Whenever we are sharing the
 * data buffer, we transfer the refcount ownership to the hdr and update
 * the appropriate kstats.
 */
static void
arc_share_buf(arc_buf_hdr_t *hdr, arc_buf_t *buf)
{
        ASSERT(arc_can_share(hdr, buf));
        ASSERT3P(hdr->b_l1hdr.b_pabd, ==, NULL);
        ASSERT(!ARC_BUF_ENCRYPTED(buf));
        ASSERT(HDR_EMPTY_OR_LOCKED(hdr));

        /*
         * Start sharing the data buffer. We transfer the
         * refcount ownership to the hdr since it always owns
         * the refcount whenever an arc_buf_t is shared.
         */
        zfs_refcount_transfer_ownership_many(&hdr->b_l1hdr.b_state->arcs_size,
            arc_hdr_size(hdr), buf, hdr);
        hdr->b_l1hdr.b_pabd = abd_get_from_buf(buf->b_data, arc_buf_size(buf));
        abd_take_ownership_of_buf(hdr->b_l1hdr.b_pabd,
            HDR_ISTYPE_METADATA(hdr));
        arc_hdr_set_flags(hdr, ARC_FLAG_SHARED_DATA);
        buf->b_flags |= ARC_BUF_FLAG_SHARED;

        /*
         * Since we've transferred ownership to the hdr we need
         * to increment its compressed and uncompressed kstats and
         * decrement the overhead size.
         */
        ARCSTAT_INCR(arcstat_compressed_size, arc_hdr_size(hdr));
        ARCSTAT_INCR(arcstat_uncompressed_size, HDR_GET_LSIZE(hdr));
        ARCSTAT_INCR(arcstat_overhead_size, -arc_buf_size(buf));
}

static void
arc_unshare_buf(arc_buf_hdr_t *hdr, arc_buf_t *buf)
{
        ASSERT(arc_buf_is_shared(buf));
        ASSERT3P(hdr->b_l1hdr.b_pabd, !=, NULL);
        ASSERT(HDR_EMPTY_OR_LOCKED(hdr));

        /*
         * We are no longer sharing this buffer so we need
         * to transfer its ownership to the rightful owner.
         */
        zfs_refcount_transfer_ownership_many(&hdr->b_l1hdr.b_state->arcs_size,
            arc_hdr_size(hdr), hdr, buf);
        arc_hdr_clear_flags(hdr, ARC_FLAG_SHARED_DATA);
        abd_release_ownership_of_buf(hdr->b_l1hdr.b_pabd);
        abd_put(hdr->b_l1hdr.b_pabd);
        hdr->b_l1hdr.b_pabd = NULL;
        buf->b_flags &= ~ARC_BUF_FLAG_SHARED;

        /*
         * Since the buffer is no longer shared between
         * the arc buf and the hdr, count it as overhead.
         */
        ARCSTAT_INCR(arcstat_compressed_size, -arc_hdr_size(hdr));
        ARCSTAT_INCR(arcstat_uncompressed_size, -HDR_GET_LSIZE(hdr));
        ARCSTAT_INCR(arcstat_overhead_size, arc_buf_size(buf));
}

/*
 * Remove an arc_buf_t from the hdr's buf list and return the last
 * arc_buf_t on the list. If no buffers remain on the list then return
 * NULL.
 */
static arc_buf_t *
arc_buf_remove(arc_buf_hdr_t *hdr, arc_buf_t *buf)
{
        ASSERT(HDR_HAS_L1HDR(hdr));
        ASSERT(HDR_EMPTY_OR_LOCKED(hdr));

        arc_buf_t **bufp = &hdr->b_l1hdr.b_buf;
        arc_buf_t *lastbuf = NULL;

        /*
         * Remove the buf from the hdr list and locate the last
         * remaining buffer on the list.
         */
        while (*bufp != NULL) {
                if (*bufp == buf)
                        *bufp = buf->b_next;

                /*
                 * If we've removed a buffer in the middle of
                 * the list then update the lastbuf and update
                 * bufp.
                 */
                if (*bufp != NULL) {
                        lastbuf = *bufp;
                        bufp = &(*bufp)->b_next;
                }
        }
        buf->b_next = NULL;
        ASSERT3P(lastbuf, !=, buf);
        IMPLY(hdr->b_l1hdr.b_bufcnt > 0, lastbuf != NULL);
        IMPLY(hdr->b_l1hdr.b_bufcnt > 0, hdr->b_l1hdr.b_buf != NULL);
        IMPLY(lastbuf != NULL, ARC_BUF_LAST(lastbuf));

        return (lastbuf);
}

/*
 * Free up buf->b_data and pull the arc_buf_t off of the arc_buf_hdr_t's
 * list and free it.
 */
static void
arc_buf_destroy_impl(arc_buf_t *buf)
{
        arc_buf_hdr_t *hdr = buf->b_hdr;

        /*
         * Free up the data associated with the buf but only if we're not
         * sharing this with the hdr. If we are sharing it with the hdr, the
         * hdr is responsible for doing the free.
         */
        if (buf->b_data != NULL) {
                /*
                 * We're about to change the hdr's b_flags. We must either
                 * hold the hash_lock or be undiscoverable.
                 */
                ASSERT(HDR_EMPTY_OR_LOCKED(hdr));

                arc_cksum_verify(buf);
                arc_buf_unwatch(buf);

                if (arc_buf_is_shared(buf)) {
                        arc_hdr_clear_flags(hdr, ARC_FLAG_SHARED_DATA);
                } else {
                        uint64_t size = arc_buf_size(buf);
                        arc_free_data_buf(hdr, buf->b_data, size, buf);
                        ARCSTAT_INCR(arcstat_overhead_size, -size);
                }
                buf->b_data = NULL;

                ASSERT(hdr->b_l1hdr.b_bufcnt > 0);
                hdr->b_l1hdr.b_bufcnt -= 1;

                if (ARC_BUF_ENCRYPTED(buf)) {
                        hdr->b_crypt_hdr.b_ebufcnt -= 1;

                        /*
                         * If we have no more encrypted buffers and we've
                         * already gotten a copy of the decrypted data we can
                         * free b_rabd to save some space.
                         */
                        if (hdr->b_crypt_hdr.b_ebufcnt == 0 &&
                            HDR_HAS_RABD(hdr) && hdr->b_l1hdr.b_pabd != NULL &&
                            !HDR_IO_IN_PROGRESS(hdr)) {
                                arc_hdr_free_abd(hdr, B_TRUE);
                        }
                }
        }

        arc_buf_t *lastbuf = arc_buf_remove(hdr, buf);

        if (ARC_BUF_SHARED(buf) && !ARC_BUF_COMPRESSED(buf)) {
                /*
                 * If the current arc_buf_t is sharing its data buffer with the
                 * hdr, then reassign the hdr's b_pabd to share it with the new
                 * buffer at the end of the list. The shared buffer is always
                 * the last one on the hdr's buffer list.
                 *
                 * There is an equivalent case for compressed bufs, but since
                 * they aren't guaranteed to be the last buf in the list and
                 * that is an exceedingly rare case, we just allow that space be
                 * wasted temporarily. We must also be careful not to share
                 * encrypted buffers, since they cannot be shared.
                 */
                if (lastbuf != NULL && !ARC_BUF_ENCRYPTED(lastbuf)) {
                        /* Only one buf can be shared at once */
                        VERIFY(!arc_buf_is_shared(lastbuf));
                        /* hdr is uncompressed so can't have compressed buf */
                        VERIFY(!ARC_BUF_COMPRESSED(lastbuf));

                        ASSERT3P(hdr->b_l1hdr.b_pabd, !=, NULL);
                        arc_hdr_free_abd(hdr, B_FALSE);

                        /*
                         * We must setup a new shared block between the
                         * last buffer and the hdr. The data would have
                         * been allocated by the arc buf so we need to transfer
                         * ownership to the hdr since it's now being shared.
                         */
                        arc_share_buf(hdr, lastbuf);
                }
        } else if (HDR_SHARED_DATA(hdr)) {
                /*
                 * Uncompressed shared buffers are always at the end
                 * of the list. Compressed buffers don't have the
                 * same requirements. This makes it hard to
                 * simply assert that the lastbuf is shared so
                 * we rely on the hdr's compression flags to determine
                 * if we have a compressed, shared buffer.
                 */
                ASSERT3P(lastbuf, !=, NULL);
                ASSERT(arc_buf_is_shared(lastbuf) ||
                    arc_hdr_get_compress(hdr) != ZIO_COMPRESS_OFF);
        }

        /*
         * Free the checksum if we're removing the last uncompressed buf from
         * this hdr.
         */
        if (!arc_hdr_has_uncompressed_buf(hdr)) {
                arc_cksum_free(hdr);
        }

        /* clean up the buf */
        buf->b_hdr = NULL;
        kmem_cache_free(buf_cache, buf);
}

static void
arc_hdr_alloc_abd(arc_buf_hdr_t *hdr, int alloc_flags)
{
        uint64_t size;
        boolean_t alloc_rdata = ((alloc_flags & ARC_HDR_ALLOC_RDATA) != 0);
        boolean_t do_adapt = ((alloc_flags & ARC_HDR_DO_ADAPT) != 0);

        ASSERT3U(HDR_GET_LSIZE(hdr), >, 0);
        ASSERT(HDR_HAS_L1HDR(hdr));
        ASSERT(!HDR_SHARED_DATA(hdr) || alloc_rdata);
        IMPLY(alloc_rdata, HDR_PROTECTED(hdr));

        if (alloc_rdata) {
                size = HDR_GET_PSIZE(hdr);
                ASSERT3P(hdr->b_crypt_hdr.b_rabd, ==, NULL);
                hdr->b_crypt_hdr.b_rabd = arc_get_data_abd(hdr, size, hdr,
                    do_adapt);
                ASSERT3P(hdr->b_crypt_hdr.b_rabd, !=, NULL);
                ARCSTAT_INCR(arcstat_raw_size, size);
        } else {
                size = arc_hdr_size(hdr);
                ASSERT3P(hdr->b_l1hdr.b_pabd, ==, NULL);
                hdr->b_l1hdr.b_pabd = arc_get_data_abd(hdr, size, hdr,
                    do_adapt);
                ASSERT3P(hdr->b_l1hdr.b_pabd, !=, NULL);
        }

        ARCSTAT_INCR(arcstat_compressed_size, size);
        ARCSTAT_INCR(arcstat_uncompressed_size, HDR_GET_LSIZE(hdr));
}

static void
arc_hdr_free_abd(arc_buf_hdr_t *hdr, boolean_t free_rdata)
{
        uint64_t size = (free_rdata) ? HDR_GET_PSIZE(hdr) : arc_hdr_size(hdr);

        ASSERT(HDR_HAS_L1HDR(hdr));
        ASSERT(hdr->b_l1hdr.b_pabd != NULL || HDR_HAS_RABD(hdr));
        IMPLY(free_rdata, HDR_HAS_RABD(hdr));

        /*
         * If the hdr is currently being written to the l2arc then
         * we defer freeing the data by adding it to the l2arc_free_on_write
         * list. The l2arc will free the data once it's finished
         * writing it to the l2arc device.
         */
        if (HDR_L2_WRITING(hdr)) {
                arc_hdr_free_on_write(hdr, free_rdata);
                ARCSTAT_BUMP(arcstat_l2_free_on_write);
        } else if (free_rdata) {
                arc_free_data_abd(hdr, hdr->b_crypt_hdr.b_rabd, size, hdr);
        } else {
                arc_free_data_abd(hdr, hdr->b_l1hdr.b_pabd, size, hdr);
        }

        if (free_rdata) {
                hdr->b_crypt_hdr.b_rabd = NULL;
                ARCSTAT_INCR(arcstat_raw_size, -size);
        } else {
                hdr->b_l1hdr.b_pabd = NULL;
        }

        if (hdr->b_l1hdr.b_pabd == NULL && !HDR_HAS_RABD(hdr))
                hdr->b_l1hdr.b_byteswap = DMU_BSWAP_NUMFUNCS;

        ARCSTAT_INCR(arcstat_compressed_size, -size);
        ARCSTAT_INCR(arcstat_uncompressed_size, -HDR_GET_LSIZE(hdr));
}

static arc_buf_hdr_t *
arc_hdr_alloc(uint64_t spa, int32_t psize, int32_t lsize,
    boolean_t protected, enum zio_compress compression_type, uint8_t complevel,
    arc_buf_contents_t type, boolean_t alloc_rdata)
{
        arc_buf_hdr_t *hdr;
        int flags = ARC_HDR_DO_ADAPT;

        VERIFY(type == ARC_BUFC_DATA || type == ARC_BUFC_METADATA);
        if (protected) {
                hdr = kmem_cache_alloc(hdr_full_crypt_cache, KM_PUSHPAGE);
        } else {
                hdr = kmem_cache_alloc(hdr_full_cache, KM_PUSHPAGE);
        }
        flags |= alloc_rdata ? ARC_HDR_ALLOC_RDATA : 0;

        ASSERT(HDR_EMPTY(hdr));
        ASSERT3P(hdr->b_l1hdr.b_freeze_cksum, ==, NULL);
        HDR_SET_PSIZE(hdr, psize);
        HDR_SET_LSIZE(hdr, lsize);
        hdr->b_spa = spa;
        hdr->b_type = type;
        hdr->b_flags = 0;
        arc_hdr_set_flags(hdr, arc_bufc_to_flags(type) | ARC_FLAG_HAS_L1HDR);
        arc_hdr_set_compress(hdr, compression_type);
        hdr->b_complevel = complevel;
        if (protected)
                arc_hdr_set_flags(hdr, ARC_FLAG_PROTECTED);

        hdr->b_l1hdr.b_state = arc_anon;
        hdr->b_l1hdr.b_arc_access = 0;
        hdr->b_l1hdr.b_bufcnt = 0;
        hdr->b_l1hdr.b_buf = NULL;

        /*
         * Allocate the hdr's buffer. This will contain either
         * the compressed or uncompressed data depending on the block
         * it references and compressed arc enablement.
         */
        arc_hdr_alloc_abd(hdr, flags);
        ASSERT(zfs_refcount_is_zero(&hdr->b_l1hdr.b_refcnt));

        return (hdr);
}

/*
 * Transition between the two allocation states for the arc_buf_hdr struct.
 * The arc_buf_hdr struct can be allocated with (hdr_full_cache) or without
 * (hdr_l2only_cache) the fields necessary for the L1 cache - the smaller
 * version is used when a cache buffer is only in the L2ARC in order to reduce
 * memory usage.
 */
static arc_buf_hdr_t *
arc_hdr_realloc(arc_buf_hdr_t *hdr, kmem_cache_t *old, kmem_cache_t *new)
{
        ASSERT(HDR_HAS_L2HDR(hdr));

        arc_buf_hdr_t *nhdr;
        l2arc_dev_t *dev = hdr->b_l2hdr.b_dev;

        ASSERT((old == hdr_full_cache && new == hdr_l2only_cache) ||
            (old == hdr_l2only_cache && new == hdr_full_cache));

        /*
         * if the caller wanted a new full header and the header is to be
         * encrypted we will actually allocate the header from the full crypt
         * cache instead. The same applies to freeing from the old cache.
         */
        if (HDR_PROTECTED(hdr) && new == hdr_full_cache)
                new = hdr_full_crypt_cache;
        if (HDR_PROTECTED(hdr) && old == hdr_full_cache)
                old = hdr_full_crypt_cache;

        nhdr = kmem_cache_alloc(new, KM_PUSHPAGE);

        ASSERT(MUTEX_HELD(HDR_LOCK(hdr)));
        buf_hash_remove(hdr);

        bcopy(hdr, nhdr, HDR_L2ONLY_SIZE);

        if (new == hdr_full_cache || new == hdr_full_crypt_cache) {
                arc_hdr_set_flags(nhdr, ARC_FLAG_HAS_L1HDR);
                /*
                 * arc_access and arc_change_state need to be aware that a
                 * header has just come out of L2ARC, so we set its state to
                 * l2c_only even though it's about to change.
                 */
                nhdr->b_l1hdr.b_state = arc_l2c_only;

                /* Verify previous threads set to NULL before freeing */
                ASSERT3P(nhdr->b_l1hdr.b_pabd, ==, NULL);
                ASSERT(!HDR_HAS_RABD(hdr));
        } else {
                ASSERT3P(hdr->b_l1hdr.b_buf, ==, NULL);
                ASSERT0(hdr->b_l1hdr.b_bufcnt);
                ASSERT3P(hdr->b_l1hdr.b_freeze_cksum, ==, NULL);

                /*
                 * If we've reached here, We must have been called from
                 * arc_evict_hdr(), as such we should have already been
                 * removed from any ghost list we were previously on
                 * (which protects us from racing with arc_evict_state),
                 * thus no locking is needed during this check.
                 */
                ASSERT(!multilist_link_active(&hdr->b_l1hdr.b_arc_node));

                /*
                 * A buffer must not be moved into the arc_l2c_only
                 * state if it's not finished being written out to the
                 * l2arc device. Otherwise, the b_l1hdr.b_pabd field
                 * might try to be accessed, even though it was removed.
                 */
                VERIFY(!HDR_L2_WRITING(hdr));
                VERIFY3P(hdr->b_l1hdr.b_pabd, ==, NULL);
                ASSERT(!HDR_HAS_RABD(hdr));

                arc_hdr_clear_flags(nhdr, ARC_FLAG_HAS_L1HDR);
        }
        /*
         * The header has been reallocated so we need to re-insert it into any
         * lists it was on.
         */
        (void) buf_hash_insert(nhdr, NULL);

        ASSERT(list_link_active(&hdr->b_l2hdr.b_l2node));

        mutex_enter(&dev->l2ad_mtx);

        /*
         * We must place the realloc'ed header back into the list at
         * the same spot. Otherwise, if it's placed earlier in the list,
         * l2arc_write_buffers() could find it during the function's
         * write phase, and try to write it out to the l2arc.
         */
        list_insert_after(&dev->l2ad_buflist, hdr, nhdr);
        list_remove(&dev->l2ad_buflist, hdr);

        mutex_exit(&dev->l2ad_mtx);

        /*
         * Since we're using the pointer address as the tag when
         * incrementing and decrementing the l2ad_alloc refcount, we
         * must remove the old pointer (that we're about to destroy) and
         * add the new pointer to the refcount. Otherwise we'd remove
         * the wrong pointer address when calling arc_hdr_destroy() later.
         */

        (void) zfs_refcount_remove_many(&dev->l2ad_alloc,
            arc_hdr_size(hdr), hdr);
        (void) zfs_refcount_add_many(&dev->l2ad_alloc,
            arc_hdr_size(nhdr), nhdr);

        buf_discard_identity(hdr);
        kmem_cache_free(old, hdr);

        return (nhdr);
}

/*
 * This function allows an L1 header to be reallocated as a crypt
 * header and vice versa. If we are going to a crypt header, the
 * new fields will be zeroed out.
 */
static arc_buf_hdr_t *
arc_hdr_realloc_crypt(arc_buf_hdr_t *hdr, boolean_t need_crypt)
{
        arc_buf_hdr_t *nhdr;
        arc_buf_t *buf;
        kmem_cache_t *ncache, *ocache;
        unsigned nsize, osize;

        /*
         * This function requires that hdr is in the arc_anon state.
         * Therefore it won't have any L2ARC data for us to worry
         * about copying.
         */
        ASSERT(HDR_HAS_L1HDR(hdr));
        ASSERT(!HDR_HAS_L2HDR(hdr));
        ASSERT3U(!!HDR_PROTECTED(hdr), !=, need_crypt);
        ASSERT3P(hdr->b_l1hdr.b_state, ==, arc_anon);
        ASSERT(!multilist_link_active(&hdr->b_l1hdr.b_arc_node));
        ASSERT(!list_link_active(&hdr->b_l2hdr.b_l2node));
        ASSERT3P(hdr->b_hash_next, ==, NULL);

        if (need_crypt) {
                ncache = hdr_full_crypt_cache;
                nsize = sizeof (hdr->b_crypt_hdr);
                ocache = hdr_full_cache;
                osize = HDR_FULL_SIZE;
        } else {
                ncache = hdr_full_cache;
                nsize = HDR_FULL_SIZE;
                ocache = hdr_full_crypt_cache;
                osize = sizeof (hdr->b_crypt_hdr);
        }

        nhdr = kmem_cache_alloc(ncache, KM_PUSHPAGE);

        /*
         * Copy all members that aren't locks or condvars to the new header.
         * No lists are pointing to us (as we asserted above), so we don't
         * need to worry about the list nodes.
         */
        nhdr->b_dva = hdr->b_dva;
        nhdr->b_birth = hdr->b_birth;
        nhdr->b_type = hdr->b_type;
        nhdr->b_flags = hdr->b_flags;
        nhdr->b_psize = hdr->b_psize;
        nhdr->b_lsize = hdr->b_lsize;
        nhdr->b_spa = hdr->b_spa;
        nhdr->b_l1hdr.b_freeze_cksum = hdr->b_l1hdr.b_freeze_cksum;
        nhdr->b_l1hdr.b_bufcnt = hdr->b_l1hdr.b_bufcnt;
        nhdr->b_l1hdr.b_byteswap = hdr->b_l1hdr.b_byteswap;
        nhdr->b_l1hdr.b_state = hdr->b_l1hdr.b_state;
        nhdr->b_l1hdr.b_arc_access = hdr->b_l1hdr.b_arc_access;
        nhdr->b_l1hdr.b_mru_hits = hdr->b_l1hdr.b_mru_hits;
        nhdr->b_l1hdr.b_mru_ghost_hits = hdr->b_l1hdr.b_mru_ghost_hits;
        nhdr->b_l1hdr.b_mfu_hits = hdr->b_l1hdr.b_mfu_hits;
        nhdr->b_l1hdr.b_mfu_ghost_hits = hdr->b_l1hdr.b_mfu_ghost_hits;
        nhdr->b_l1hdr.b_l2_hits = hdr->b_l1hdr.b_l2_hits;
        nhdr->b_l1hdr.b_acb = hdr->b_l1hdr.b_acb;
        nhdr->b_l1hdr.b_pabd = hdr->b_l1hdr.b_pabd;

        /*
         * This zfs_refcount_add() exists only to ensure that the individual
         * arc buffers always point to a header that is referenced, avoiding
         * a small race condition that could trigger ASSERTs.
         */
        (void) zfs_refcount_add(&nhdr->b_l1hdr.b_refcnt, FTAG);
        nhdr->b_l1hdr.b_buf = hdr->b_l1hdr.b_buf;
        for (buf = nhdr->b_l1hdr.b_buf; buf != NULL; buf = buf->b_next) {
                mutex_enter(&buf->b_evict_lock);
                buf->b_hdr = nhdr;
                mutex_exit(&buf->b_evict_lock);
        }

        zfs_refcount_transfer(&nhdr->b_l1hdr.b_refcnt, &hdr->b_l1hdr.b_refcnt);
        (void) zfs_refcount_remove(&nhdr->b_l1hdr.b_refcnt, FTAG);
        ASSERT0(zfs_refcount_count(&hdr->b_l1hdr.b_refcnt));

        if (need_crypt) {
                arc_hdr_set_flags(nhdr, ARC_FLAG_PROTECTED);
        } else {
                arc_hdr_clear_flags(nhdr, ARC_FLAG_PROTECTED);
        }

        /* unset all members of the original hdr */
        bzero(&hdr->b_dva, sizeof (dva_t));
        hdr->b_birth = 0;
        hdr->b_type = ARC_BUFC_INVALID;
        hdr->b_flags = 0;
        hdr->b_psize = 0;
        hdr->b_lsize = 0;
        hdr->b_spa = 0;
        hdr->b_l1hdr.b_freeze_cksum = NULL;
        hdr->b_l1hdr.b_buf = NULL;
        hdr->b_l1hdr.b_bufcnt = 0;
        hdr->b_l1hdr.b_byteswap = 0;
        hdr->b_l1hdr.b_state = NULL;
        hdr->b_l1hdr.b_arc_access = 0;
        hdr->b_l1hdr.b_mru_hits = 0;
        hdr->b_l1hdr.b_mru_ghost_hits = 0;
        hdr->b_l1hdr.b_mfu_hits = 0;
        hdr->b_l1hdr.b_mfu_ghost_hits = 0;
        hdr->b_l1hdr.b_l2_hits = 0;
        hdr->b_l1hdr.b_acb = NULL;
        hdr->b_l1hdr.b_pabd = NULL;

        if (ocache == hdr_full_crypt_cache) {
                ASSERT(!HDR_HAS_RABD(hdr));
                hdr->b_crypt_hdr.b_ot = DMU_OT_NONE;
                hdr->b_crypt_hdr.b_ebufcnt = 0;
                hdr->b_crypt_hdr.b_dsobj = 0;
                bzero(hdr->b_crypt_hdr.b_salt, ZIO_DATA_SALT_LEN);
                bzero(hdr->b_crypt_hdr.b_iv, ZIO_DATA_IV_LEN);
                bzero(hdr->b_crypt_hdr.b_mac, ZIO_DATA_MAC_LEN);
        }

        buf_discard_identity(hdr);
        kmem_cache_free(ocache, hdr);

        return (nhdr);
}

/*
 * This function is used by the send / receive code to convert a newly
 * allocated arc_buf_t to one that is suitable for a raw encrypted write. It
 * is also used to allow the root objset block to be updated without altering
 * its embedded MACs. Both block types will always be uncompressed so we do not
 * have to worry about compression type or psize.
 */
void
arc_convert_to_raw(arc_buf_t *buf, uint64_t dsobj, boolean_t byteorder,
    dmu_object_type_t ot, const uint8_t *salt, const uint8_t *iv,
    const uint8_t *mac)
{
        arc_buf_hdr_t *hdr = buf->b_hdr;

        ASSERT(ot == DMU_OT_DNODE || ot == DMU_OT_OBJSET);
        ASSERT(HDR_HAS_L1HDR(hdr));
        ASSERT3P(hdr->b_l1hdr.b_state, ==, arc_anon);

        buf->b_flags |= (ARC_BUF_FLAG_COMPRESSED | ARC_BUF_FLAG_ENCRYPTED);
        if (!HDR_PROTECTED(hdr))
                hdr = arc_hdr_realloc_crypt(hdr, B_TRUE);
        hdr->b_crypt_hdr.b_dsobj = dsobj;
        hdr->b_crypt_hdr.b_ot = ot;
        hdr->b_l1hdr.b_byteswap = (byteorder == ZFS_HOST_BYTEORDER) ?
            DMU_BSWAP_NUMFUNCS : DMU_OT_BYTESWAP(ot);
        if (!arc_hdr_has_uncompressed_buf(hdr))
                arc_cksum_free(hdr);

        if (salt != NULL)
                bcopy(salt, hdr->b_crypt_hdr.b_salt, ZIO_DATA_SALT_LEN);
        if (iv != NULL)
                bcopy(iv, hdr->b_crypt_hdr.b_iv, ZIO_DATA_IV_LEN);
        if (mac != NULL)
                bcopy(mac, hdr->b_crypt_hdr.b_mac, ZIO_DATA_MAC_LEN);
}

/*
 * Allocate a new arc_buf_hdr_t and arc_buf_t and return the buf to the caller.
 * The buf is returned thawed since we expect the consumer to modify it.
 */
arc_buf_t *
arc_alloc_buf(spa_t *spa, void *tag, arc_buf_contents_t type, int32_t size)
{
        arc_buf_hdr_t *hdr = arc_hdr_alloc(spa_load_guid(spa), size, size,
            B_FALSE, ZIO_COMPRESS_OFF, 0, type, B_FALSE);

        arc_buf_t *buf = NULL;
        VERIFY0(arc_buf_alloc_impl(hdr, spa, NULL, tag, B_FALSE, B_FALSE,
            B_FALSE, B_FALSE, &buf));
        arc_buf_thaw(buf);

        return (buf);
}

/*
 * Allocate a compressed buf in the same manner as arc_alloc_buf. Don't use this
 * for bufs containing metadata.
 */
arc_buf_t *
arc_alloc_compressed_buf(spa_t *spa, void *tag, uint64_t psize, uint64_t lsize,
    enum zio_compress compression_type, uint8_t complevel)
{
        ASSERT3U(lsize, >, 0);
        ASSERT3U(lsize, >=, psize);
        ASSERT3U(compression_type, >, ZIO_COMPRESS_OFF);
        ASSERT3U(compression_type, <, ZIO_COMPRESS_FUNCTIONS);

        arc_buf_hdr_t *hdr = arc_hdr_alloc(spa_load_guid(spa), psize, lsize,
            B_FALSE, compression_type, complevel, ARC_BUFC_DATA, B_FALSE);

        arc_buf_t *buf = NULL;
        VERIFY0(arc_buf_alloc_impl(hdr, spa, NULL, tag, B_FALSE,
            B_TRUE, B_FALSE, B_FALSE, &buf));
        arc_buf_thaw(buf);
        ASSERT3P(hdr->b_l1hdr.b_freeze_cksum, ==, NULL);

        if (!arc_buf_is_shared(buf)) {
                /*
                 * To ensure that the hdr has the correct data in it if we call
                 * arc_untransform() on this buf before it's been written to
                 * disk, it's easiest if we just set up sharing between the
                 * buf and the hdr.
                 */
                arc_hdr_free_abd(hdr, B_FALSE);
                arc_share_buf(hdr, buf);
        }

        return (buf);
}

arc_buf_t *
arc_alloc_raw_buf(spa_t *spa, void *tag, uint64_t dsobj, boolean_t byteorder,
    const uint8_t *salt, const uint8_t *iv, const uint8_t *mac,
    dmu_object_type_t ot, uint64_t psize, uint64_t lsize,
    enum zio_compress compression_type, uint8_t complevel)
{
        arc_buf_hdr_t *hdr;
        arc_buf_t *buf;
        arc_buf_contents_t type = DMU_OT_IS_METADATA(ot) ?
            ARC_BUFC_METADATA : ARC_BUFC_DATA;

        ASSERT3U(lsize, >, 0);
        ASSERT3U(lsize, >=, psize);
        ASSERT3U(compression_type, >=, ZIO_COMPRESS_OFF);
        ASSERT3U(compression_type, <, ZIO_COMPRESS_FUNCTIONS);

        hdr = arc_hdr_alloc(spa_load_guid(spa), psize, lsize, B_TRUE,
            compression_type, complevel, type, B_TRUE);

        hdr->b_crypt_hdr.b_dsobj = dsobj;
        hdr->b_crypt_hdr.b_ot = ot;
        hdr->b_l1hdr.b_byteswap = (byteorder == ZFS_HOST_BYTEORDER) ?
            DMU_BSWAP_NUMFUNCS : DMU_OT_BYTESWAP(ot);
        bcopy(salt, hdr->b_crypt_hdr.b_salt, ZIO_DATA_SALT_LEN);
        bcopy(iv, hdr->b_crypt_hdr.b_iv, ZIO_DATA_IV_LEN);
        bcopy(mac, hdr->b_crypt_hdr.b_mac, ZIO_DATA_MAC_LEN);

        /*
         * This buffer will be considered encrypted even if the ot is not an
         * encrypted type. It will become authenticated instead in
         * arc_write_ready().
         */
        buf = NULL;
        VERIFY0(arc_buf_alloc_impl(hdr, spa, NULL, tag, B_TRUE, B_TRUE,
            B_FALSE, B_FALSE, &buf));
        arc_buf_thaw(buf);
        ASSERT3P(hdr->b_l1hdr.b_freeze_cksum, ==, NULL);

        return (buf);
}

static void
arc_hdr_l2hdr_destroy(arc_buf_hdr_t *hdr)
{
        l2arc_buf_hdr_t *l2hdr = &hdr->b_l2hdr;
        l2arc_dev_t *dev = l2hdr->b_dev;
        uint64_t psize = HDR_GET_PSIZE(hdr);
        uint64_t asize = vdev_psize_to_asize(dev->l2ad_vdev, psize);

        ASSERT(MUTEX_HELD(&dev->l2ad_mtx));
        ASSERT(HDR_HAS_L2HDR(hdr));

        list_remove(&dev->l2ad_buflist, hdr);

        ARCSTAT_INCR(arcstat_l2_psize, -psize);
        ARCSTAT_INCR(arcstat_l2_lsize, -HDR_GET_LSIZE(hdr));

        vdev_space_update(dev->l2ad_vdev, -asize, 0, 0);

        (void) zfs_refcount_remove_many(&dev->l2ad_alloc, arc_hdr_size(hdr),
            hdr);
        arc_hdr_clear_flags(hdr, ARC_FLAG_HAS_L2HDR);
}

static void
arc_hdr_destroy(arc_buf_hdr_t *hdr)
{
        if (HDR_HAS_L1HDR(hdr)) {
                ASSERT(hdr->b_l1hdr.b_buf == NULL ||
                    hdr->b_l1hdr.b_bufcnt > 0);
                ASSERT(zfs_refcount_is_zero(&hdr->b_l1hdr.b_refcnt));
                ASSERT3P(hdr->b_l1hdr.b_state, ==, arc_anon);
        }
        ASSERT(!HDR_IO_IN_PROGRESS(hdr));
        ASSERT(!HDR_IN_HASH_TABLE(hdr));

        if (HDR_HAS_L2HDR(hdr)) {
                l2arc_dev_t *dev = hdr->b_l2hdr.b_dev;
                boolean_t buflist_held = MUTEX_HELD(&dev->l2ad_mtx);

                if (!buflist_held)
                        mutex_enter(&dev->l2ad_mtx);

                /*
                 * Even though we checked this conditional above, we
                 * need to check this again now that we have the
                 * l2ad_mtx. This is because we could be racing with
                 * another thread calling l2arc_evict() which might have
                 * destroyed this header's L2 portion as we were waiting
                 * to acquire the l2ad_mtx. If that happens, we don't
                 * want to re-destroy the header's L2 portion.
                 */
                if (HDR_HAS_L2HDR(hdr))
                        arc_hdr_l2hdr_destroy(hdr);

                if (!buflist_held)
                        mutex_exit(&dev->l2ad_mtx);
        }

        /*
         * The header's identify can only be safely discarded once it is no
         * longer discoverable.  This requires removing it from the hash table
         * and the l2arc header list.  After this point the hash lock can not
         * be used to protect the header.
         */
        if (!HDR_EMPTY(hdr))
                buf_discard_identity(hdr);

        if (HDR_HAS_L1HDR(hdr)) {
                arc_cksum_free(hdr);

                while (hdr->b_l1hdr.b_buf != NULL)
                        arc_buf_destroy_impl(hdr->b_l1hdr.b_buf);

                if (hdr->b_l1hdr.b_pabd != NULL)
                        arc_hdr_free_abd(hdr, B_FALSE);

                if (HDR_HAS_RABD(hdr))
                        arc_hdr_free_abd(hdr, B_TRUE);
        }

        ASSERT3P(hdr->b_hash_next, ==, NULL);
        if (HDR_HAS_L1HDR(hdr)) {
                ASSERT(!multilist_link_active(&hdr->b_l1hdr.b_arc_node));
                ASSERT3P(hdr->b_l1hdr.b_acb, ==, NULL);

                if (!HDR_PROTECTED(hdr)) {
                        kmem_cache_free(hdr_full_cache, hdr);
                } else {
                        kmem_cache_free(hdr_full_crypt_cache, hdr);
                }
        } else {
                kmem_cache_free(hdr_l2only_cache, hdr);
        }
}

void
arc_buf_destroy(arc_buf_t *buf, void* tag)
{
        arc_buf_hdr_t *hdr = buf->b_hdr;

        if (hdr->b_l1hdr.b_state == arc_anon) {
                ASSERT3U(hdr->b_l1hdr.b_bufcnt, ==, 1);
                ASSERT(!HDR_IO_IN_PROGRESS(hdr));
                VERIFY0(remove_reference(hdr, NULL, tag));
                arc_hdr_destroy(hdr);
                return;
        }

        kmutex_t *hash_lock = HDR_LOCK(hdr);
        mutex_enter(hash_lock);

        ASSERT3P(hdr, ==, buf->b_hdr);
        ASSERT(hdr->b_l1hdr.b_bufcnt > 0);
        ASSERT3P(hash_lock, ==, HDR_LOCK(hdr));
        ASSERT3P(hdr->b_l1hdr.b_state, !=, arc_anon);
        ASSERT3P(buf->b_data, !=, NULL);

        (void) remove_reference(hdr, hash_lock, tag);
        arc_buf_destroy_impl(buf);
        mutex_exit(hash_lock);
}

/*
 * Evict the arc_buf_hdr that is provided as a parameter. The resultant
 * state of the header is dependent on its state prior to entering this
 * function. The following transitions are possible:
 *
 *    - arc_mru -> arc_mru_ghost
 *    - arc_mfu -> arc_mfu_ghost
 *    - arc_mru_ghost -> arc_l2c_only
 *    - arc_mru_ghost -> deleted
 *    - arc_mfu_ghost -> arc_l2c_only
 *    - arc_mfu_ghost -> deleted
 */
static int64_t
arc_evict_hdr(arc_buf_hdr_t *hdr, kmutex_t *hash_lock)
{
        arc_state_t *evicted_state, *state;
        int64_t bytes_evicted = 0;
        int min_lifetime = HDR_PRESCIENT_PREFETCH(hdr) ?
            arc_min_prescient_prefetch_ms : arc_min_prefetch_ms;

        ASSERT(MUTEX_HELD(hash_lock));
        ASSERT(HDR_HAS_L1HDR(hdr));

        state = hdr->b_l1hdr.b_state;
        if (GHOST_STATE(state)) {
                ASSERT(!HDR_IO_IN_PROGRESS(hdr));
                ASSERT3P(hdr->b_l1hdr.b_buf, ==, NULL);

                /*
                 * l2arc_write_buffers() relies on a header's L1 portion
                 * (i.e. its b_pabd field) during it's write phase.
                 * Thus, we cannot push a header onto the arc_l2c_only
                 * state (removing its L1 piece) until the header is
                 * done being written to the l2arc.
                 */
                if (HDR_HAS_L2HDR(hdr) && HDR_L2_WRITING(hdr)) {
                        ARCSTAT_BUMP(arcstat_evict_l2_skip);
                        return (bytes_evicted);
                }

                ARCSTAT_BUMP(arcstat_deleted);
                bytes_evicted += HDR_GET_LSIZE(hdr);

                DTRACE_PROBE1(arc__delete, arc_buf_hdr_t *, hdr);

                if (HDR_HAS_L2HDR(hdr)) {
                        ASSERT(hdr->b_l1hdr.b_pabd == NULL);
                        ASSERT(!HDR_HAS_RABD(hdr));
                        /*
                         * This buffer is cached on the 2nd Level ARC;
                         * don't destroy the header.
                         */
                        arc_change_state(arc_l2c_only, hdr, hash_lock);
                        /*
                         * dropping from L1+L2 cached to L2-only,
                         * realloc to remove the L1 header.
                         */
                        hdr = arc_hdr_realloc(hdr, hdr_full_cache,
                            hdr_l2only_cache);
                } else {
                        arc_change_state(arc_anon, hdr, hash_lock);
                        arc_hdr_destroy(hdr);
                }
                return (bytes_evicted);
        }

        ASSERT(state == arc_mru || state == arc_mfu);
        evicted_state = (state == arc_mru) ? arc_mru_ghost : arc_mfu_ghost;

        /* prefetch buffers have a minimum lifespan */
        if (HDR_IO_IN_PROGRESS(hdr) ||
            ((hdr->b_flags & (ARC_FLAG_PREFETCH | ARC_FLAG_INDIRECT)) &&
            ddi_get_lbolt() - hdr->b_l1hdr.b_arc_access <
            MSEC_TO_TICK(min_lifetime))) {
                ARCSTAT_BUMP(arcstat_evict_skip);
                return (bytes_evicted);
        }

        ASSERT0(zfs_refcount_count(&hdr->b_l1hdr.b_refcnt));
        while (hdr->b_l1hdr.b_buf) {
                arc_buf_t *buf = hdr->b_l1hdr.b_buf;
                if (!mutex_tryenter(&buf->b_evict_lock)) {
                        ARCSTAT_BUMP(arcstat_mutex_miss);
                        break;
                }
                if (buf->b_data != NULL)
                        bytes_evicted += HDR_GET_LSIZE(hdr);
                mutex_exit(&buf->b_evict_lock);
                arc_buf_destroy_impl(buf);
        }

        if (HDR_HAS_L2HDR(hdr)) {
                ARCSTAT_INCR(arcstat_evict_l2_cached, HDR_GET_LSIZE(hdr));
        } else {
                if (l2arc_write_eligible(hdr->b_spa, hdr)) {
                        ARCSTAT_INCR(arcstat_evict_l2_eligible,
                            HDR_GET_LSIZE(hdr));
                } else {
                        ARCSTAT_INCR(arcstat_evict_l2_ineligible,
                            HDR_GET_LSIZE(hdr));
                }
        }

        if (hdr->b_l1hdr.b_bufcnt == 0) {
                arc_cksum_free(hdr);

                bytes_evicted += arc_hdr_size(hdr);

                /*
                 * If this hdr is being evicted and has a compressed
                 * buffer then we discard it here before we change states.
                 * This ensures that the accounting is updated correctly
                 * in arc_free_data_impl().
                 */
                if (hdr->b_l1hdr.b_pabd != NULL)
                        arc_hdr_free_abd(hdr, B_FALSE);

                if (HDR_HAS_RABD(hdr))
                        arc_hdr_free_abd(hdr, B_TRUE);

                arc_change_state(evicted_state, hdr, hash_lock);
                ASSERT(HDR_IN_HASH_TABLE(hdr));
                arc_hdr_set_flags(hdr, ARC_FLAG_IN_HASH_TABLE);
                DTRACE_PROBE1(arc__evict, arc_buf_hdr_t *, hdr);
        }

        return (bytes_evicted);
}

static void
arc_set_need_free(void)
{
        ASSERT(MUTEX_HELD(&arc_evict_lock));
        int64_t remaining = arc_free_memory() - arc_sys_free / 2;
        arc_evict_waiter_t *aw = list_tail(&arc_evict_waiters);
        if (aw == NULL) {
                arc_need_free = MAX(-remaining, 0);
        } else {
                arc_need_free =
                    MAX(-remaining, (int64_t)(aw->aew_count - arc_evict_count));
        }
}

static uint64_t
arc_evict_state_impl(multilist_t *ml, int idx, arc_buf_hdr_t *marker,
    uint64_t spa, int64_t bytes)
{
        multilist_sublist_t *mls;
        uint64_t bytes_evicted = 0;
        arc_buf_hdr_t *hdr;
        kmutex_t *hash_lock;
        int evict_count = 0;

        ASSERT3P(marker, !=, NULL);
        IMPLY(bytes < 0, bytes == ARC_EVICT_ALL);

        mls = multilist_sublist_lock(ml, idx);

        for (hdr = multilist_sublist_prev(mls, marker); hdr != NULL;
            hdr = multilist_sublist_prev(mls, marker)) {
                if ((bytes != ARC_EVICT_ALL && bytes_evicted >= bytes) ||
                    (evict_count >= zfs_arc_evict_batch_limit))
                        break;

                /*
                 * To keep our iteration location, move the marker
                 * forward. Since we're not holding hdr's hash lock, we
                 * must be very careful and not remove 'hdr' from the
                 * sublist. Otherwise, other consumers might mistake the
                 * 'hdr' as not being on a sublist when they call the
                 * multilist_link_active() function (they all rely on
                 * the hash lock protecting concurrent insertions and
                 * removals). multilist_sublist_move_forward() was
                 * specifically implemented to ensure this is the case
                 * (only 'marker' will be removed and re-inserted).
                 */
                multilist_sublist_move_forward(mls, marker);

                /*
                 * The only case where the b_spa field should ever be
                 * zero, is the marker headers inserted by
                 * arc_evict_state(). It's possible for multiple threads
                 * to be calling arc_evict_state() concurrently (e.g.
                 * dsl_pool_close() and zio_inject_fault()), so we must
                 * skip any markers we see from these other threads.
                 */
                if (hdr->b_spa == 0)
                        continue;

                /* we're only interested in evicting buffers of a certain spa */
                if (spa != 0 && hdr->b_spa != spa) {
                        ARCSTAT_BUMP(arcstat_evict_skip);
                        continue;
                }

                hash_lock = HDR_LOCK(hdr);

                /*
                 * We aren't calling this function from any code path
                 * that would already be holding a hash lock, so we're
                 * asserting on this assumption to be defensive in case
                 * this ever changes. Without this check, it would be
                 * possible to incorrectly increment arcstat_mutex_miss
                 * below (e.g. if the code changed such that we called
                 * this function with a hash lock held).
                 */
                ASSERT(!MUTEX_HELD(hash_lock));

                if (mutex_tryenter(hash_lock)) {
                        uint64_t evicted = arc_evict_hdr(hdr, hash_lock);
                        mutex_exit(hash_lock);

                        bytes_evicted += evicted;

                        /*
                         * If evicted is zero, arc_evict_hdr() must have
                         * decided to skip this header, don't increment
                         * evict_count in this case.
                         */
                        if (evicted != 0)
                                evict_count++;

                } else {
                        ARCSTAT_BUMP(arcstat_mutex_miss);
                }
        }

        multilist_sublist_unlock(mls);

        /*
         * Increment the count of evicted bytes, and wake up any threads that
         * are waiting for the count to reach this value.  Since the list is
         * ordered by ascending aew_count, we pop off the beginning of the
         * list until we reach the end, or a waiter that's past the current
         * "count".  Doing this outside the loop reduces the number of times
         * we need to acquire the global arc_evict_lock.
         *
         * Only wake when there's sufficient free memory in the system
         * (specifically, arc_sys_free/2, which by default is a bit more than
         * 1/64th of RAM).  See the comments in arc_wait_for_eviction().
         */
        mutex_enter(&arc_evict_lock);
        arc_evict_count += bytes_evicted;

        if ((int64_t)(arc_free_memory() - arc_sys_free / 2) > 0) {
                arc_evict_waiter_t *aw;
                while ((aw = list_head(&arc_evict_waiters)) != NULL &&
                    aw->aew_count <= arc_evict_count) {
                        list_remove(&arc_evict_waiters, aw);
                        cv_broadcast(&aw->aew_cv);
                }
        }
        arc_set_need_free();
        mutex_exit(&arc_evict_lock);

        /*
         * If the ARC size is reduced from arc_c_max to arc_c_min (especially
         * if the average cached block is small), eviction can be on-CPU for
         * many seconds.  To ensure that other threads that may be bound to
         * this CPU are able to make progress, make a voluntary preemption
         * call here.
         */
        cond_resched();

        return (bytes_evicted);
}

/*
 * Evict buffers from the given arc state, until we've removed the
 * specified number of bytes. Move the removed buffers to the
 * appropriate evict state.
 *
 * This function makes a "best effort". It skips over any buffers
 * it can't get a hash_lock on, and so, may not catch all candidates.
 * It may also return without evicting as much space as requested.
 *
 * If bytes is specified using the special value ARC_EVICT_ALL, this
 * will evict all available (i.e. unlocked and evictable) buffers from
 * the given arc state; which is used by arc_flush().
 */
static uint64_t
arc_evict_state(arc_state_t *state, uint64_t spa, int64_t bytes,
    arc_buf_contents_t type)
{
        uint64_t total_evicted = 0;
        multilist_t *ml = state->arcs_list[type];
        int num_sublists;
        arc_buf_hdr_t **markers;

        IMPLY(bytes < 0, bytes == ARC_EVICT_ALL);

        num_sublists = multilist_get_num_sublists(ml);

        /*
         * If we've tried to evict from each sublist, made some
         * progress, but still have not hit the target number of bytes
         * to evict, we want to keep trying. The markers allow us to
         * pick up where we left off for each individual sublist, rather
         * than starting from the tail each time.
         */
        markers = kmem_zalloc(sizeof (*markers) * num_sublists, KM_SLEEP);
        for (int i = 0; i < num_sublists; i++) {
                multilist_sublist_t *mls;

                markers[i] = kmem_cache_alloc(hdr_full_cache, KM_SLEEP);

                /*
                 * A b_spa of 0 is used to indicate that this header is
                 * a marker. This fact is used in arc_evict_type() and
                 * arc_evict_state_impl().
                 */
                markers[i]->b_spa = 0;

                mls = multilist_sublist_lock(ml, i);
                multilist_sublist_insert_tail(mls, markers[i]);
                multilist_sublist_unlock(mls);
        }

        /*
         * While we haven't hit our target number of bytes to evict, or
         * we're evicting all available buffers.
         */
        while (total_evicted < bytes || bytes == ARC_EVICT_ALL) {
                int sublist_idx = multilist_get_random_index(ml);
                uint64_t scan_evicted = 0;

                /*
                 * Try to reduce pinned dnodes with a floor of arc_dnode_limit.
                 * Request that 10% of the LRUs be scanned by the superblock
                 * shrinker.
                 */
                if (type == ARC_BUFC_DATA && aggsum_compare(&astat_dnode_size,
                    arc_dnode_size_limit) > 0) {
                        arc_prune_async((aggsum_upper_bound(&astat_dnode_size) -
                            arc_dnode_size_limit) / sizeof (dnode_t) /
                            zfs_arc_dnode_reduce_percent);
                }

                /*
                 * Start eviction using a randomly selected sublist,
                 * this is to try and evenly balance eviction across all
                 * sublists. Always starting at the same sublist
                 * (e.g. index 0) would cause evictions to favor certain
                 * sublists over others.
                 */
                for (int i = 0; i < num_sublists; i++) {
                        uint64_t bytes_remaining;
                        uint64_t bytes_evicted;

                        if (bytes == ARC_EVICT_ALL)
                                bytes_remaining = ARC_EVICT_ALL;
                        else if (total_evicted < bytes)
                                bytes_remaining = bytes - total_evicted;
                        else
                                break;

                        bytes_evicted = arc_evict_state_impl(ml, sublist_idx,
                            markers[sublist_idx], spa, bytes_remaining);

                        scan_evicted += bytes_evicted;
                        total_evicted += bytes_evicted;

                        /* we've reached the end, wrap to the beginning */
                        if (++sublist_idx >= num_sublists)
                                sublist_idx = 0;
                }

                /*
                 * If we didn't evict anything during this scan, we have
                 * no reason to believe we'll evict more during another
                 * scan, so break the loop.
                 */
                if (scan_evicted == 0) {
                        /* This isn't possible, let's make that obvious */
                        ASSERT3S(bytes, !=, 0);

                        /*
                         * When bytes is ARC_EVICT_ALL, the only way to
                         * break the loop is when scan_evicted is zero.
                         * In that case, we actually have evicted enough,
                         * so we don't want to increment the kstat.
                         */
                        if (bytes != ARC_EVICT_ALL) {
                                ASSERT3S(total_evicted, <, bytes);
                                ARCSTAT_BUMP(arcstat_evict_not_enough);
                        }

                        break;
                }
        }

        for (int i = 0; i < num_sublists; i++) {
                multilist_sublist_t *mls = multilist_sublist_lock(ml, i);
                multilist_sublist_remove(mls, markers[i]);
                multilist_sublist_unlock(mls);

                kmem_cache_free(hdr_full_cache, markers[i]);
        }
        kmem_free(markers, sizeof (*markers) * num_sublists);

        return (total_evicted);
}

/*
 * Flush all "evictable" data of the given type from the arc state
 * specified. This will not evict any "active" buffers (i.e. referenced).
 *
 * When 'retry' is set to B_FALSE, the function will make a single pass
 * over the state and evict any buffers that it can. Since it doesn't
 * continually retry the eviction, it might end up leaving some buffers
 * in the ARC due to lock misses.
 *
 * When 'retry' is set to B_TRUE, the function will continually retry the
 * eviction until *all* evictable buffers have been removed from the
 * state. As a result, if concurrent insertions into the state are
 * allowed (e.g. if the ARC isn't shutting down), this function might
 * wind up in an infinite loop, continually trying to evict buffers.
 */
static uint64_t
arc_flush_state(arc_state_t *state, uint64_t spa, arc_buf_contents_t type,
    boolean_t retry)
{
        uint64_t evicted = 0;

        while (zfs_refcount_count(&state->arcs_esize[type]) != 0) {
                evicted += arc_evict_state(state, spa, ARC_EVICT_ALL, type);

                if (!retry)
                        break;
        }

        return (evicted);
}

/*
 * Evict the specified number of bytes from the state specified,
 * restricting eviction to the spa and type given. This function
 * prevents us from trying to evict more from a state's list than
 * is "evictable", and to skip evicting altogether when passed a
 * negative value for "bytes". In contrast, arc_evict_state() will
 * evict everything it can, when passed a negative value for "bytes".
 */
static uint64_t
arc_evict_impl(arc_state_t *state, uint64_t spa, int64_t bytes,
    arc_buf_contents_t type)
{
        int64_t delta;

        if (bytes > 0 && zfs_refcount_count(&state->arcs_esize[type]) > 0) {
                delta = MIN(zfs_refcount_count(&state->arcs_esize[type]),
                    bytes);
                return (arc_evict_state(state, spa, delta, type));
        }

        return (0);
}

/*
 * The goal of this function is to evict enough meta data buffers from the
 * ARC in order to enforce the arc_meta_limit.  Achieving this is slightly
 * more complicated than it appears because it is common for data buffers
 * to have holds on meta data buffers.  In addition, dnode meta data buffers
 * will be held by the dnodes in the block preventing them from being freed.
 * This means we can't simply traverse the ARC and expect to always find
 * enough unheld meta data buffer to release.
 *
 * Therefore, this function has been updated to make alternating passes
 * over the ARC releasing data buffers and then newly unheld meta data
 * buffers.  This ensures forward progress is maintained and meta_used
 * will decrease.  Normally this is sufficient, but if required the ARC
 * will call the registered prune callbacks causing dentry and inodes to
 * be dropped from the VFS cache.  This will make dnode meta data buffers
 * available for reclaim.
 */
static uint64_t
arc_evict_meta_balanced(uint64_t meta_used)
{
        int64_t delta, prune = 0, adjustmnt;
        uint64_t total_evicted = 0;
        arc_buf_contents_t type = ARC_BUFC_DATA;
        int restarts = MAX(zfs_arc_meta_adjust_restarts, 0);

restart:
        /*
         * This slightly differs than the way we evict from the mru in
         * arc_evict because we don't have a "target" value (i.e. no
         * "meta" arc_p). As a result, I think we can completely
         * cannibalize the metadata in the MRU before we evict the
         * metadata from the MFU. I think we probably need to implement a
         * "metadata arc_p" value to do this properly.
         */
        adjustmnt = meta_used - arc_meta_limit;

        if (adjustmnt > 0 &&
            zfs_refcount_count(&arc_mru->arcs_esize[type]) > 0) {
                delta = MIN(zfs_refcount_count(&arc_mru->arcs_esize[type]),
                    adjustmnt);
                total_evicted += arc_evict_impl(arc_mru, 0, delta, type);
                adjustmnt -= delta;
        }

        /*
         * We can't afford to recalculate adjustmnt here. If we do,
         * new metadata buffers can sneak into the MRU or ANON lists,
         * thus penalize the MFU metadata. Although the fudge factor is
         * small, it has been empirically shown to be significant for
         * certain workloads (e.g. creating many empty directories). As
         * such, we use the original calculation for adjustmnt, and
         * simply decrement the amount of data evicted from the MRU.
         */

        if (adjustmnt > 0 &&
            zfs_refcount_count(&arc_mfu->arcs_esize[type]) > 0) {
                delta = MIN(zfs_refcount_count(&arc_mfu->arcs_esize[type]),
                    adjustmnt);
                total_evicted += arc_evict_impl(arc_mfu, 0, delta, type);
        }

        adjustmnt = meta_used - arc_meta_limit;

        if (adjustmnt > 0 &&
            zfs_refcount_count(&arc_mru_ghost->arcs_esize[type]) > 0) {
                delta = MIN(adjustmnt,
                    zfs_refcount_count(&arc_mru_ghost->arcs_esize[type]));
                total_evicted += arc_evict_impl(arc_mru_ghost, 0, delta, type);
                adjustmnt -= delta;
        }

        if (adjustmnt > 0 &&
            zfs_refcount_count(&arc_mfu_ghost->arcs_esize[type]) > 0) {
                delta = MIN(adjustmnt,
                    zfs_refcount_count(&arc_mfu_ghost->arcs_esize[type]));
                total_evicted += arc_evict_impl(arc_mfu_ghost, 0, delta, type);
        }

        /*
         * If after attempting to make the requested adjustment to the ARC
         * the meta limit is still being exceeded then request that the
         * higher layers drop some cached objects which have holds on ARC
         * meta buffers.  Requests to the upper layers will be made with
         * increasingly large scan sizes until the ARC is below the limit.
         */
        if (meta_used > arc_meta_limit) {
                if (type == ARC_BUFC_DATA) {
                        type = ARC_BUFC_METADATA;
                } else {
                        type = ARC_BUFC_DATA;

                        if (zfs_arc_meta_prune) {
                                prune += zfs_arc_meta_prune;
                                arc_prune_async(prune);
                        }
                }

                if (restarts > 0) {
                        restarts--;
                        goto restart;
                }
        }
        return (total_evicted);
}

/*
 * Evict metadata buffers from the cache, such that arc_meta_used is
 * capped by the arc_meta_limit tunable.
 */
static uint64_t
arc_evict_meta_only(uint64_t meta_used)
{
        uint64_t total_evicted = 0;
        int64_t target;

        /*
         * If we're over the meta limit, we want to evict enough
         * metadata to get back under the meta limit. We don't want to
         * evict so much that we drop the MRU below arc_p, though. If
         * we're over the meta limit more than we're over arc_p, we
         * evict some from the MRU here, and some from the MFU below.
         */
        target = MIN((int64_t)(meta_used - arc_meta_limit),
            (int64_t)(zfs_refcount_count(&arc_anon->arcs_size) +
            zfs_refcount_count(&arc_mru->arcs_size) - arc_p));

        total_evicted += arc_evict_impl(arc_mru, 0, target, ARC_BUFC_METADATA);

        /*
         * Similar to the above, we want to evict enough bytes to get us
         * below the meta limit, but not so much as to drop us below the
         * space allotted to the MFU (which is defined as arc_c - arc_p).
         */
        target = MIN((int64_t)(meta_used - arc_meta_limit),
            (int64_t)(zfs_refcount_count(&arc_mfu->arcs_size) -
            (arc_c - arc_p)));

        total_evicted += arc_evict_impl(arc_mfu, 0, target, ARC_BUFC_METADATA);

        return (total_evicted);
}

static uint64_t
arc_evict_meta(uint64_t meta_used)
{
        if (zfs_arc_meta_strategy == ARC_STRATEGY_META_ONLY)
                return (arc_evict_meta_only(meta_used));
        else
                return (arc_evict_meta_balanced(meta_used));
}

/*
 * Return the type of the oldest buffer in the given arc state
 *
 * This function will select a random sublist of type ARC_BUFC_DATA and
 * a random sublist of type ARC_BUFC_METADATA. The tail of each sublist
 * is compared, and the type which contains the "older" buffer will be
 * returned.
 */
static arc_buf_contents_t
arc_evict_type(arc_state_t *state)
{
        multilist_t *data_ml = state->arcs_list[ARC_BUFC_DATA];
        multilist_t *meta_ml = state->arcs_list[ARC_BUFC_METADATA];
        int data_idx = multilist_get_random_index(data_ml);
        int meta_idx = multilist_get_random_index(meta_ml);
        multilist_sublist_t *data_mls;
        multilist_sublist_t *meta_mls;
        arc_buf_contents_t type;
        arc_buf_hdr_t *data_hdr;
        arc_buf_hdr_t *meta_hdr;

        /*
         * We keep the sublist lock until we're finished, to prevent
         * the headers from being destroyed via arc_evict_state().
         */
        data_mls = multilist_sublist_lock(data_ml, data_idx);
        meta_mls = multilist_sublist_lock(meta_ml, meta_idx);

        /*
         * These two loops are to ensure we skip any markers that
         * might be at the tail of the lists due to arc_evict_state().
         */

        for (data_hdr = multilist_sublist_tail(data_mls); data_hdr != NULL;
            data_hdr = multilist_sublist_prev(data_mls, data_hdr)) {
                if (data_hdr->b_spa != 0)
                        break;
        }

        for (meta_hdr = multilist_sublist_tail(meta_mls); meta_hdr != NULL;
            meta_hdr = multilist_sublist_prev(meta_mls, meta_hdr)) {
                if (meta_hdr->b_spa != 0)
                        break;
        }

        if (data_hdr == NULL && meta_hdr == NULL) {
                type = ARC_BUFC_DATA;
        } else if (data_hdr == NULL) {
                ASSERT3P(meta_hdr, !=, NULL);
                type = ARC_BUFC_METADATA;
        } else if (meta_hdr == NULL) {
                ASSERT3P(data_hdr, !=, NULL);
                type = ARC_BUFC_DATA;
        } else {
                ASSERT3P(data_hdr, !=, NULL);
                ASSERT3P(meta_hdr, !=, NULL);

                /* The headers can't be on the sublist without an L1 header */
                ASSERT(HDR_HAS_L1HDR(data_hdr));
                ASSERT(HDR_HAS_L1HDR(meta_hdr));

                if (data_hdr->b_l1hdr.b_arc_access <
                    meta_hdr->b_l1hdr.b_arc_access) {
                        type = ARC_BUFC_DATA;
                } else {
                        type = ARC_BUFC_METADATA;
                }
        }

        multilist_sublist_unlock(meta_mls);
        multilist_sublist_unlock(data_mls);

        return (type);
}

/*
 * Evict buffers from the cache, such that arc_size is capped by arc_c.
 */
static uint64_t
arc_evict(void)
{
        uint64_t total_evicted = 0;
        uint64_t bytes;
        int64_t target;
        uint64_t asize = aggsum_value(&arc_size);
        uint64_t ameta = aggsum_value(&arc_meta_used);

        /*
         * If we're over arc_meta_limit, we want to correct that before
         * potentially evicting data buffers below.
         */
        total_evicted += arc_evict_meta(ameta);

        /*
         * Adjust MRU size
         *
         * If we're over the target cache size, we want to evict enough
         * from the list to get back to our target size. We don't want
         * to evict too much from the MRU, such that it drops below
         * arc_p. So, if we're over our target cache size more than
         * the MRU is over arc_p, we'll evict enough to get back to
         * arc_p here, and then evict more from the MFU below.
         */
        target = MIN((int64_t)(asize - arc_c),
            (int64_t)(zfs_refcount_count(&arc_anon->arcs_size) +
            zfs_refcount_count(&arc_mru->arcs_size) + ameta - arc_p));

        /*
         * If we're below arc_meta_min, always prefer to evict data.
         * Otherwise, try to satisfy the requested number of bytes to
         * evict from the type which contains older buffers; in an
         * effort to keep newer buffers in the cache regardless of their
         * type. If we cannot satisfy the number of bytes from this
         * type, spill over into the next type.
         */
        if (arc_evict_type(arc_mru) == ARC_BUFC_METADATA &&
            ameta > arc_meta_min) {
                bytes = arc_evict_impl(arc_mru, 0, target, ARC_BUFC_METADATA);
                total_evicted += bytes;

                /*
                 * If we couldn't evict our target number of bytes from
                 * metadata, we try to get the rest from data.
                 */
                target -= bytes;

                total_evicted +=
                    arc_evict_impl(arc_mru, 0, target, ARC_BUFC_DATA);
        } else {
                bytes = arc_evict_impl(arc_mru, 0, target, ARC_BUFC_DATA);
                total_evicted += bytes;

                /*
                 * If we couldn't evict our target number of bytes from
                 * data, we try to get the rest from metadata.
                 */
                target -= bytes;

                total_evicted +=
                    arc_evict_impl(arc_mru, 0, target, ARC_BUFC_METADATA);
        }

        /*
         * Re-sum ARC stats after the first round of evictions.
         */
        asize = aggsum_value(&arc_size);
        ameta = aggsum_value(&arc_meta_used);


        /*
         * Adjust MFU size
         *
         * Now that we've tried to evict enough from the MRU to get its
         * size back to arc_p, if we're still above the target cache
         * size, we evict the rest from the MFU.
         */
        target = asize - arc_c;

        if (arc_evict_type(arc_mfu) == ARC_BUFC_METADATA &&
            ameta > arc_meta_min) {
                bytes = arc_evict_impl(arc_mfu, 0, target, ARC_BUFC_METADATA);
                total_evicted += bytes;

                /*
                 * If we couldn't evict our target number of bytes from
                 * metadata, we try to get the rest from data.
                 */
                target -= bytes;

                total_evicted +=
                    arc_evict_impl(arc_mfu, 0, target, ARC_BUFC_DATA);
        } else {
                bytes = arc_evict_impl(arc_mfu, 0, target, ARC_BUFC_DATA);
                total_evicted += bytes;

                /*
                 * If we couldn't evict our target number of bytes from
                 * data, we try to get the rest from data.
                 */
                target -= bytes;

                total_evicted +=
                    arc_evict_impl(arc_mfu, 0, target, ARC_BUFC_METADATA);
        }

        /*
         * Adjust ghost lists
         *
         * In addition to the above, the ARC also defines target values
         * for the ghost lists. The sum of the mru list and mru ghost
         * list should never exceed the target size of the cache, and
         * the sum of the mru list, mfu list, mru ghost list, and mfu
         * ghost list should never exceed twice the target size of the
         * cache. The following logic enforces these limits on the ghost
         * caches, and evicts from them as needed.
         */
        target = zfs_refcount_count(&arc_mru->arcs_size) +
            zfs_refcount_count(&arc_mru_ghost->arcs_size) - arc_c;

        bytes = arc_evict_impl(arc_mru_ghost, 0, target, ARC_BUFC_DATA);
        total_evicted += bytes;

        target -= bytes;

        total_evicted +=
            arc_evict_impl(arc_mru_ghost, 0, target, ARC_BUFC_METADATA);

        /*
         * We assume the sum of the mru list and mfu list is less than
         * or equal to arc_c (we enforced this above), which means we
         * can use the simpler of the two equations below:
         *
         *      mru + mfu + mru ghost + mfu ghost <= 2 * arc_c
         *                  mru ghost + mfu ghost <= arc_c
         */
        target = zfs_refcount_count(&arc_mru_ghost->arcs_size) +
            zfs_refcount_count(&arc_mfu_ghost->arcs_size) - arc_c;

        bytes = arc_evict_impl(arc_mfu_ghost, 0, target, ARC_BUFC_DATA);
        total_evicted += bytes;

        target -= bytes;

        total_evicted +=
            arc_evict_impl(arc_mfu_ghost, 0, target, ARC_BUFC_METADATA);

        return (total_evicted);
}

void
arc_flush(spa_t *spa, boolean_t retry)
{
        uint64_t guid = 0;

        /*
         * If retry is B_TRUE, a spa must not be specified since we have
         * no good way to determine if all of a spa's buffers have been
         * evicted from an arc state.
         */
        ASSERT(!retry || spa == 0);

        if (spa != NULL)
                guid = spa_load_guid(spa);

        (void) arc_flush_state(arc_mru, guid, ARC_BUFC_DATA, retry);
        (void) arc_flush_state(arc_mru, guid, ARC_BUFC_METADATA, retry);

        (void) arc_flush_state(arc_mfu, guid, ARC_BUFC_DATA, retry);
        (void) arc_flush_state(arc_mfu, guid, ARC_BUFC_METADATA, retry);

        (void) arc_flush_state(arc_mru_ghost, guid, ARC_BUFC_DATA, retry);
        (void) arc_flush_state(arc_mru_ghost, guid, ARC_BUFC_METADATA, retry);

        (void) arc_flush_state(arc_mfu_ghost, guid, ARC_BUFC_DATA, retry);
        (void) arc_flush_state(arc_mfu_ghost, guid, ARC_BUFC_METADATA, retry);
}

void
arc_reduce_target_size(int64_t to_free)
{
        uint64_t asize = aggsum_value(&arc_size);

        /*
         * All callers want the ARC to actually evict (at least) this much
         * memory.  Therefore we reduce from the lower of the current size and
         * the target size.  This way, even if arc_c is much higher than
         * arc_size (as can be the case after many calls to arc_freed(), we will
         * immediately have arc_c < arc_size and therefore the arc_evict_zthr
         * will evict.
         */
        uint64_t c = MIN(arc_c, asize);

        if (c > to_free && c - to_free > arc_c_min) {
                arc_c = c - to_free;
                atomic_add_64(&arc_p, -(arc_p >> arc_shrink_shift));
                if (arc_p > arc_c)
                        arc_p = (arc_c >> 1);
                ASSERT(arc_c >= arc_c_min);
                ASSERT((int64_t)arc_p >= 0);
        } else {
                arc_c = arc_c_min;
        }

        if (asize > arc_c) {
                /* See comment in arc_evict_cb_check() on why lock+flag */
                mutex_enter(&arc_evict_lock);
                arc_evict_needed = B_TRUE;
                mutex_exit(&arc_evict_lock);
                zthr_wakeup(arc_evict_zthr);
        }
}

/*
 * Determine if the system is under memory pressure and is asking
 * to reclaim memory. A return value of B_TRUE indicates that the system
 * is under memory pressure and that the arc should adjust accordingly.
 */
boolean_t
arc_reclaim_needed(void)
{
        return (arc_available_memory() < 0);
}

void
arc_kmem_reap_soon(void)
{
        size_t                  i;
        kmem_cache_t            *prev_cache = NULL;
        kmem_cache_t            *prev_data_cache = NULL;
        extern kmem_cache_t     *zio_buf_cache[];
        extern kmem_cache_t     *zio_data_buf_cache[];

#ifdef _KERNEL
        if ((aggsum_compare(&arc_meta_used, arc_meta_limit) >= 0) &&
            zfs_arc_meta_prune) {
                /*
                 * We are exceeding our meta-data cache limit.
                 * Prune some entries to release holds on meta-data.
                 */
                arc_prune_async(zfs_arc_meta_prune);
        }
#if defined(_ILP32)
        /*
         * Reclaim unused memory from all kmem caches.
         */
        kmem_reap();
#endif
#endif

        for (i = 0; i < SPA_MAXBLOCKSIZE >> SPA_MINBLOCKSHIFT; i++) {
#if defined(_ILP32)
                /* reach upper limit of cache size on 32-bit */
                if (zio_buf_cache[i] == NULL)
                        break;
#endif
                if (zio_buf_cache[i] != prev_cache) {
                        prev_cache = zio_buf_cache[i];
                        kmem_cache_reap_now(zio_buf_cache[i]);
                }
                if (zio_data_buf_cache[i] != prev_data_cache) {
                        prev_data_cache = zio_data_buf_cache[i];
                        kmem_cache_reap_now(zio_data_buf_cache[i]);
                }
        }
        kmem_cache_reap_now(buf_cache);
        kmem_cache_reap_now(hdr_full_cache);
        kmem_cache_reap_now(hdr_l2only_cache);
        kmem_cache_reap_now(zfs_btree_leaf_cache);
        abd_cache_reap_now();
}

/* ARGSUSED */
static boolean_t
arc_evict_cb_check(void *arg, zthr_t *zthr)
{
        /*
         * This is necessary so that any changes which may have been made to
         * many of the zfs_arc_* module parameters will be propagated to
         * their actual internal variable counterparts. Without this,
         * changing those module params at runtime would have no effect.
         */
        arc_tuning_update(B_FALSE);

        /*
         * This is necessary in order to keep the kstat information
         * up to date for tools that display kstat data such as the
         * mdb ::arc dcmd and the Linux crash utility.  These tools
         * typically do not call kstat's update function, but simply
         * dump out stats from the most recent update.  Without
         * this call, these commands may show stale stats for the
         * anon, mru, mru_ghost, mfu, and mfu_ghost lists. Even
         * with this change, the data might be up to 1 second
         * out of date(the arc_evict_zthr has a maximum sleep
         * time of 1 second); but that should suffice.  The
         * arc_state_t structures can be queried directly if more
         * accurate information is needed.
         */
        if (arc_ksp != NULL)
                arc_ksp->ks_update(arc_ksp, KSTAT_READ);

        /*
         * We have to rely on arc_wait_for_eviction() to tell us when to
         * evict, rather than checking if we are overflowing here, so that we
         * are sure to not leave arc_wait_for_eviction() waiting on aew_cv.
         * If we have become "not overflowing" since arc_wait_for_eviction()
         * checked, we need to wake it up.  We could broadcast the CV here,
         * but arc_wait_for_eviction() may have not yet gone to sleep.  We
         * would need to use a mutex to ensure that this function doesn't
         * broadcast until arc_wait_for_eviction() has gone to sleep (e.g.
         * the arc_evict_lock).  However, the lock ordering of such a lock
         * would necessarily be incorrect with respect to the zthr_lock,
         * which is held before this function is called, and is held by
         * arc_wait_for_eviction() when it calls zthr_wakeup().
         */
        return (arc_evict_needed);
}

/*
 * Keep arc_size under arc_c by running arc_evict which evicts data
 * from the ARC.
 */
/* ARGSUSED */
static void
arc_evict_cb(void *arg, zthr_t *zthr)
{
        uint64_t evicted = 0;
        fstrans_cookie_t cookie = spl_fstrans_mark();

        /* Evict from cache */
        evicted = arc_evict();

        /*
         * If evicted is zero, we couldn't evict anything
         * via arc_evict(). This could be due to hash lock
         * collisions, but more likely due to the majority of
         * arc buffers being unevictable. Therefore, even if
         * arc_size is above arc_c, another pass is unlikely to
         * be helpful and could potentially cause us to enter an
         * infinite loop.  Additionally, zthr_iscancelled() is
         * checked here so that if the arc is shutting down, the
         * broadcast will wake any remaining arc evict waiters.
         */
        mutex_enter(&arc_evict_lock);
        arc_evict_needed = !zthr_iscancelled(arc_evict_zthr) &&
            evicted > 0 && aggsum_compare(&arc_size, arc_c) > 0;
        if (!arc_evict_needed) {
                /*
                 * We're either no longer overflowing, or we
                 * can't evict anything more, so we should wake
                 * arc_get_data_impl() sooner.
                 */
                arc_evict_waiter_t *aw;
                while ((aw = list_remove_head(&arc_evict_waiters)) != NULL) {
                        cv_broadcast(&aw->aew_cv);
                }
                arc_set_need_free();
        }
        mutex_exit(&arc_evict_lock);
        spl_fstrans_unmark(cookie);
}

/* ARGSUSED */
static boolean_t
arc_reap_cb_check(void *arg, zthr_t *zthr)
{
        int64_t free_memory = arc_available_memory();
        static int reap_cb_check_counter = 0;

        /*
         * If a kmem reap is already active, don't schedule more.  We must
         * check for this because kmem_cache_reap_soon() won't actually
         * block on the cache being reaped (this is to prevent callers from
         * becoming implicitly blocked by a system-wide kmem reap -- which,
         * on a system with many, many full magazines, can take minutes).
         */
        if (!kmem_cache_reap_active() && free_memory < 0) {

                arc_no_grow = B_TRUE;
                arc_warm = B_TRUE;
                /*
                 * Wait at least zfs_grow_retry (default 5) seconds
                 * before considering growing.
                 */
                arc_growtime = gethrtime() + SEC2NSEC(arc_grow_retry);
                return (B_TRUE);
        } else if (free_memory < arc_c >> arc_no_grow_shift) {
                arc_no_grow = B_TRUE;
        } else if (gethrtime() >= arc_growtime) {
                arc_no_grow = B_FALSE;
        }

        /*
         * Called unconditionally every 60 seconds to reclaim unused
         * zstd compression and decompression context. This is done
         * here to avoid the need for an independent thread.
         */
        if (!((reap_cb_check_counter++) % 60))
                zfs_zstd_cache_reap_now();

        return (B_FALSE);
}

/*
 * Keep enough free memory in the system by reaping the ARC's kmem
 * caches.  To cause more slabs to be reapable, we may reduce the
 * target size of the cache (arc_c), causing the arc_evict_cb()
 * to free more buffers.
 */
/* ARGSUSED */
static void
arc_reap_cb(void *arg, zthr_t *zthr)
{
        int64_t free_memory;
        fstrans_cookie_t cookie = spl_fstrans_mark();

        /*
         * Kick off asynchronous kmem_reap()'s of all our caches.
         */
        arc_kmem_reap_soon();

        /*
         * Wait at least arc_kmem_cache_reap_retry_ms between
         * arc_kmem_reap_soon() calls. Without this check it is possible to
         * end up in a situation where we spend lots of time reaping
         * caches, while we're near arc_c_min.  Waiting here also gives the
         * subsequent free memory check a chance of finding that the
         * asynchronous reap has already freed enough memory, and we don't
         * need to call arc_reduce_target_size().
         */
        delay((hz * arc_kmem_cache_reap_retry_ms + 999) / 1000);

        /*
         * Reduce the target size as needed to maintain the amount of free
         * memory in the system at a fraction of the arc_size (1/128th by
         * default).  If oversubscribed (free_memory < 0) then reduce the
         * target arc_size by the deficit amount plus the fractional
         * amount.  If free memory is positive but less then the fractional
         * amount, reduce by what is needed to hit the fractional amount.
         */
        free_memory = arc_available_memory();

        int64_t to_free =
            (arc_c >> arc_shrink_shift) - free_memory;
        if (to_free > 0) {
                arc_reduce_target_size(to_free);
        }
        spl_fstrans_unmark(cookie);
}

#ifdef _KERNEL
/*
 * Determine the amount of memory eligible for eviction contained in the
 * ARC. All clean data reported by the ghost lists can always be safely
 * evicted. Due to arc_c_min, the same does not hold for all clean data
 * contained by the regular mru and mfu lists.
 *
 * In the case of the regular mru and mfu lists, we need to report as
 * much clean data as possible, such that evicting that same reported
 * data will not bring arc_size below arc_c_min. Thus, in certain
 * circumstances, the total amount of clean data in the mru and mfu
 * lists might not actually be evictable.
 *
 * The following two distinct cases are accounted for:
 *
 * 1. The sum of the amount of dirty data contained by both the mru and
 *    mfu lists, plus the ARC's other accounting (e.g. the anon list),
 *    is greater than or equal to arc_c_min.
 *    (i.e. amount of dirty data >= arc_c_min)
 *
 *    This is the easy case; all clean data contained by the mru and mfu
 *    lists is evictable. Evicting all clean data can only drop arc_size
 *    to the amount of dirty data, which is greater than arc_c_min.
 *
 * 2. The sum of the amount of dirty data contained by both the mru and
 *    mfu lists, plus the ARC's other accounting (e.g. the anon list),
 *    is less than arc_c_min.
 *    (i.e. arc_c_min > amount of dirty data)
 *
 *    2.1. arc_size is greater than or equal arc_c_min.
 *         (i.e. arc_size >= arc_c_min > amount of dirty data)
 *
 *         In this case, not all clean data from the regular mru and mfu
 *         lists is actually evictable; we must leave enough clean data
 *         to keep arc_size above arc_c_min. Thus, the maximum amount of
 *         evictable data from the two lists combined, is exactly the
 *         difference between arc_size and arc_c_min.
 *
 *    2.2. arc_size is less than arc_c_min
 *         (i.e. arc_c_min > arc_size > amount of dirty data)
 *
 *         In this case, none of the data contained in the mru and mfu
 *         lists is evictable, even if it's clean. Since arc_size is
 *         already below arc_c_min, evicting any more would only
 *         increase this negative difference.
 */

#endif /* _KERNEL */

/*
 * Adapt arc info given the number of bytes we are trying to add and
 * the state that we are coming from.  This function is only called
 * when we are adding new content to the cache.
 */
static void
arc_adapt(int bytes, arc_state_t *state)
{
        int mult;
        uint64_t arc_p_min = (arc_c >> arc_p_min_shift);
        int64_t mrug_size = zfs_refcount_count(&arc_mru_ghost->arcs_size);
        int64_t mfug_size = zfs_refcount_count(&arc_mfu_ghost->arcs_size);

        ASSERT(bytes > 0);
        /*
         * Adapt the target size of the MRU list:
         *      - if we just hit in the MRU ghost list, then increase
         *        the target size of the MRU list.
         *      - if we just hit in the MFU ghost list, then increase
         *        the target size of the MFU list by decreasing the
         *        target size of the MRU list.
         */
        if (state == arc_mru_ghost) {
                mult = (mrug_size >= mfug_size) ? 1 : (mfug_size / mrug_size);
                if (!zfs_arc_p_dampener_disable)
                        mult = MIN(mult, 10); /* avoid wild arc_p adjustment */

                arc_p = MIN(arc_c - arc_p_min, arc_p + bytes * mult);
        } else if (state == arc_mfu_ghost) {
                uint64_t delta;

                mult = (mfug_size >= mrug_size) ? 1 : (mrug_size / mfug_size);
                if (!zfs_arc_p_dampener_disable)
                        mult = MIN(mult, 10);

                delta = MIN(bytes * mult, arc_p);
                arc_p = MAX(arc_p_min, arc_p - delta);
        }
        ASSERT((int64_t)arc_p >= 0);

        /*
         * Wake reap thread if we do not have any available memory
         */
        if (arc_reclaim_needed()) {
                zthr_wakeup(arc_reap_zthr);
                return;
        }

        if (arc_no_grow)
                return;

        if (arc_c >= arc_c_max)
                return;

        /*
         * If we're within (2 * maxblocksize) bytes of the target
         * cache size, increment the target cache size
         */
        ASSERT3U(arc_c, >=, 2ULL << SPA_MAXBLOCKSHIFT);
        if (aggsum_upper_bound(&arc_size) >=
            arc_c - (2ULL << SPA_MAXBLOCKSHIFT)) {
                atomic_add_64(&arc_c, (int64_t)bytes);
                if (arc_c > arc_c_max)
                        arc_c = arc_c_max;
                else if (state == arc_anon)
                        atomic_add_64(&arc_p, (int64_t)bytes);
                if (arc_p > arc_c)
                        arc_p = arc_c;
        }
        ASSERT((int64_t)arc_p >= 0);
}

/*
 * Check if arc_size has grown past our upper threshold, determined by
 * zfs_arc_overflow_shift.
 */
boolean_t
arc_is_overflowing(void)
{
        /* Always allow at least one block of overflow */
        int64_t overflow = MAX(SPA_MAXBLOCKSIZE,
            arc_c >> zfs_arc_overflow_shift);

        /*
         * We just compare the lower bound here for performance reasons. Our
         * primary goals are to make sure that the arc never grows without
         * bound, and that it can reach its maximum size. This check
         * accomplishes both goals. The maximum amount we could run over by is
         * 2 * aggsum_borrow_multiplier * NUM_CPUS * the average size of a block
         * in the ARC. In practice, that's in the tens of MB, which is low
         * enough to be safe.
         */
        return (aggsum_lower_bound(&arc_size) >= (int64_t)arc_c + overflow);
}

static abd_t *
arc_get_data_abd(arc_buf_hdr_t *hdr, uint64_t size, void *tag,
    boolean_t do_adapt)
{
        arc_buf_contents_t type = arc_buf_type(hdr);

        arc_get_data_impl(hdr, size, tag, do_adapt);
        if (type == ARC_BUFC_METADATA) {
                return (abd_alloc(size, B_TRUE));
        } else {
                ASSERT(type == ARC_BUFC_DATA);
                return (abd_alloc(size, B_FALSE));
        }
}

static void *
arc_get_data_buf(arc_buf_hdr_t *hdr, uint64_t size, void *tag)
{
        arc_buf_contents_t type = arc_buf_type(hdr);

        arc_get_data_impl(hdr, size, tag, B_TRUE);
        if (type == ARC_BUFC_METADATA) {
                return (zio_buf_alloc(size));
        } else {
                ASSERT(type == ARC_BUFC_DATA);
                return (zio_data_buf_alloc(size));
        }
}

/*
 * Wait for the specified amount of data (in bytes) to be evicted from the
 * ARC, and for there to be sufficient free memory in the system.  Waiting for
 * eviction ensures that the memory used by the ARC decreases.  Waiting for
 * free memory ensures that the system won't run out of free pages, regardless
 * of ARC behavior and settings.  See arc_lowmem_init().
 */
void
arc_wait_for_eviction(uint64_t amount)
{
        mutex_enter(&arc_evict_lock);
        if (arc_is_overflowing()) {
                arc_evict_needed = B_TRUE;
                zthr_wakeup(arc_evict_zthr);

                if (amount != 0) {
                        arc_evict_waiter_t aw;
                        list_link_init(&aw.aew_node);
                        cv_init(&aw.aew_cv, NULL, CV_DEFAULT, NULL);

                        arc_evict_waiter_t *last =
                            list_tail(&arc_evict_waiters);
                        if (last != NULL) {
                                ASSERT3U(last->aew_count, >, arc_evict_count);
                                aw.aew_count = last->aew_count + amount;
                        } else {
                                aw.aew_count = arc_evict_count + amount;
                        }

                        list_insert_tail(&arc_evict_waiters, &aw);

                        arc_set_need_free();

                        DTRACE_PROBE3(arc__wait__for__eviction,
                            uint64_t, amount,
                            uint64_t, arc_evict_count,
                            uint64_t, aw.aew_count);

                        /*
                         * We will be woken up either when arc_evict_count
                         * reaches aew_count, or when the ARC is no longer
                         * overflowing and eviction completes.
                         */
                        cv_wait(&aw.aew_cv, &arc_evict_lock);

                        /*
                         * In case of "false" wakeup, we will still be on the
                         * list.
                         */
                        if (list_link_active(&aw.aew_node))
                                list_remove(&arc_evict_waiters, &aw);

                        cv_destroy(&aw.aew_cv);
                }
        }
        mutex_exit(&arc_evict_lock);
}

/*
 * Allocate a block and return it to the caller. If we are hitting the
 * hard limit for the cache size, we must sleep, waiting for the eviction
 * thread to catch up. If we're past the target size but below the hard
 * limit, we'll only signal the reclaim thread and continue on.
 */
static void
arc_get_data_impl(arc_buf_hdr_t *hdr, uint64_t size, void *tag,
    boolean_t do_adapt)
{
        arc_state_t *state = hdr->b_l1hdr.b_state;
        arc_buf_contents_t type = arc_buf_type(hdr);

        if (do_adapt)
                arc_adapt(size, state);

        /*
         * If arc_size is currently overflowing, we must be adding data
         * faster than we are evicting.  To ensure we don't compound the
         * problem by adding more data and forcing arc_size to grow even
         * further past it's target size, we wait for the eviction thread to
         * make some progress.  We also wait for there to be sufficient free
         * memory in the system, as measured by arc_free_memory().
         *
         * Specifically, we wait for zfs_arc_eviction_pct percent of the
         * requested size to be evicted.  This should be more than 100%, to
         * ensure that that progress is also made towards getting arc_size
         * under arc_c.  See the comment above zfs_arc_eviction_pct.
         *
         * We do the overflowing check without holding the arc_evict_lock to
         * reduce lock contention in this hot path.  Note that
         * arc_wait_for_eviction() will acquire the lock and check again to
         * ensure we are truly overflowing before blocking.
         */
        if (arc_is_overflowing()) {
                arc_wait_for_eviction(size *
                    zfs_arc_eviction_pct / 100);
        }

        VERIFY3U(hdr->b_type, ==, type);
        if (type == ARC_BUFC_METADATA) {
                arc_space_consume(size, ARC_SPACE_META);
        } else {
                arc_space_consume(size, ARC_SPACE_DATA);
        }

        /*
         * Update the state size.  Note that ghost states have a
         * "ghost size" and so don't need to be updated.
         */
        if (!GHOST_STATE(state)) {

                (void) zfs_refcount_add_many(&state->arcs_size, size, tag);

                /*
                 * If this is reached via arc_read, the link is
                 * protected by the hash lock. If reached via
                 * arc_buf_alloc, the header should not be accessed by
                 * any other thread. And, if reached via arc_read_done,
                 * the hash lock will protect it if it's found in the
                 * hash table; otherwise no other thread should be
                 * trying to [add|remove]_reference it.
                 */
                if (multilist_link_active(&hdr->b_l1hdr.b_arc_node)) {
                        ASSERT(zfs_refcount_is_zero(&hdr->b_l1hdr.b_refcnt));
                        (void) zfs_refcount_add_many(&state->arcs_esize[type],
                            size, tag);
                }

                /*
                 * If we are growing the cache, and we are adding anonymous
                 * data, and we have outgrown arc_p, update arc_p
                 */
                if (aggsum_upper_bound(&arc_size) < arc_c &&
                    hdr->b_l1hdr.b_state == arc_anon &&
                    (zfs_refcount_count(&arc_anon->arcs_size) +
                    zfs_refcount_count(&arc_mru->arcs_size) > arc_p))
                        arc_p = MIN(arc_c, arc_p + size);
        }
}

static void
arc_free_data_abd(arc_buf_hdr_t *hdr, abd_t *abd, uint64_t size, void *tag)
{
        arc_free_data_impl(hdr, size, tag);
        abd_free(abd);
}

static void
arc_free_data_buf(arc_buf_hdr_t *hdr, void *buf, uint64_t size, void *tag)
{
        arc_buf_contents_t type = arc_buf_type(hdr);

        arc_free_data_impl(hdr, size, tag);
        if (type == ARC_BUFC_METADATA) {
                zio_buf_free(buf, size);
        } else {
                ASSERT(type == ARC_BUFC_DATA);
                zio_data_buf_free(buf, size);
        }
}

/*
 * Free the arc data buffer.
 */
static void
arc_free_data_impl(arc_buf_hdr_t *hdr, uint64_t size, void *tag)
{
        arc_state_t *state = hdr->b_l1hdr.b_state;
        arc_buf_contents_t type = arc_buf_type(hdr);

        /* protected by hash lock, if in the hash table */
        if (multilist_link_active(&hdr->b_l1hdr.b_arc_node)) {
                ASSERT(zfs_refcount_is_zero(&hdr->b_l1hdr.b_refcnt));
                ASSERT(state != arc_anon && state != arc_l2c_only);

                (void) zfs_refcount_remove_many(&state->arcs_esize[type],
                    size, tag);
        }
        (void) zfs_refcount_remove_many(&state->arcs_size, size, tag);

        VERIFY3U(hdr->b_type, ==, type);
        if (type == ARC_BUFC_METADATA) {
                arc_space_return(size, ARC_SPACE_META);
        } else {
                ASSERT(type == ARC_BUFC_DATA);
                arc_space_return(size, ARC_SPACE_DATA);
        }
}

/*
 * This routine is called whenever a buffer is accessed.
 * NOTE: the hash lock is dropped in this function.
 */
static void
arc_access(arc_buf_hdr_t *hdr, kmutex_t *hash_lock)
{
        clock_t now;

        ASSERT(MUTEX_HELD(hash_lock));
        ASSERT(HDR_HAS_L1HDR(hdr));

        if (hdr->b_l1hdr.b_state == arc_anon) {
                /*
                 * This buffer is not in the cache, and does not
                 * appear in our "ghost" list.  Add the new buffer
                 * to the MRU state.
                 */

                ASSERT0(hdr->b_l1hdr.b_arc_access);
                hdr->b_l1hdr.b_arc_access = ddi_get_lbolt();
                DTRACE_PROBE1(new_state__mru, arc_buf_hdr_t *, hdr);
                arc_change_state(arc_mru, hdr, hash_lock);

        } else if (hdr->b_l1hdr.b_state == arc_mru) {
                now = ddi_get_lbolt();

                /*
                 * If this buffer is here because of a prefetch, then either:
                 * - clear the flag if this is a "referencing" read
                 *   (any subsequent access will bump this into the MFU state).
                 * or
                 * - move the buffer to the head of the list if this is
                 *   another prefetch (to make it less likely to be evicted).
                 */
                if (HDR_PREFETCH(hdr) || HDR_PRESCIENT_PREFETCH(hdr)) {
                        if (zfs_refcount_count(&hdr->b_l1hdr.b_refcnt) == 0) {
                                /* link protected by hash lock */
                                ASSERT(multilist_link_active(
                                    &hdr->b_l1hdr.b_arc_node));
                        } else {
                                arc_hdr_clear_flags(hdr,
                                    ARC_FLAG_PREFETCH |
                                    ARC_FLAG_PRESCIENT_PREFETCH);
                                atomic_inc_32(&hdr->b_l1hdr.b_mru_hits);
                                ARCSTAT_BUMP(arcstat_mru_hits);
                        }
                        hdr->b_l1hdr.b_arc_access = now;
                        return;
                }

                /*
                 * This buffer has been "accessed" only once so far,
                 * but it is still in the cache. Move it to the MFU
                 * state.
                 */
                if (ddi_time_after(now, hdr->b_l1hdr.b_arc_access +
                    ARC_MINTIME)) {
                        /*
                         * More than 125ms have passed since we
                         * instantiated this buffer.  Move it to the
                         * most frequently used state.
                         */
                        hdr->b_l1hdr.b_arc_access = now;
                        DTRACE_PROBE1(new_state__mfu, arc_buf_hdr_t *, hdr);
                        arc_change_state(arc_mfu, hdr, hash_lock);
                }
                atomic_inc_32(&hdr->b_l1hdr.b_mru_hits);
                ARCSTAT_BUMP(arcstat_mru_hits);
        } else if (hdr->b_l1hdr.b_state == arc_mru_ghost) {
                arc_state_t     *new_state;
                /*
                 * This buffer has been "accessed" recently, but
                 * was evicted from the cache.  Move it to the
                 * MFU state.
                 */

                if (HDR_PREFETCH(hdr) || HDR_PRESCIENT_PREFETCH(hdr)) {
                        new_state = arc_mru;
                        if (zfs_refcount_count(&hdr->b_l1hdr.b_refcnt) > 0) {
                                arc_hdr_clear_flags(hdr,
                                    ARC_FLAG_PREFETCH |
                                    ARC_FLAG_PRESCIENT_PREFETCH);
                        }
                        DTRACE_PROBE1(new_state__mru, arc_buf_hdr_t *, hdr);
                } else {
                        new_state = arc_mfu;
                        DTRACE_PROBE1(new_state__mfu, arc_buf_hdr_t *, hdr);
                }

                hdr->b_l1hdr.b_arc_access = ddi_get_lbolt();
                arc_change_state(new_state, hdr, hash_lock);

                atomic_inc_32(&hdr->b_l1hdr.b_mru_ghost_hits);
                ARCSTAT_BUMP(arcstat_mru_ghost_hits);
        } else if (hdr->b_l1hdr.b_state == arc_mfu) {
                /*
                 * This buffer has been accessed more than once and is
                 * still in the cache.  Keep it in the MFU state.
                 *
                 * NOTE: an add_reference() that occurred when we did
                 * the arc_read() will have kicked this off the list.
                 * If it was a prefetch, we will explicitly move it to
                 * the head of the list now.
                 */

                atomic_inc_32(&hdr->b_l1hdr.b_mfu_hits);
                ARCSTAT_BUMP(arcstat_mfu_hits);
                hdr->b_l1hdr.b_arc_access = ddi_get_lbolt();
        } else if (hdr->b_l1hdr.b_state == arc_mfu_ghost) {
                arc_state_t     *new_state = arc_mfu;
                /*
                 * This buffer has been accessed more than once but has
                 * been evicted from the cache.  Move it back to the
                 * MFU state.
                 */

                if (HDR_PREFETCH(hdr) || HDR_PRESCIENT_PREFETCH(hdr)) {
                        /*
                         * This is a prefetch access...
                         * move this block back to the MRU state.
                         */
                        new_state = arc_mru;
                }

                hdr->b_l1hdr.b_arc_access = ddi_get_lbolt();
                DTRACE_PROBE1(new_state__mfu, arc_buf_hdr_t *, hdr);
                arc_change_state(new_state, hdr, hash_lock);

                atomic_inc_32(&hdr->b_l1hdr.b_mfu_ghost_hits);
                ARCSTAT_BUMP(arcstat_mfu_ghost_hits);
        } else if (hdr->b_l1hdr.b_state == arc_l2c_only) {
                /*
                 * This buffer is on the 2nd Level ARC.
                 */

                hdr->b_l1hdr.b_arc_access = ddi_get_lbolt();
                DTRACE_PROBE1(new_state__mfu, arc_buf_hdr_t *, hdr);
                arc_change_state(arc_mfu, hdr, hash_lock);
        } else {
                cmn_err(CE_PANIC, "invalid arc state 0x%p",
                    hdr->b_l1hdr.b_state);
        }
}

/*
 * This routine is called by dbuf_hold() to update the arc_access() state
 * which otherwise would be skipped for entries in the dbuf cache.
 */
void
arc_buf_access(arc_buf_t *buf)
{
        mutex_enter(&buf->b_evict_lock);
        arc_buf_hdr_t *hdr = buf->b_hdr;

        /*
         * Avoid taking the hash_lock when possible as an optimization.
         * The header must be checked again under the hash_lock in order
         * to handle the case where it is concurrently being released.
         */
        if (hdr->b_l1hdr.b_state == arc_anon || HDR_EMPTY(hdr)) {
                mutex_exit(&buf->b_evict_lock);
                return;
        }

        kmutex_t *hash_lock = HDR_LOCK(hdr);
        mutex_enter(hash_lock);

        if (hdr->b_l1hdr.b_state == arc_anon || HDR_EMPTY(hdr)) {
                mutex_exit(hash_lock);
                mutex_exit(&buf->b_evict_lock);
                ARCSTAT_BUMP(arcstat_access_skip);
                return;
        }

        mutex_exit(&buf->b_evict_lock);

        ASSERT(hdr->b_l1hdr.b_state == arc_mru ||
            hdr->b_l1hdr.b_state == arc_mfu);

        DTRACE_PROBE1(arc__hit, arc_buf_hdr_t *, hdr);
        arc_access(hdr, hash_lock);
        mutex_exit(hash_lock);

        ARCSTAT_BUMP(arcstat_hits);
        ARCSTAT_CONDSTAT(!HDR_PREFETCH(hdr) && !HDR_PRESCIENT_PREFETCH(hdr),
            demand, prefetch, !HDR_ISTYPE_METADATA(hdr), data, metadata, hits);
}

/* a generic arc_read_done_func_t which you can use */
/* ARGSUSED */
void
arc_bcopy_func(zio_t *zio, const zbookmark_phys_t *zb, const blkptr_t *bp,
    arc_buf_t *buf, void *arg)
{
        if (buf == NULL)
                return;

        bcopy(buf->b_data, arg, arc_buf_size(buf));
        arc_buf_destroy(buf, arg);
}

/* a generic arc_read_done_func_t */
/* ARGSUSED */
void
arc_getbuf_func(zio_t *zio, const zbookmark_phys_t *zb, const blkptr_t *bp,
    arc_buf_t *buf, void *arg)
{
        arc_buf_t **bufp = arg;

        if (buf == NULL) {
                ASSERT(zio == NULL || zio->io_error != 0);
                *bufp = NULL;
        } else {
                ASSERT(zio == NULL || zio->io_error == 0);
                *bufp = buf;
                ASSERT(buf->b_data != NULL);
        }
}

static void
arc_hdr_verify(arc_buf_hdr_t *hdr, blkptr_t *bp)
{
        if (BP_IS_HOLE(bp) || BP_IS_EMBEDDED(bp)) {
                ASSERT3U(HDR_GET_PSIZE(hdr), ==, 0);
                ASSERT3U(arc_hdr_get_compress(hdr), ==, ZIO_COMPRESS_OFF);
        } else {
                if (HDR_COMPRESSION_ENABLED(hdr)) {
                        ASSERT3U(arc_hdr_get_compress(hdr), ==,
                            BP_GET_COMPRESS(bp));
                }
                ASSERT3U(HDR_GET_LSIZE(hdr), ==, BP_GET_LSIZE(bp));
                ASSERT3U(HDR_GET_PSIZE(hdr), ==, BP_GET_PSIZE(bp));
                ASSERT3U(!!HDR_PROTECTED(hdr), ==, BP_IS_PROTECTED(bp));
        }
}

static void
arc_read_done(zio_t *zio)
{
        blkptr_t        *bp = zio->io_bp;
        arc_buf_hdr_t   *hdr = zio->io_private;
        kmutex_t        *hash_lock = NULL;
        arc_callback_t  *callback_list;
        arc_callback_t  *acb;
        boolean_t       freeable = B_FALSE;

        /*
         * The hdr was inserted into hash-table and removed from lists
         * prior to starting I/O.  We should find this header, since
         * it's in the hash table, and it should be legit since it's
         * not possible to evict it during the I/O.  The only possible
         * reason for it not to be found is if we were freed during the
         * read.
         */
        if (HDR_IN_HASH_TABLE(hdr)) {
                arc_buf_hdr_t *found;

                ASSERT3U(hdr->b_birth, ==, BP_PHYSICAL_BIRTH(zio->io_bp));
                ASSERT3U(hdr->b_dva.dva_word[0], ==,
                    BP_IDENTITY(zio->io_bp)->dva_word[0]);
                ASSERT3U(hdr->b_dva.dva_word[1], ==,
                    BP_IDENTITY(zio->io_bp)->dva_word[1]);

                found = buf_hash_find(hdr->b_spa, zio->io_bp, &hash_lock);

                ASSERT((found == hdr &&
                    DVA_EQUAL(&hdr->b_dva, BP_IDENTITY(zio->io_bp))) ||
                    (found == hdr && HDR_L2_READING(hdr)));
                ASSERT3P(hash_lock, !=, NULL);
        }

        if (BP_IS_PROTECTED(bp)) {
                hdr->b_crypt_hdr.b_ot = BP_GET_TYPE(bp);
                hdr->b_crypt_hdr.b_dsobj = zio->io_bookmark.zb_objset;
                zio_crypt_decode_params_bp(bp, hdr->b_crypt_hdr.b_salt,
                    hdr->b_crypt_hdr.b_iv);

                if (BP_GET_TYPE(bp) == DMU_OT_INTENT_LOG) {
                        void *tmpbuf;

                        tmpbuf = abd_borrow_buf_copy(zio->io_abd,
                            sizeof (zil_chain_t));
                        zio_crypt_decode_mac_zil(tmpbuf,
                            hdr->b_crypt_hdr.b_mac);
                        abd_return_buf(zio->io_abd, tmpbuf,
                            sizeof (zil_chain_t));
                } else {
                        zio_crypt_decode_mac_bp(bp, hdr->b_crypt_hdr.b_mac);
                }
        }

        if (zio->io_error == 0) {
                /* byteswap if necessary */
                if (BP_SHOULD_BYTESWAP(zio->io_bp)) {
                        if (BP_GET_LEVEL(zio->io_bp) > 0) {
                                hdr->b_l1hdr.b_byteswap = DMU_BSWAP_UINT64;
                        } else {
                                hdr->b_l1hdr.b_byteswap =
                                    DMU_OT_BYTESWAP(BP_GET_TYPE(zio->io_bp));
                        }
                } else {
                        hdr->b_l1hdr.b_byteswap = DMU_BSWAP_NUMFUNCS;
                }
                if (!HDR_L2_READING(hdr)) {
                        hdr->b_complevel = zio->io_prop.zp_complevel;
                }
        }

        arc_hdr_clear_flags(hdr, ARC_FLAG_L2_EVICTED);
        if (l2arc_noprefetch && HDR_PREFETCH(hdr))
                arc_hdr_clear_flags(hdr, ARC_FLAG_L2CACHE);

        callback_list = hdr->b_l1hdr.b_acb;
        ASSERT3P(callback_list, !=, NULL);

        if (hash_lock && zio->io_error == 0 &&
            hdr->b_l1hdr.b_state == arc_anon) {
                /*
                 * Only call arc_access on anonymous buffers.  This is because
                 * if we've issued an I/O for an evicted buffer, we've already
                 * called arc_access (to prevent any simultaneous readers from
                 * getting confused).
                 */
                arc_access(hdr, hash_lock);
        }

        /*
         * If a read request has a callback (i.e. acb_done is not NULL), then we
         * make a buf containing the data according to the parameters which were
         * passed in. The implementation of arc_buf_alloc_impl() ensures that we
         * aren't needlessly decompressing the data multiple times.
         */
        int callback_cnt = 0;
        for (acb = callback_list; acb != NULL; acb = acb->acb_next) {
                if (!acb->acb_done)
                        continue;

                callback_cnt++;

                if (zio->io_error != 0)
                        continue;

                int error = arc_buf_alloc_impl(hdr, zio->io_spa,
                    &acb->acb_zb, acb->acb_private, acb->acb_encrypted,
                    acb->acb_compressed, acb->acb_noauth, B_TRUE,
                    &acb->acb_buf);

                /*
                 * Assert non-speculative zios didn't fail because an
                 * encryption key wasn't loaded
                 */
                ASSERT((zio->io_flags & ZIO_FLAG_SPECULATIVE) ||
                    error != EACCES);

                /*
                 * If we failed to decrypt, report an error now (as the zio
                 * layer would have done if it had done the transforms).
                 */
                if (error == ECKSUM) {
                        ASSERT(BP_IS_PROTECTED(bp));
                        error = SET_ERROR(EIO);
                        if ((zio->io_flags & ZIO_FLAG_SPECULATIVE) == 0) {
                                spa_log_error(zio->io_spa, &acb->acb_zb);
                                (void) zfs_ereport_post(
                                    FM_EREPORT_ZFS_AUTHENTICATION,
                                    zio->io_spa, NULL, &acb->acb_zb, zio, 0);
                        }
                }

                if (error != 0) {
                        /*
                         * Decompression or decryption failed.  Set
                         * io_error so that when we call acb_done
                         * (below), we will indicate that the read
                         * failed. Note that in the unusual case
                         * where one callback is compressed and another
                         * uncompressed, we will mark all of them
                         * as failed, even though the uncompressed
                         * one can't actually fail.  In this case,
                         * the hdr will not be anonymous, because
                         * if there are multiple callbacks, it's
                         * because multiple threads found the same
                         * arc buf in the hash table.
                         */
                        zio->io_error = error;
                }
        }

        /*
         * If there are multiple callbacks, we must have the hash lock,
         * because the only way for multiple threads to find this hdr is
         * in the hash table.  This ensures that if there are multiple
         * callbacks, the hdr is not anonymous.  If it were anonymous,
         * we couldn't use arc_buf_destroy() in the error case below.
         */
        ASSERT(callback_cnt < 2 || hash_lock != NULL);

        hdr->b_l1hdr.b_acb = NULL;
        arc_hdr_clear_flags(hdr, ARC_FLAG_IO_IN_PROGRESS);
        if (callback_cnt == 0)
                ASSERT(hdr->b_l1hdr.b_pabd != NULL || HDR_HAS_RABD(hdr));

        ASSERT(zfs_refcount_is_zero(&hdr->b_l1hdr.b_refcnt) ||
            callback_list != NULL);

        if (zio->io_error == 0) {
                arc_hdr_verify(hdr, zio->io_bp);
        } else {
                arc_hdr_set_flags(hdr, ARC_FLAG_IO_ERROR);
                if (hdr->b_l1hdr.b_state != arc_anon)
                        arc_change_state(arc_anon, hdr, hash_lock);
                if (HDR_IN_HASH_TABLE(hdr))
                        buf_hash_remove(hdr);
                freeable = zfs_refcount_is_zero(&hdr->b_l1hdr.b_refcnt);
        }

        /*
         * Broadcast before we drop the hash_lock to avoid the possibility
         * that the hdr (and hence the cv) might be freed before we get to
         * the cv_broadcast().
         */
        cv_broadcast(&hdr->b_l1hdr.b_cv);

        if (hash_lock != NULL) {
                mutex_exit(hash_lock);
        } else {
                /*
                 * This block was freed while we waited for the read to
                 * complete.  It has been removed from the hash table and
                 * moved to the anonymous state (so that it won't show up
                 * in the cache).
                 */
                ASSERT3P(hdr->b_l1hdr.b_state, ==, arc_anon);
                freeable = zfs_refcount_is_zero(&hdr->b_l1hdr.b_refcnt);
        }

        /* execute each callback and free its structure */
        while ((acb = callback_list) != NULL) {
                if (acb->acb_done != NULL) {
                        if (zio->io_error != 0 && acb->acb_buf != NULL) {
                                /*
                                 * If arc_buf_alloc_impl() fails during
                                 * decompression, the buf will still be
                                 * allocated, and needs to be freed here.
                                 */
                                arc_buf_destroy(acb->acb_buf,
                                    acb->acb_private);
                                acb->acb_buf = NULL;
                        }
                        acb->acb_done(zio, &zio->io_bookmark, zio->io_bp,
                            acb->acb_buf, acb->acb_private);
                }

                if (acb->acb_zio_dummy != NULL) {
                        acb->acb_zio_dummy->io_error = zio->io_error;
                        zio_nowait(acb->acb_zio_dummy);
                }

                callback_list = acb->acb_next;
                kmem_free(acb, sizeof (arc_callback_t));
        }

        if (freeable)
                arc_hdr_destroy(hdr);
}

/*
 * "Read" the block at the specified DVA (in bp) via the
 * cache.  If the block is found in the cache, invoke the provided
 * callback immediately and return.  Note that the `zio' parameter
 * in the callback will be NULL in this case, since no IO was
 * required.  If the block is not in the cache pass the read request
 * on to the spa with a substitute callback function, so that the
 * requested block will be added to the cache.
 *
 * If a read request arrives for a block that has a read in-progress,
 * either wait for the in-progress read to complete (and return the
 * results); or, if this is a read with a "done" func, add a record
 * to the read to invoke the "done" func when the read completes,
 * and return; or just return.
 *
 * arc_read_done() will invoke all the requested "done" functions
 * for readers of this block.
 */
int
arc_read(zio_t *pio, spa_t *spa, const blkptr_t *bp,
    arc_read_done_func_t *done, void *private, zio_priority_t priority,
    int zio_flags, arc_flags_t *arc_flags, const zbookmark_phys_t *zb)
{
        arc_buf_hdr_t *hdr = NULL;
        kmutex_t *hash_lock = NULL;
        zio_t *rzio;
        uint64_t guid = spa_load_guid(spa);
        boolean_t compressed_read = (zio_flags & ZIO_FLAG_RAW_COMPRESS) != 0;
        boolean_t encrypted_read = BP_IS_ENCRYPTED(bp) &&
            (zio_flags & ZIO_FLAG_RAW_ENCRYPT) != 0;
        boolean_t noauth_read = BP_IS_AUTHENTICATED(bp) &&
            (zio_flags & ZIO_FLAG_RAW_ENCRYPT) != 0;
        boolean_t embedded_bp = !!BP_IS_EMBEDDED(bp);
        int rc = 0;

        ASSERT(!embedded_bp ||
            BPE_GET_ETYPE(bp) == BP_EMBEDDED_TYPE_DATA);
        ASSERT(!BP_IS_HOLE(bp));
        ASSERT(!BP_IS_REDACTED(bp));

        /*
         * Normally SPL_FSTRANS will already be set since kernel threads which
         * expect to call the DMU interfaces will set it when created.  System
         * calls are similarly handled by setting/cleaning the bit in the
         * registered callback (module/os/.../zfs/zpl_*).
         *
         * External consumers such as Lustre which call the exported DMU
         * interfaces may not have set SPL_FSTRANS.  To avoid a deadlock
         * on the hash_lock always set and clear the bit.
         */
        fstrans_cookie_t cookie = spl_fstrans_mark();
top:
        if (!embedded_bp) {
                /*
                 * Embedded BP's have no DVA and require no I/O to "read".
                 * Create an anonymous arc buf to back it.
                 */
                hdr = buf_hash_find(guid, bp, &hash_lock);
        }

        /*
         * Determine if we have an L1 cache hit or a cache miss. For simplicity
         * we maintain encrypted data separately from compressed / uncompressed
         * data. If the user is requesting raw encrypted data and we don't have
         * that in the header we will read from disk to guarantee that we can
         * get it even if the encryption keys aren't loaded.
         */
        if (hdr != NULL && HDR_HAS_L1HDR(hdr) && (HDR_HAS_RABD(hdr) ||
            (hdr->b_l1hdr.b_pabd != NULL && !encrypted_read))) {
                arc_buf_t *buf = NULL;
                *arc_flags |= ARC_FLAG_CACHED;

                if (HDR_IO_IN_PROGRESS(hdr)) {
                        zio_t *head_zio = hdr->b_l1hdr.b_acb->acb_zio_head;

                        if (*arc_flags & ARC_FLAG_CACHED_ONLY) {
                                mutex_exit(hash_lock);
                                ARCSTAT_BUMP(arcstat_cached_only_in_progress);
                                rc = SET_ERROR(ENOENT);
                                goto out;
                        }

                        ASSERT3P(head_zio, !=, NULL);
                        if ((hdr->b_flags & ARC_FLAG_PRIO_ASYNC_READ) &&
                            priority == ZIO_PRIORITY_SYNC_READ) {
                                /*
                                 * This is a sync read that needs to wait for
                                 * an in-flight async read. Request that the
                                 * zio have its priority upgraded.
                                 */
                                zio_change_priority(head_zio, priority);
                                DTRACE_PROBE1(arc__async__upgrade__sync,
                                    arc_buf_hdr_t *, hdr);
                                ARCSTAT_BUMP(arcstat_async_upgrade_sync);
                        }
                        if (hdr->b_flags & ARC_FLAG_PREDICTIVE_PREFETCH) {
                                arc_hdr_clear_flags(hdr,
                                    ARC_FLAG_PREDICTIVE_PREFETCH);
                        }

                        if (*arc_flags & ARC_FLAG_WAIT) {
                                cv_wait(&hdr->b_l1hdr.b_cv, hash_lock);
                                mutex_exit(hash_lock);
                                goto top;
                        }
                        ASSERT(*arc_flags & ARC_FLAG_NOWAIT);

                        if (done) {
                                arc_callback_t *acb = NULL;

                                acb = kmem_zalloc(sizeof (arc_callback_t),
                                    KM_SLEEP);
                                acb->acb_done = done;
                                acb->acb_private = private;
                                acb->acb_compressed = compressed_read;
                                acb->acb_encrypted = encrypted_read;
                                acb->acb_noauth = noauth_read;
                                acb->acb_zb = *zb;
                                if (pio != NULL)
                                        acb->acb_zio_dummy = zio_null(pio,
                                            spa, NULL, NULL, NULL, zio_flags);

                                ASSERT3P(acb->acb_done, !=, NULL);
                                acb->acb_zio_head = head_zio;
                                acb->acb_next = hdr->b_l1hdr.b_acb;
                                hdr->b_l1hdr.b_acb = acb;
                                mutex_exit(hash_lock);
                                goto out;
                        }
                        mutex_exit(hash_lock);
                        goto out;
                }

                ASSERT(hdr->b_l1hdr.b_state == arc_mru ||
                    hdr->b_l1hdr.b_state == arc_mfu);

                if (done) {
                        if (hdr->b_flags & ARC_FLAG_PREDICTIVE_PREFETCH) {
                                /*
                                 * This is a demand read which does not have to
                                 * wait for i/o because we did a predictive
                                 * prefetch i/o for it, which has completed.
                                 */
                                DTRACE_PROBE1(
                                    arc__demand__hit__predictive__prefetch,
                                    arc_buf_hdr_t *, hdr);
                                ARCSTAT_BUMP(
                                    arcstat_demand_hit_predictive_prefetch);
                                arc_hdr_clear_flags(hdr,
                                    ARC_FLAG_PREDICTIVE_PREFETCH);
                        }

                        if (hdr->b_flags & ARC_FLAG_PRESCIENT_PREFETCH) {
                                ARCSTAT_BUMP(
                                    arcstat_demand_hit_prescient_prefetch);
                                arc_hdr_clear_flags(hdr,
                                    ARC_FLAG_PRESCIENT_PREFETCH);
                        }

                        ASSERT(!embedded_bp || !BP_IS_HOLE(bp));

                        /* Get a buf with the desired data in it. */
                        rc = arc_buf_alloc_impl(hdr, spa, zb, private,
                            encrypted_read, compressed_read, noauth_read,
                            B_TRUE, &buf);
                        if (rc == ECKSUM) {
                                /*
                                 * Convert authentication and decryption errors
                                 * to EIO (and generate an ereport if needed)
                                 * before leaving the ARC.
                                 */
                                rc = SET_ERROR(EIO);
                                if ((zio_flags & ZIO_FLAG_SPECULATIVE) == 0) {
                                        spa_log_error(spa, zb);
                                        (void) zfs_ereport_post(
                                            FM_EREPORT_ZFS_AUTHENTICATION,
                                            spa, NULL, zb, NULL, 0);
                                }
                        }
                        if (rc != 0) {
                                (void) remove_reference(hdr, hash_lock,
                                    private);
                                arc_buf_destroy_impl(buf);
                                buf = NULL;
                        }

                        /* assert any errors weren't due to unloaded keys */
                        ASSERT((zio_flags & ZIO_FLAG_SPECULATIVE) ||
                            rc != EACCES);
                } else if (*arc_flags & ARC_FLAG_PREFETCH &&
                    zfs_refcount_count(&hdr->b_l1hdr.b_refcnt) == 0) {
                        arc_hdr_set_flags(hdr, ARC_FLAG_PREFETCH);
                }
                DTRACE_PROBE1(arc__hit, arc_buf_hdr_t *, hdr);
                arc_access(hdr, hash_lock);
                if (*arc_flags & ARC_FLAG_PRESCIENT_PREFETCH)
                        arc_hdr_set_flags(hdr, ARC_FLAG_PRESCIENT_PREFETCH);
                if (*arc_flags & ARC_FLAG_L2CACHE)
                        arc_hdr_set_flags(hdr, ARC_FLAG_L2CACHE);
                mutex_exit(hash_lock);
                ARCSTAT_BUMP(arcstat_hits);
                ARCSTAT_CONDSTAT(!HDR_PREFETCH(hdr),
                    demand, prefetch, !HDR_ISTYPE_METADATA(hdr),
                    data, metadata, hits);

                if (done)
                        done(NULL, zb, bp, buf, private);
        } else {
                uint64_t lsize = BP_GET_LSIZE(bp);
                uint64_t psize = BP_GET_PSIZE(bp);
                arc_callback_t *acb;
                vdev_t *vd = NULL;
                uint64_t addr = 0;
                boolean_t devw = B_FALSE;
                uint64_t size;
                abd_t *hdr_abd;
                int alloc_flags = encrypted_read ? ARC_HDR_ALLOC_RDATA : 0;

                if (*arc_flags & ARC_FLAG_CACHED_ONLY) {
                        rc = SET_ERROR(ENOENT);
                        if (hash_lock != NULL)
                                mutex_exit(hash_lock);
                        goto out;
                }

                /*
                 * Gracefully handle a damaged logical block size as a
                 * checksum error.
                 */
                if (lsize > spa_maxblocksize(spa)) {
                        rc = SET_ERROR(ECKSUM);
                        if (hash_lock != NULL)
                                mutex_exit(hash_lock);
                        goto out;
                }

                if (hdr == NULL) {
                        /*
                         * This block is not in the cache or it has
                         * embedded data.
                         */
                        arc_buf_hdr_t *exists = NULL;
                        arc_buf_contents_t type = BP_GET_BUFC_TYPE(bp);
                        hdr = arc_hdr_alloc(spa_load_guid(spa), psize, lsize,
                            BP_IS_PROTECTED(bp), BP_GET_COMPRESS(bp), 0, type,
                            encrypted_read);

                        if (!embedded_bp) {
                                hdr->b_dva = *BP_IDENTITY(bp);
                                hdr->b_birth = BP_PHYSICAL_BIRTH(bp);
                                exists = buf_hash_insert(hdr, &hash_lock);
                        }
                        if (exists != NULL) {
                                /* somebody beat us to the hash insert */
                                mutex_exit(hash_lock);
                                buf_discard_identity(hdr);
                                arc_hdr_destroy(hdr);
                                goto top; /* restart the IO request */
                        }
                } else {
                        /*
                         * This block is in the ghost cache or encrypted data
                         * was requested and we didn't have it. If it was
                         * L2-only (and thus didn't have an L1 hdr),
                         * we realloc the header to add an L1 hdr.
                         */
                        if (!HDR_HAS_L1HDR(hdr)) {
                                hdr = arc_hdr_realloc(hdr, hdr_l2only_cache,
                                    hdr_full_cache);
                        }

                        if (GHOST_STATE(hdr->b_l1hdr.b_state)) {
                                ASSERT3P(hdr->b_l1hdr.b_pabd, ==, NULL);
                                ASSERT(!HDR_HAS_RABD(hdr));
                                ASSERT(!HDR_IO_IN_PROGRESS(hdr));
                                ASSERT0(zfs_refcount_count(
                                    &hdr->b_l1hdr.b_refcnt));
                                ASSERT3P(hdr->b_l1hdr.b_buf, ==, NULL);
                                ASSERT3P(hdr->b_l1hdr.b_freeze_cksum, ==, NULL);
                        } else if (HDR_IO_IN_PROGRESS(hdr)) {
                                /*
                                 * If this header already had an IO in progress
                                 * and we are performing another IO to fetch
                                 * encrypted data we must wait until the first
                                 * IO completes so as not to confuse
                                 * arc_read_done(). This should be very rare
                                 * and so the performance impact shouldn't
                                 * matter.
                                 */
                                cv_wait(&hdr->b_l1hdr.b_cv, hash_lock);
                                mutex_exit(hash_lock);
                                goto top;
                        }

                        /*
                         * This is a delicate dance that we play here.
                         * This hdr might be in the ghost list so we access
                         * it to move it out of the ghost list before we
                         * initiate the read. If it's a prefetch then
                         * it won't have a callback so we'll remove the
                         * reference that arc_buf_alloc_impl() created. We
                         * do this after we've called arc_access() to
                         * avoid hitting an assert in remove_reference().
                         */
                        arc_adapt(arc_hdr_size(hdr), hdr->b_l1hdr.b_state);
                        arc_access(hdr, hash_lock);
                        arc_hdr_alloc_abd(hdr, alloc_flags);
                }

                if (encrypted_read) {
                        ASSERT(HDR_HAS_RABD(hdr));
                        size = HDR_GET_PSIZE(hdr);
                        hdr_abd = hdr->b_crypt_hdr.b_rabd;
                        zio_flags |= ZIO_FLAG_RAW;
                } else {
                        ASSERT3P(hdr->b_l1hdr.b_pabd, !=, NULL);
                        size = arc_hdr_size(hdr);
                        hdr_abd = hdr->b_l1hdr.b_pabd;

                        if (arc_hdr_get_compress(hdr) != ZIO_COMPRESS_OFF) {
                                zio_flags |= ZIO_FLAG_RAW_COMPRESS;
                        }

                        /*
                         * For authenticated bp's, we do not ask the ZIO layer
                         * to authenticate them since this will cause the entire
                         * IO to fail if the key isn't loaded. Instead, we
                         * defer authentication until arc_buf_fill(), which will
                         * verify the data when the key is available.
                         */
                        if (BP_IS_AUTHENTICATED(bp))
                                zio_flags |= ZIO_FLAG_RAW_ENCRYPT;
                }

                if (*arc_flags & ARC_FLAG_PREFETCH &&
                    zfs_refcount_is_zero(&hdr->b_l1hdr.b_refcnt))
                        arc_hdr_set_flags(hdr, ARC_FLAG_PREFETCH);
                if (*arc_flags & ARC_FLAG_PRESCIENT_PREFETCH)
                        arc_hdr_set_flags(hdr, ARC_FLAG_PRESCIENT_PREFETCH);
                if (*arc_flags & ARC_FLAG_L2CACHE)
                        arc_hdr_set_flags(hdr, ARC_FLAG_L2CACHE);
                if (BP_IS_AUTHENTICATED(bp))
                        arc_hdr_set_flags(hdr, ARC_FLAG_NOAUTH);
                if (BP_GET_LEVEL(bp) > 0)
                        arc_hdr_set_flags(hdr, ARC_FLAG_INDIRECT);
                if (*arc_flags & ARC_FLAG_PREDICTIVE_PREFETCH)
                        arc_hdr_set_flags(hdr, ARC_FLAG_PREDICTIVE_PREFETCH);
                ASSERT(!GHOST_STATE(hdr->b_l1hdr.b_state));

                acb = kmem_zalloc(sizeof (arc_callback_t), KM_SLEEP);
                acb->acb_done = done;
                acb->acb_private = private;
                acb->acb_compressed = compressed_read;
                acb->acb_encrypted = encrypted_read;
                acb->acb_noauth = noauth_read;
                acb->acb_zb = *zb;

                ASSERT3P(hdr->b_l1hdr.b_acb, ==, NULL);
                hdr->b_l1hdr.b_acb = acb;
                arc_hdr_set_flags(hdr, ARC_FLAG_IO_IN_PROGRESS);

                if (HDR_HAS_L2HDR(hdr) &&
                    (vd = hdr->b_l2hdr.b_dev->l2ad_vdev) != NULL) {
                        devw = hdr->b_l2hdr.b_dev->l2ad_writing;
                        addr = hdr->b_l2hdr.b_daddr;
                        /*
                         * Lock out L2ARC device removal.
                         */
                        if (vdev_is_dead(vd) ||
                            !spa_config_tryenter(spa, SCL_L2ARC, vd, RW_READER))
                                vd = NULL;
                }

                /*
                 * We count both async reads and scrub IOs as asynchronous so
                 * that both can be upgraded in the event of a cache hit while
                 * the read IO is still in-flight.
                 */
                if (priority == ZIO_PRIORITY_ASYNC_READ ||
                    priority == ZIO_PRIORITY_SCRUB)
                        arc_hdr_set_flags(hdr, ARC_FLAG_PRIO_ASYNC_READ);
                else
                        arc_hdr_clear_flags(hdr, ARC_FLAG_PRIO_ASYNC_READ);

                /*
                 * At this point, we have a level 1 cache miss or a blkptr
                 * with embedded data.  Try again in L2ARC if possible.
                 */
                ASSERT3U(HDR_GET_LSIZE(hdr), ==, lsize);

                /*
                 * Skip ARC stat bump for block pointers with embedded
                 * data. The data are read from the blkptr itself via
                 * decode_embedded_bp_compressed().
                 */
                if (!embedded_bp) {
                        DTRACE_PROBE4(arc__miss, arc_buf_hdr_t *, hdr,
                            blkptr_t *, bp, uint64_t, lsize,
                            zbookmark_phys_t *, zb);
                        ARCSTAT_BUMP(arcstat_misses);
                        ARCSTAT_CONDSTAT(!HDR_PREFETCH(hdr),
                            demand, prefetch, !HDR_ISTYPE_METADATA(hdr), data,
                            metadata, misses);
                }

                if (vd != NULL && l2arc_ndev != 0 && !(l2arc_norw && devw)) {
                        /*
                         * Read from the L2ARC if the following are true:
                         * 1. The L2ARC vdev was previously cached.
                         * 2. This buffer still has L2ARC metadata.
                         * 3. This buffer isn't currently writing to the L2ARC.
                         * 4. The L2ARC entry wasn't evicted, which may
                         *    also have invalidated the vdev.
                         * 5. This isn't prefetch and l2arc_noprefetch is set.
                         */
                        if (HDR_HAS_L2HDR(hdr) &&
                            !HDR_L2_WRITING(hdr) && !HDR_L2_EVICTED(hdr) &&
                            !(l2arc_noprefetch && HDR_PREFETCH(hdr))) {
                                l2arc_read_callback_t *cb;
                                abd_t *abd;
                                uint64_t asize;

                                DTRACE_PROBE1(l2arc__hit, arc_buf_hdr_t *, hdr);
                                ARCSTAT_BUMP(arcstat_l2_hits);
                                atomic_inc_32(&hdr->b_l2hdr.b_hits);

                                cb = kmem_zalloc(sizeof (l2arc_read_callback_t),
                                    KM_SLEEP);
                                cb->l2rcb_hdr = hdr;
                                cb->l2rcb_bp = *bp;
                                cb->l2rcb_zb = *zb;
                                cb->l2rcb_flags = zio_flags;

                                /*
                                 * When Compressed ARC is disabled, but the
                                 * L2ARC block is compressed, arc_hdr_size()
                                 * will have returned LSIZE rather than PSIZE.
                                 */
                                if (HDR_GET_COMPRESS(hdr) != ZIO_COMPRESS_OFF &&
                                    !HDR_COMPRESSION_ENABLED(hdr) &&
                                    HDR_GET_PSIZE(hdr) != 0) {
                                        size = HDR_GET_PSIZE(hdr);
                                }

                                asize = vdev_psize_to_asize(vd, size);
                                if (asize != size) {
                                        abd = abd_alloc_for_io(asize,
                                            HDR_ISTYPE_METADATA(hdr));
                                        cb->l2rcb_abd = abd;
                                } else {
                                        abd = hdr_abd;
                                }

                                ASSERT(addr >= VDEV_LABEL_START_SIZE &&
                                    addr + asize <= vd->vdev_psize -
                                    VDEV_LABEL_END_SIZE);

                                /*
                                 * l2arc read.  The SCL_L2ARC lock will be
                                 * released by l2arc_read_done().
                                 * Issue a null zio if the underlying buffer
                                 * was squashed to zero size by compression.
                                 */
                                ASSERT3U(arc_hdr_get_compress(hdr), !=,
                                    ZIO_COMPRESS_EMPTY);
                                rzio = zio_read_phys(pio, vd, addr,
                                    asize, abd,
                                    ZIO_CHECKSUM_OFF,
                                    l2arc_read_done, cb, priority,
                                    zio_flags | ZIO_FLAG_DONT_CACHE |
                                    ZIO_FLAG_CANFAIL |
                                    ZIO_FLAG_DONT_PROPAGATE |
                                    ZIO_FLAG_DONT_RETRY, B_FALSE);
                                acb->acb_zio_head = rzio;

                                if (hash_lock != NULL)
                                        mutex_exit(hash_lock);

                                DTRACE_PROBE2(l2arc__read, vdev_t *, vd,
                                    zio_t *, rzio);
                                ARCSTAT_INCR(arcstat_l2_read_bytes,
                                    HDR_GET_PSIZE(hdr));

                                if (*arc_flags & ARC_FLAG_NOWAIT) {
                                        zio_nowait(rzio);
                                        goto out;
                                }

                                ASSERT(*arc_flags & ARC_FLAG_WAIT);
                                if (zio_wait(rzio) == 0)
                                        goto out;

                                /* l2arc read error; goto zio_read() */
                                if (hash_lock != NULL)
                                        mutex_enter(hash_lock);
                        } else {
                                DTRACE_PROBE1(l2arc__miss,
                                    arc_buf_hdr_t *, hdr);
                                ARCSTAT_BUMP(arcstat_l2_misses);
                                if (HDR_L2_WRITING(hdr))
                                        ARCSTAT_BUMP(arcstat_l2_rw_clash);
                                spa_config_exit(spa, SCL_L2ARC, vd);
                        }
                } else {
                        if (vd != NULL)
                                spa_config_exit(spa, SCL_L2ARC, vd);
                        /*
                         * Skip ARC stat bump for block pointers with
                         * embedded data. The data are read from the blkptr
                         * itself via decode_embedded_bp_compressed().
                         */
                        if (l2arc_ndev != 0 && !embedded_bp) {
                                DTRACE_PROBE1(l2arc__miss,
                                    arc_buf_hdr_t *, hdr);
                                ARCSTAT_BUMP(arcstat_l2_misses);
                        }
                }

                rzio = zio_read(pio, spa, bp, hdr_abd, size,
                    arc_read_done, hdr, priority, zio_flags, zb);
                acb->acb_zio_head = rzio;

                if (hash_lock != NULL)
                        mutex_exit(hash_lock);

                if (*arc_flags & ARC_FLAG_WAIT) {
                        rc = zio_wait(rzio);
                        goto out;
                }

                ASSERT(*arc_flags & ARC_FLAG_NOWAIT);
                zio_nowait(rzio);
        }

out:
        /* embedded bps don't actually go to disk */
        if (!embedded_bp)
                spa_read_history_add(spa, zb, *arc_flags);
        spl_fstrans_unmark(cookie);
        return (rc);
}

arc_prune_t *
arc_add_prune_callback(arc_prune_func_t *func, void *private)
{
        arc_prune_t *p;

        p = kmem_alloc(sizeof (*p), KM_SLEEP);
        p->p_pfunc = func;
        p->p_private = private;
        list_link_init(&p->p_node);
        zfs_refcount_create(&p->p_refcnt);

        mutex_enter(&arc_prune_mtx);
        zfs_refcount_add(&p->p_refcnt, &arc_prune_list);
        list_insert_head(&arc_prune_list, p);
        mutex_exit(&arc_prune_mtx);

        return (p);
}

void
arc_remove_prune_callback(arc_prune_t *p)
{
        boolean_t wait = B_FALSE;
        mutex_enter(&arc_prune_mtx);
        list_remove(&arc_prune_list, p);
        if (zfs_refcount_remove(&p->p_refcnt, &arc_prune_list) > 0)
                wait = B_TRUE;
        mutex_exit(&arc_prune_mtx);

        /* wait for arc_prune_task to finish */
        if (wait)
                taskq_wait_outstanding(arc_prune_taskq, 0);
        ASSERT0(zfs_refcount_count(&p->p_refcnt));
        zfs_refcount_destroy(&p->p_refcnt);
        kmem_free(p, sizeof (*p));
}

/*
 * Notify the arc that a block was freed, and thus will never be used again.
 */
void
arc_freed(spa_t *spa, const blkptr_t *bp)
{
        arc_buf_hdr_t *hdr;
        kmutex_t *hash_lock;
        uint64_t guid = spa_load_guid(spa);

        ASSERT(!BP_IS_EMBEDDED(bp));

        hdr = buf_hash_find(guid, bp, &hash_lock);
        if (hdr == NULL)
                return;

        /*
         * We might be trying to free a block that is still doing I/O
         * (i.e. prefetch) or has a reference (i.e. a dedup-ed,
         * dmu_sync-ed block). If this block is being prefetched, then it
         * would still have the ARC_FLAG_IO_IN_PROGRESS flag set on the hdr
         * until the I/O completes. A block may also have a reference if it is
         * part of a dedup-ed, dmu_synced write. The dmu_sync() function would
         * have written the new block to its final resting place on disk but
         * without the dedup flag set. This would have left the hdr in the MRU
         * state and discoverable. When the txg finally syncs it detects that
         * the block was overridden in open context and issues an override I/O.
         * Since this is a dedup block, the override I/O will determine if the
         * block is already in the DDT. If so, then it will replace the io_bp
         * with the bp from the DDT and allow the I/O to finish. When the I/O
         * reaches the done callback, dbuf_write_override_done, it will
         * check to see if the io_bp and io_bp_override are identical.
         * If they are not, then it indicates that the bp was replaced with
         * the bp in the DDT and the override bp is freed. This allows
         * us to arrive here with a reference on a block that is being
         * freed. So if we have an I/O in progress, or a reference to
         * this hdr, then we don't destroy the hdr.
         */
        if (!HDR_HAS_L1HDR(hdr) || (!HDR_IO_IN_PROGRESS(hdr) &&
            zfs_refcount_is_zero(&hdr->b_l1hdr.b_refcnt))) {
                arc_change_state(arc_anon, hdr, hash_lock);
                arc_hdr_destroy(hdr);
                mutex_exit(hash_lock);
        } else {
                mutex_exit(hash_lock);
        }

}

/*
 * Release this buffer from the cache, making it an anonymous buffer.  This
 * must be done after a read and prior to modifying the buffer contents.
 * If the buffer has more than one reference, we must make
 * a new hdr for the buffer.
 */
void
arc_release(arc_buf_t *buf, void *tag)
{
        arc_buf_hdr_t *hdr = buf->b_hdr;

        /*
         * It would be nice to assert that if its DMU metadata (level >
         * 0 || it's the dnode file), then it must be syncing context.
         * But we don't know that information at this level.
         */

        mutex_enter(&buf->b_evict_lock);

        ASSERT(HDR_HAS_L1HDR(hdr));

        /*
         * We don't grab the hash lock prior to this check, because if
         * the buffer's header is in the arc_anon state, it won't be
         * linked into the hash table.
         */
        if (hdr->b_l1hdr.b_state == arc_anon) {
                mutex_exit(&buf->b_evict_lock);
                ASSERT(!HDR_IO_IN_PROGRESS(hdr));
                ASSERT(!HDR_IN_HASH_TABLE(hdr));
                ASSERT(!HDR_HAS_L2HDR(hdr));
                ASSERT(HDR_EMPTY(hdr));

                ASSERT3U(hdr->b_l1hdr.b_bufcnt, ==, 1);
                ASSERT3S(zfs_refcount_count(&hdr->b_l1hdr.b_refcnt), ==, 1);
                ASSERT(!list_link_active(&hdr->b_l1hdr.b_arc_node));

                hdr->b_l1hdr.b_arc_access = 0;

                /*
                 * If the buf is being overridden then it may already
                 * have a hdr that is not empty.
                 */
                buf_discard_identity(hdr);
                arc_buf_thaw(buf);

                return;
        }

        kmutex_t *hash_lock = HDR_LOCK(hdr);
        mutex_enter(hash_lock);

        /*
         * This assignment is only valid as long as the hash_lock is
         * held, we must be careful not to reference state or the
         * b_state field after dropping the lock.
         */
        arc_state_t *state = hdr->b_l1hdr.b_state;
        ASSERT3P(hash_lock, ==, HDR_LOCK(hdr));
        ASSERT3P(state, !=, arc_anon);

        /* this buffer is not on any list */
        ASSERT3S(zfs_refcount_count(&hdr->b_l1hdr.b_refcnt), >, 0);

        if (HDR_HAS_L2HDR(hdr)) {
                mutex_enter(&hdr->b_l2hdr.b_dev->l2ad_mtx);

                /*
                 * We have to recheck this conditional again now that
                 * we're holding the l2ad_mtx to prevent a race with
                 * another thread which might be concurrently calling
                 * l2arc_evict(). In that case, l2arc_evict() might have
                 * destroyed the header's L2 portion as we were waiting
                 * to acquire the l2ad_mtx.
                 */
                if (HDR_HAS_L2HDR(hdr))
                        arc_hdr_l2hdr_destroy(hdr);

                mutex_exit(&hdr->b_l2hdr.b_dev->l2ad_mtx);
        }

        /*
         * Do we have more than one buf?
         */
        if (hdr->b_l1hdr.b_bufcnt > 1) {
                arc_buf_hdr_t *nhdr;
                uint64_t spa = hdr->b_spa;
                uint64_t psize = HDR_GET_PSIZE(hdr);
                uint64_t lsize = HDR_GET_LSIZE(hdr);
                boolean_t protected = HDR_PROTECTED(hdr);
                enum zio_compress compress = arc_hdr_get_compress(hdr);
                arc_buf_contents_t type = arc_buf_type(hdr);
                VERIFY3U(hdr->b_type, ==, type);

                ASSERT(hdr->b_l1hdr.b_buf != buf || buf->b_next != NULL);
                (void) remove_reference(hdr, hash_lock, tag);

                if (arc_buf_is_shared(buf) && !ARC_BUF_COMPRESSED(buf)) {
                        ASSERT3P(hdr->b_l1hdr.b_buf, !=, buf);
                        ASSERT(ARC_BUF_LAST(buf));
                }

                /*
                 * Pull the data off of this hdr and attach it to
                 * a new anonymous hdr. Also find the last buffer
                 * in the hdr's buffer list.
                 */
                arc_buf_t *lastbuf = arc_buf_remove(hdr, buf);
                ASSERT3P(lastbuf, !=, NULL);

                /*
                 * If the current arc_buf_t and the hdr are sharing their data
                 * buffer, then we must stop sharing that block.
                 */
                if (arc_buf_is_shared(buf)) {
                        ASSERT3P(hdr->b_l1hdr.b_buf, !=, buf);
                        VERIFY(!arc_buf_is_shared(lastbuf));

                        /*
                         * First, sever the block sharing relationship between
                         * buf and the arc_buf_hdr_t.
                         */
                        arc_unshare_buf(hdr, buf);

                        /*
                         * Now we need to recreate the hdr's b_pabd. Since we
                         * have lastbuf handy, we try to share with it, but if
                         * we can't then we allocate a new b_pabd and copy the
                         * data from buf into it.
                         */
                        if (arc_can_share(hdr, lastbuf)) {
                                arc_share_buf(hdr, lastbuf);
                        } else {
                                arc_hdr_alloc_abd(hdr, ARC_HDR_DO_ADAPT);
                                abd_copy_from_buf(hdr->b_l1hdr.b_pabd,
                                    buf->b_data, psize);
                        }
                        VERIFY3P(lastbuf->b_data, !=, NULL);
                } else if (HDR_SHARED_DATA(hdr)) {
                        /*
                         * Uncompressed shared buffers are always at the end
                         * of the list. Compressed buffers don't have the
                         * same requirements. This makes it hard to
                         * simply assert that the lastbuf is shared so
                         * we rely on the hdr's compression flags to determine
                         * if we have a compressed, shared buffer.
                         */
                        ASSERT(arc_buf_is_shared(lastbuf) ||
                            arc_hdr_get_compress(hdr) != ZIO_COMPRESS_OFF);
                        ASSERT(!ARC_BUF_SHARED(buf));
                }

                ASSERT(hdr->b_l1hdr.b_pabd != NULL || HDR_HAS_RABD(hdr));
                ASSERT3P(state, !=, arc_l2c_only);

                (void) zfs_refcount_remove_many(&state->arcs_size,
                    arc_buf_size(buf), buf);

                if (zfs_refcount_is_zero(&hdr->b_l1hdr.b_refcnt)) {
                        ASSERT3P(state, !=, arc_l2c_only);
                        (void) zfs_refcount_remove_many(
                            &state->arcs_esize[type],
                            arc_buf_size(buf), buf);
                }

                hdr->b_l1hdr.b_bufcnt -= 1;
                if (ARC_BUF_ENCRYPTED(buf))
                        hdr->b_crypt_hdr.b_ebufcnt -= 1;

                arc_cksum_verify(buf);
                arc_buf_unwatch(buf);

                /* if this is the last uncompressed buf free the checksum */
                if (!arc_hdr_has_uncompressed_buf(hdr))
                        arc_cksum_free(hdr);

                mutex_exit(hash_lock);

                /*
                 * Allocate a new hdr. The new hdr will contain a b_pabd
                 * buffer which will be freed in arc_write().
                 */
                nhdr = arc_hdr_alloc(spa, psize, lsize, protected,
                    compress, hdr->b_complevel, type, HDR_HAS_RABD(hdr));
                ASSERT3P(nhdr->b_l1hdr.b_buf, ==, NULL);
                ASSERT0(nhdr->b_l1hdr.b_bufcnt);
                ASSERT0(zfs_refcount_count(&nhdr->b_l1hdr.b_refcnt));
                VERIFY3U(nhdr->b_type, ==, type);
                ASSERT(!HDR_SHARED_DATA(nhdr));

                nhdr->b_l1hdr.b_buf = buf;
                nhdr->b_l1hdr.b_bufcnt = 1;
                if (ARC_BUF_ENCRYPTED(buf))
                        nhdr->b_crypt_hdr.b_ebufcnt = 1;
                nhdr->b_l1hdr.b_mru_hits = 0;
                nhdr->b_l1hdr.b_mru_ghost_hits = 0;
                nhdr->b_l1hdr.b_mfu_hits = 0;
                nhdr->b_l1hdr.b_mfu_ghost_hits = 0;
                nhdr->b_l1hdr.b_l2_hits = 0;
                (void) zfs_refcount_add(&nhdr->b_l1hdr.b_refcnt, tag);
                buf->b_hdr = nhdr;

                mutex_exit(&buf->b_evict_lock);
                (void) zfs_refcount_add_many(&arc_anon->arcs_size,
                    arc_buf_size(buf), buf);
        } else {
                mutex_exit(&buf->b_evict_lock);
                ASSERT(zfs_refcount_count(&hdr->b_l1hdr.b_refcnt) == 1);
                /* protected by hash lock, or hdr is on arc_anon */
                ASSERT(!multilist_link_active(&hdr->b_l1hdr.b_arc_node));
                ASSERT(!HDR_IO_IN_PROGRESS(hdr));
                hdr->b_l1hdr.b_mru_hits = 0;
                hdr->b_l1hdr.b_mru_ghost_hits = 0;
                hdr->b_l1hdr.b_mfu_hits = 0;
                hdr->b_l1hdr.b_mfu_ghost_hits = 0;
                hdr->b_l1hdr.b_l2_hits = 0;
                arc_change_state(arc_anon, hdr, hash_lock);
                hdr->b_l1hdr.b_arc_access = 0;

                mutex_exit(hash_lock);
                buf_discard_identity(hdr);
                arc_buf_thaw(buf);
        }
}

int
arc_released(arc_buf_t *buf)
{
        int released;

        mutex_enter(&buf->b_evict_lock);
        released = (buf->b_data != NULL &&
            buf->b_hdr->b_l1hdr.b_state == arc_anon);
        mutex_exit(&buf->b_evict_lock);
        return (released);
}

#ifdef ZFS_DEBUG
int
arc_referenced(arc_buf_t *buf)
{
        int referenced;

        mutex_enter(&buf->b_evict_lock);
        referenced = (zfs_refcount_count(&buf->b_hdr->b_l1hdr.b_refcnt));
        mutex_exit(&buf->b_evict_lock);
        return (referenced);
}
#endif

static void
arc_write_ready(zio_t *zio)
{
        arc_write_callback_t *callback = zio->io_private;
        arc_buf_t *buf = callback->awcb_buf;
        arc_buf_hdr_t *hdr = buf->b_hdr;
        blkptr_t *bp = zio->io_bp;
        uint64_t psize = BP_IS_HOLE(bp) ? 0 : BP_GET_PSIZE(bp);
        fstrans_cookie_t cookie = spl_fstrans_mark();

        ASSERT(HDR_HAS_L1HDR(hdr));
        ASSERT(!zfs_refcount_is_zero(&buf->b_hdr->b_l1hdr.b_refcnt));
        ASSERT(hdr->b_l1hdr.b_bufcnt > 0);

        /*
         * If we're reexecuting this zio because the pool suspended, then
         * cleanup any state that was previously set the first time the
         * callback was invoked.
         */
        if (zio->io_flags & ZIO_FLAG_REEXECUTED) {
                arc_cksum_free(hdr);
                arc_buf_unwatch(buf);
                if (hdr->b_l1hdr.b_pabd != NULL) {
                        if (arc_buf_is_shared(buf)) {
                                arc_unshare_buf(hdr, buf);
                        } else {
                                arc_hdr_free_abd(hdr, B_FALSE);
                        }
                }

                if (HDR_HAS_RABD(hdr))
                        arc_hdr_free_abd(hdr, B_TRUE);
        }
        ASSERT3P(hdr->b_l1hdr.b_pabd, ==, NULL);
        ASSERT(!HDR_HAS_RABD(hdr));
        ASSERT(!HDR_SHARED_DATA(hdr));
        ASSERT(!arc_buf_is_shared(buf));

        callback->awcb_ready(zio, buf, callback->awcb_private);

        if (HDR_IO_IN_PROGRESS(hdr))
                ASSERT(zio->io_flags & ZIO_FLAG_REEXECUTED);

        arc_hdr_set_flags(hdr, ARC_FLAG_IO_IN_PROGRESS);

        if (BP_IS_PROTECTED(bp) != !!HDR_PROTECTED(hdr))
                hdr = arc_hdr_realloc_crypt(hdr, BP_IS_PROTECTED(bp));

        if (BP_IS_PROTECTED(bp)) {
                /* ZIL blocks are written through zio_rewrite */
                ASSERT3U(BP_GET_TYPE(bp), !=, DMU_OT_INTENT_LOG);
                ASSERT(HDR_PROTECTED(hdr));

                if (BP_SHOULD_BYTESWAP(bp)) {
                        if (BP_GET_LEVEL(bp) > 0) {
                                hdr->b_l1hdr.b_byteswap = DMU_BSWAP_UINT64;
                        } else {
                                hdr->b_l1hdr.b_byteswap =
                                    DMU_OT_BYTESWAP(BP_GET_TYPE(bp));
                        }
                } else {
                        hdr->b_l1hdr.b_byteswap = DMU_BSWAP_NUMFUNCS;
                }

                hdr->b_crypt_hdr.b_ot = BP_GET_TYPE(bp);
                hdr->b_crypt_hdr.b_dsobj = zio->io_bookmark.zb_objset;
                zio_crypt_decode_params_bp(bp, hdr->b_crypt_hdr.b_salt,
                    hdr->b_crypt_hdr.b_iv);
                zio_crypt_decode_mac_bp(bp, hdr->b_crypt_hdr.b_mac);
        }

        /*
         * If this block was written for raw encryption but the zio layer
         * ended up only authenticating it, adjust the buffer flags now.
         */
        if (BP_IS_AUTHENTICATED(bp) && ARC_BUF_ENCRYPTED(buf)) {
                arc_hdr_set_flags(hdr, ARC_FLAG_NOAUTH);
                buf->b_flags &= ~ARC_BUF_FLAG_ENCRYPTED;
                if (BP_GET_COMPRESS(bp) == ZIO_COMPRESS_OFF)
                        buf->b_flags &= ~ARC_BUF_FLAG_COMPRESSED;
        } else if (BP_IS_HOLE(bp) && ARC_BUF_ENCRYPTED(buf)) {
                buf->b_flags &= ~ARC_BUF_FLAG_ENCRYPTED;
                buf->b_flags &= ~ARC_BUF_FLAG_COMPRESSED;
        }

        /* this must be done after the buffer flags are adjusted */
        arc_cksum_compute(buf);

        enum zio_compress compress;
        if (BP_IS_HOLE(bp) || BP_IS_EMBEDDED(bp)) {
                compress = ZIO_COMPRESS_OFF;
        } else {
                ASSERT3U(HDR_GET_LSIZE(hdr), ==, BP_GET_LSIZE(bp));
                compress = BP_GET_COMPRESS(bp);
        }
        HDR_SET_PSIZE(hdr, psize);
        arc_hdr_set_compress(hdr, compress);
        hdr->b_complevel = zio->io_prop.zp_complevel;

        if (zio->io_error != 0 || psize == 0)
                goto out;

        /*
         * Fill the hdr with data. If the buffer is encrypted we have no choice
         * but to copy the data into b_radb. If the hdr is compressed, the data
         * we want is available from the zio, otherwise we can take it from
         * the buf.
         *
         * We might be able to share the buf's data with the hdr here. However,
         * doing so would cause the ARC to be full of linear ABDs if we write a
         * lot of shareable data. As a compromise, we check whether scattered
         * ABDs are allowed, and assume that if they are then the user wants
         * the ARC to be primarily filled with them regardless of the data being
         * written. Therefore, if they're allowed then we allocate one and copy
         * the data into it; otherwise, we share the data directly if we can.
         */
        if (ARC_BUF_ENCRYPTED(buf)) {
                ASSERT3U(psize, >, 0);
                ASSERT(ARC_BUF_COMPRESSED(buf));
                arc_hdr_alloc_abd(hdr, ARC_HDR_DO_ADAPT|ARC_HDR_ALLOC_RDATA);
                abd_copy(hdr->b_crypt_hdr.b_rabd, zio->io_abd, psize);
        } else if (zfs_abd_scatter_enabled || !arc_can_share(hdr, buf)) {
                /*
                 * Ideally, we would always copy the io_abd into b_pabd, but the
                 * user may have disabled compressed ARC, thus we must check the
                 * hdr's compression setting rather than the io_bp's.
                 */
                if (BP_IS_ENCRYPTED(bp)) {
                        ASSERT3U(psize, >, 0);
                        arc_hdr_alloc_abd(hdr,
                            ARC_HDR_DO_ADAPT|ARC_HDR_ALLOC_RDATA);
                        abd_copy(hdr->b_crypt_hdr.b_rabd, zio->io_abd, psize);
                } else if (arc_hdr_get_compress(hdr) != ZIO_COMPRESS_OFF &&
                    !ARC_BUF_COMPRESSED(buf)) {
                        ASSERT3U(psize, >, 0);
                        arc_hdr_alloc_abd(hdr, ARC_HDR_DO_ADAPT);
                        abd_copy(hdr->b_l1hdr.b_pabd, zio->io_abd, psize);
                } else {
                        ASSERT3U(zio->io_orig_size, ==, arc_hdr_size(hdr));
                        arc_hdr_alloc_abd(hdr, ARC_HDR_DO_ADAPT);
                        abd_copy_from_buf(hdr->b_l1hdr.b_pabd, buf->b_data,
                            arc_buf_size(buf));
                }
        } else {
                ASSERT3P(buf->b_data, ==, abd_to_buf(zio->io_orig_abd));
                ASSERT3U(zio->io_orig_size, ==, arc_buf_size(buf));
                ASSERT3U(hdr->b_l1hdr.b_bufcnt, ==, 1);

                arc_share_buf(hdr, buf);
        }

out:
        arc_hdr_verify(hdr, bp);
        spl_fstrans_unmark(cookie);
}

static void
arc_write_children_ready(zio_t *zio)
{
        arc_write_callback_t *callback = zio->io_private;
        arc_buf_t *buf = callback->awcb_buf;

        callback->awcb_children_ready(zio, buf, callback->awcb_private);
}

/*
 * The SPA calls this callback for each physical write that happens on behalf
 * of a logical write.  See the comment in dbuf_write_physdone() for details.
 */
static void
arc_write_physdone(zio_t *zio)
{
        arc_write_callback_t *cb = zio->io_private;
        if (cb->awcb_physdone != NULL)
                cb->awcb_physdone(zio, cb->awcb_buf, cb->awcb_private);
}

static void
arc_write_done(zio_t *zio)
{
        arc_write_callback_t *callback = zio->io_private;
        arc_buf_t *buf = callback->awcb_buf;
        arc_buf_hdr_t *hdr = buf->b_hdr;

        ASSERT3P(hdr->b_l1hdr.b_acb, ==, NULL);

        if (zio->io_error == 0) {
                arc_hdr_verify(hdr, zio->io_bp);

                if (BP_IS_HOLE(zio->io_bp) || BP_IS_EMBEDDED(zio->io_bp)) {
                        buf_discard_identity(hdr);
                } else {
                        hdr->b_dva = *BP_IDENTITY(zio->io_bp);
                        hdr->b_birth = BP_PHYSICAL_BIRTH(zio->io_bp);
                }
        } else {
                ASSERT(HDR_EMPTY(hdr));
        }

        /*
         * If the block to be written was all-zero or compressed enough to be
         * embedded in the BP, no write was performed so there will be no
         * dva/birth/checksum.  The buffer must therefore remain anonymous
         * (and uncached).
         */
        if (!HDR_EMPTY(hdr)) {
                arc_buf_hdr_t *exists;
                kmutex_t *hash_lock;

                ASSERT3U(zio->io_error, ==, 0);

                arc_cksum_verify(buf);

                exists = buf_hash_insert(hdr, &hash_lock);
                if (exists != NULL) {
                        /*
                         * This can only happen if we overwrite for
                         * sync-to-convergence, because we remove
                         * buffers from the hash table when we arc_free().
                         */
                        if (zio->io_flags & ZIO_FLAG_IO_REWRITE) {
                                if (!BP_EQUAL(&zio->io_bp_orig, zio->io_bp))
                                        panic("bad overwrite, hdr=%p exists=%p",
                                            (void *)hdr, (void *)exists);
                                ASSERT(zfs_refcount_is_zero(
                                    &exists->b_l1hdr.b_refcnt));
                                arc_change_state(arc_anon, exists, hash_lock);
                                arc_hdr_destroy(exists);
                                mutex_exit(hash_lock);
                                exists = buf_hash_insert(hdr, &hash_lock);
                                ASSERT3P(exists, ==, NULL);
                        } else if (zio->io_flags & ZIO_FLAG_NOPWRITE) {
                                /* nopwrite */
                                ASSERT(zio->io_prop.zp_nopwrite);
                                if (!BP_EQUAL(&zio->io_bp_orig, zio->io_bp))
                                        panic("bad nopwrite, hdr=%p exists=%p",
                                            (void *)hdr, (void *)exists);
                        } else {
                                /* Dedup */
                                ASSERT(hdr->b_l1hdr.b_bufcnt == 1);
                                ASSERT(hdr->b_l1hdr.b_state == arc_anon);
                                ASSERT(BP_GET_DEDUP(zio->io_bp));
                                ASSERT(BP_GET_LEVEL(zio->io_bp) == 0);
                        }
                }
                arc_hdr_clear_flags(hdr, ARC_FLAG_IO_IN_PROGRESS);
                /* if it's not anon, we are doing a scrub */
                if (exists == NULL && hdr->b_l1hdr.b_state == arc_anon)
                        arc_access(hdr, hash_lock);
                mutex_exit(hash_lock);
        } else {
                arc_hdr_clear_flags(hdr, ARC_FLAG_IO_IN_PROGRESS);
        }

        ASSERT(!zfs_refcount_is_zero(&hdr->b_l1hdr.b_refcnt));
        callback->awcb_done(zio, buf, callback->awcb_private);

        abd_put(zio->io_abd);
        kmem_free(callback, sizeof (arc_write_callback_t));
}

zio_t *
arc_write(zio_t *pio, spa_t *spa, uint64_t txg,
    blkptr_t *bp, arc_buf_t *buf, boolean_t l2arc,
    const zio_prop_t *zp, arc_write_done_func_t *ready,
    arc_write_done_func_t *children_ready, arc_write_done_func_t *physdone,
    arc_write_done_func_t *done, void *private, zio_priority_t priority,
    int zio_flags, const zbookmark_phys_t *zb)
{
        arc_buf_hdr_t *hdr = buf->b_hdr;
        arc_write_callback_t *callback;
        zio_t *zio;
        zio_prop_t localprop = *zp;

        ASSERT3P(ready, !=, NULL);
        ASSERT3P(done, !=, NULL);
        ASSERT(!HDR_IO_ERROR(hdr));
        ASSERT(!HDR_IO_IN_PROGRESS(hdr));
        ASSERT3P(hdr->b_l1hdr.b_acb, ==, NULL);
        ASSERT3U(hdr->b_l1hdr.b_bufcnt, >, 0);
        if (l2arc)
                arc_hdr_set_flags(hdr, ARC_FLAG_L2CACHE);

        if (ARC_BUF_ENCRYPTED(buf)) {
                ASSERT(ARC_BUF_COMPRESSED(buf));
                localprop.zp_encrypt = B_TRUE;
                localprop.zp_compress = HDR_GET_COMPRESS(hdr);
                localprop.zp_complevel = hdr->b_complevel;
                localprop.zp_byteorder =
                    (hdr->b_l1hdr.b_byteswap == DMU_BSWAP_NUMFUNCS) ?
                    ZFS_HOST_BYTEORDER : !ZFS_HOST_BYTEORDER;
                bcopy(hdr->b_crypt_hdr.b_salt, localprop.zp_salt,
                    ZIO_DATA_SALT_LEN);
                bcopy(hdr->b_crypt_hdr.b_iv, localprop.zp_iv,
                    ZIO_DATA_IV_LEN);
                bcopy(hdr->b_crypt_hdr.b_mac, localprop.zp_mac,
                    ZIO_DATA_MAC_LEN);
                if (DMU_OT_IS_ENCRYPTED(localprop.zp_type)) {
                        localprop.zp_nopwrite = B_FALSE;
                        localprop.zp_copies =
                            MIN(localprop.zp_copies, SPA_DVAS_PER_BP - 1);
                }
                zio_flags |= ZIO_FLAG_RAW;
        } else if (ARC_BUF_COMPRESSED(buf)) {
                ASSERT3U(HDR_GET_LSIZE(hdr), !=, arc_buf_size(buf));
                localprop.zp_compress = HDR_GET_COMPRESS(hdr);
                localprop.zp_complevel = hdr->b_complevel;
                zio_flags |= ZIO_FLAG_RAW_COMPRESS;
        }
        callback = kmem_zalloc(sizeof (arc_write_callback_t), KM_SLEEP);
        callback->awcb_ready = ready;
        callback->awcb_children_ready = children_ready;
        callback->awcb_physdone = physdone;
        callback->awcb_done = done;
        callback->awcb_private = private;
        callback->awcb_buf = buf;

        /*
         * The hdr's b_pabd is now stale, free it now. A new data block
         * will be allocated when the zio pipeline calls arc_write_ready().
         */
        if (hdr->b_l1hdr.b_pabd != NULL) {
                /*
                 * If the buf is currently sharing the data block with
                 * the hdr then we need to break that relationship here.
                 * The hdr will remain with a NULL data pointer and the
                 * buf will take sole ownership of the block.
                 */
                if (arc_buf_is_shared(buf)) {
                        arc_unshare_buf(hdr, buf);
                } else {
                        arc_hdr_free_abd(hdr, B_FALSE);
                }
                VERIFY3P(buf->b_data, !=, NULL);
        }

        if (HDR_HAS_RABD(hdr))
                arc_hdr_free_abd(hdr, B_TRUE);

        if (!(zio_flags & ZIO_FLAG_RAW))
                arc_hdr_set_compress(hdr, ZIO_COMPRESS_OFF);

        ASSERT(!arc_buf_is_shared(buf));
        ASSERT3P(hdr->b_l1hdr.b_pabd, ==, NULL);

        zio = zio_write(pio, spa, txg, bp,
            abd_get_from_buf(buf->b_data, HDR_GET_LSIZE(hdr)),
            HDR_GET_LSIZE(hdr), arc_buf_size(buf), &localprop, arc_write_ready,
            (children_ready != NULL) ? arc_write_children_ready : NULL,
            arc_write_physdone, arc_write_done, callback,
            priority, zio_flags, zb);

        return (zio);
}

void
arc_tempreserve_clear(uint64_t reserve)
{
        atomic_add_64(&arc_tempreserve, -reserve);
        ASSERT((int64_t)arc_tempreserve >= 0);
}

int
arc_tempreserve_space(spa_t *spa, uint64_t reserve, uint64_t txg)
{
        int error;
        uint64_t anon_size;

        if (!arc_no_grow &&
            reserve > arc_c/4 &&
            reserve * 4 > (2ULL << SPA_MAXBLOCKSHIFT))
                arc_c = MIN(arc_c_max, reserve * 4);

        /*
         * Throttle when the calculated memory footprint for the TXG
         * exceeds the target ARC size.
         */
        if (reserve > arc_c) {
                DMU_TX_STAT_BUMP(dmu_tx_memory_reserve);
                return (SET_ERROR(ERESTART));
        }

        /*
         * Don't count loaned bufs as in flight dirty data to prevent long
         * network delays from blocking transactions that are ready to be
         * assigned to a txg.
         */

        /* assert that it has not wrapped around */
        ASSERT3S(atomic_add_64_nv(&arc_loaned_bytes, 0), >=, 0);

        anon_size = MAX((int64_t)(zfs_refcount_count(&arc_anon->arcs_size) -
            arc_loaned_bytes), 0);

        /*
         * Writes will, almost always, require additional memory allocations
         * in order to compress/encrypt/etc the data.  We therefore need to
         * make sure that there is sufficient available memory for this.
         */
        error = arc_memory_throttle(spa, reserve, txg);
        if (error != 0)
                return (error);

        /*
         * Throttle writes when the amount of dirty data in the cache
         * gets too large.  We try to keep the cache less than half full
         * of dirty blocks so that our sync times don't grow too large.
         *
         * In the case of one pool being built on another pool, we want
         * to make sure we don't end up throttling the lower (backing)
         * pool when the upper pool is the majority contributor to dirty
         * data. To insure we make forward progress during throttling, we
         * also check the current pool's net dirty data and only throttle
         * if it exceeds zfs_arc_pool_dirty_percent of the anonymous dirty
         * data in the cache.
         *
         * Note: if two requests come in concurrently, we might let them
         * both succeed, when one of them should fail.  Not a huge deal.
         */
        uint64_t total_dirty = reserve + arc_tempreserve + anon_size;
        uint64_t spa_dirty_anon = spa_dirty_data(spa);

        if (total_dirty > arc_c * zfs_arc_dirty_limit_percent / 100 &&
            anon_size > arc_c * zfs_arc_anon_limit_percent / 100 &&
            spa_dirty_anon > anon_size * zfs_arc_pool_dirty_percent / 100) {
#ifdef ZFS_DEBUG
                uint64_t meta_esize = zfs_refcount_count(
                    &arc_anon->arcs_esize[ARC_BUFC_METADATA]);
                uint64_t data_esize =
                    zfs_refcount_count(&arc_anon->arcs_esize[ARC_BUFC_DATA]);
                dprintf("failing, arc_tempreserve=%lluK anon_meta=%lluK "
                    "anon_data=%lluK tempreserve=%lluK arc_c=%lluK\n",
                    arc_tempreserve >> 10, meta_esize >> 10,
                    data_esize >> 10, reserve >> 10, arc_c >> 10);
#endif
                DMU_TX_STAT_BUMP(dmu_tx_dirty_throttle);
                return (SET_ERROR(ERESTART));
        }
        atomic_add_64(&arc_tempreserve, reserve);
        return (0);
}

static void
arc_kstat_update_state(arc_state_t *state, kstat_named_t *size,
    kstat_named_t *evict_data, kstat_named_t *evict_metadata)
{
        size->value.ui64 = zfs_refcount_count(&state->arcs_size);
        evict_data->value.ui64 =
            zfs_refcount_count(&state->arcs_esize[ARC_BUFC_DATA]);
        evict_metadata->value.ui64 =
            zfs_refcount_count(&state->arcs_esize[ARC_BUFC_METADATA]);
}

static int
arc_kstat_update(kstat_t *ksp, int rw)
{
        arc_stats_t *as = ksp->ks_data;

        if (rw == KSTAT_WRITE) {
                return (SET_ERROR(EACCES));
        } else {
                arc_kstat_update_state(arc_anon,
                    &as->arcstat_anon_size,
                    &as->arcstat_anon_evictable_data,
                    &as->arcstat_anon_evictable_metadata);
                arc_kstat_update_state(arc_mru,
                    &as->arcstat_mru_size,
                    &as->arcstat_mru_evictable_data,
                    &as->arcstat_mru_evictable_metadata);
                arc_kstat_update_state(arc_mru_ghost,
                    &as->arcstat_mru_ghost_size,
                    &as->arcstat_mru_ghost_evictable_data,
                    &as->arcstat_mru_ghost_evictable_metadata);
                arc_kstat_update_state(arc_mfu,
                    &as->arcstat_mfu_size,
                    &as->arcstat_mfu_evictable_data,
                    &as->arcstat_mfu_evictable_metadata);
                arc_kstat_update_state(arc_mfu_ghost,
                    &as->arcstat_mfu_ghost_size,
                    &as->arcstat_mfu_ghost_evictable_data,
                    &as->arcstat_mfu_ghost_evictable_metadata);

                ARCSTAT(arcstat_size) = aggsum_value(&arc_size);
                ARCSTAT(arcstat_meta_used) = aggsum_value(&arc_meta_used);
                ARCSTAT(arcstat_data_size) = aggsum_value(&astat_data_size);
                ARCSTAT(arcstat_metadata_size) =
                    aggsum_value(&astat_metadata_size);
                ARCSTAT(arcstat_hdr_size) = aggsum_value(&astat_hdr_size);
                ARCSTAT(arcstat_l2_hdr_size) = aggsum_value(&astat_l2_hdr_size);
                ARCSTAT(arcstat_dbuf_size) = aggsum_value(&astat_dbuf_size);
#if defined(COMPAT_FREEBSD11)
                ARCSTAT(arcstat_other_size) = aggsum_value(&astat_bonus_size) +
                    aggsum_value(&astat_dnode_size) +
                    aggsum_value(&astat_dbuf_size);
#endif
                ARCSTAT(arcstat_dnode_size) = aggsum_value(&astat_dnode_size);
                ARCSTAT(arcstat_bonus_size) = aggsum_value(&astat_bonus_size);
                ARCSTAT(arcstat_abd_chunk_waste_size) =
                    aggsum_value(&astat_abd_chunk_waste_size);

                as->arcstat_memory_all_bytes.value.ui64 =
                    arc_all_memory();
                as->arcstat_memory_free_bytes.value.ui64 =
                    arc_free_memory();
                as->arcstat_memory_available_bytes.value.i64 =
                    arc_available_memory();
        }

        return (0);
}

/*
 * This function *must* return indices evenly distributed between all
 * sublists of the multilist. This is needed due to how the ARC eviction
 * code is laid out; arc_evict_state() assumes ARC buffers are evenly
 * distributed between all sublists and uses this assumption when
 * deciding which sublist to evict from and how much to evict from it.
 */
static unsigned int
arc_state_multilist_index_func(multilist_t *ml, void *obj)
{
        arc_buf_hdr_t *hdr = obj;

        /*
         * We rely on b_dva to generate evenly distributed index
         * numbers using buf_hash below. So, as an added precaution,
         * let's make sure we never add empty buffers to the arc lists.
         */
        ASSERT(!HDR_EMPTY(hdr));

        /*
         * The assumption here, is the hash value for a given
         * arc_buf_hdr_t will remain constant throughout its lifetime
         * (i.e. its b_spa, b_dva, and b_birth fields don't change).
         * Thus, we don't need to store the header's sublist index
         * on insertion, as this index can be recalculated on removal.
         *
         * Also, the low order bits of the hash value are thought to be
         * distributed evenly. Otherwise, in the case that the multilist
         * has a power of two number of sublists, each sublists' usage
         * would not be evenly distributed.
         */
        return (buf_hash(hdr->b_spa, &hdr->b_dva, hdr->b_birth) %
            multilist_get_num_sublists(ml));
}

#define WARN_IF_TUNING_IGNORED(tuning, value, do_warn) do {     \
        if ((do_warn) && (tuning) && ((tuning) != (value))) {   \
                cmn_err(CE_WARN,                                \
                    "ignoring tunable %s (using %llu instead)", \
                    (#tuning), (value));                        \
        }                                                       \
} while (0)

/*
 * Called during module initialization and periodically thereafter to
 * apply reasonable changes to the exposed performance tunings.  Can also be
 * called explicitly by param_set_arc_*() functions when ARC tunables are
 * updated manually.  Non-zero zfs_* values which differ from the currently set
 * values will be applied.
 */
void
arc_tuning_update(boolean_t verbose)
{
        uint64_t allmem = arc_all_memory();
        unsigned long limit;

        /* Valid range: 32M - <arc_c_max> */
        if ((zfs_arc_min) && (zfs_arc_min != arc_c_min) &&
            (zfs_arc_min >= 2ULL << SPA_MAXBLOCKSHIFT) &&
            (zfs_arc_min <= arc_c_max)) {
                arc_c_min = zfs_arc_min;
                arc_c = MAX(arc_c, arc_c_min);
        }
        WARN_IF_TUNING_IGNORED(zfs_arc_min, arc_c_min, verbose);

        /* Valid range: 64M - <all physical memory> */
        if ((zfs_arc_max) && (zfs_arc_max != arc_c_max) &&
            (zfs_arc_max >= 64 << 20) && (zfs_arc_max < allmem) &&
            (zfs_arc_max > arc_c_min)) {
                arc_c_max = zfs_arc_max;
                arc_c = MIN(arc_c, arc_c_max);
                arc_p = (arc_c >> 1);
                if (arc_meta_limit > arc_c_max)
                        arc_meta_limit = arc_c_max;
                if (arc_dnode_size_limit > arc_meta_limit)
                        arc_dnode_size_limit = arc_meta_limit;
        }
        WARN_IF_TUNING_IGNORED(zfs_arc_max, arc_c_max, verbose);

        /* Valid range: 16M - <arc_c_max> */
        if ((zfs_arc_meta_min) && (zfs_arc_meta_min != arc_meta_min) &&
            (zfs_arc_meta_min >= 1ULL << SPA_MAXBLOCKSHIFT) &&
            (zfs_arc_meta_min <= arc_c_max)) {
                arc_meta_min = zfs_arc_meta_min;
                if (arc_meta_limit < arc_meta_min)
                        arc_meta_limit = arc_meta_min;
                if (arc_dnode_size_limit < arc_meta_min)
                        arc_dnode_size_limit = arc_meta_min;
        }
        WARN_IF_TUNING_IGNORED(zfs_arc_meta_min, arc_meta_min, verbose);

        /* Valid range: <arc_meta_min> - <arc_c_max> */
        limit = zfs_arc_meta_limit ? zfs_arc_meta_limit :
            MIN(zfs_arc_meta_limit_percent, 100) * arc_c_max / 100;
        if ((limit != arc_meta_limit) &&
            (limit >= arc_meta_min) &&
            (limit <= arc_c_max))
                arc_meta_limit = limit;
        WARN_IF_TUNING_IGNORED(zfs_arc_meta_limit, arc_meta_limit, verbose);

        /* Valid range: <arc_meta_min> - <arc_meta_limit> */
        limit = zfs_arc_dnode_limit ? zfs_arc_dnode_limit :
            MIN(zfs_arc_dnode_limit_percent, 100) * arc_meta_limit / 100;
        if ((limit != arc_dnode_size_limit) &&
            (limit >= arc_meta_min) &&
            (limit <= arc_meta_limit))
                arc_dnode_size_limit = limit;
        WARN_IF_TUNING_IGNORED(zfs_arc_dnode_limit, arc_dnode_size_limit,
            verbose);

        /* Valid range: 1 - N */
        if (zfs_arc_grow_retry)
                arc_grow_retry = zfs_arc_grow_retry;

        /* Valid range: 1 - N */
        if (zfs_arc_shrink_shift) {
                arc_shrink_shift = zfs_arc_shrink_shift;
                arc_no_grow_shift = MIN(arc_no_grow_shift, arc_shrink_shift -1);
        }

        /* Valid range: 1 - N */
        if (zfs_arc_p_min_shift)
                arc_p_min_shift = zfs_arc_p_min_shift;

        /* Valid range: 1 - N ms */
        if (zfs_arc_min_prefetch_ms)
                arc_min_prefetch_ms = zfs_arc_min_prefetch_ms;

        /* Valid range: 1 - N ms */
        if (zfs_arc_min_prescient_prefetch_ms) {
                arc_min_prescient_prefetch_ms =
                    zfs_arc_min_prescient_prefetch_ms;
        }

        /* Valid range: 0 - 100 */
        if ((zfs_arc_lotsfree_percent >= 0) &&
            (zfs_arc_lotsfree_percent <= 100))
                arc_lotsfree_percent = zfs_arc_lotsfree_percent;
        WARN_IF_TUNING_IGNORED(zfs_arc_lotsfree_percent, arc_lotsfree_percent,
            verbose);

        /* Valid range: 0 - <all physical memory> */
        if ((zfs_arc_sys_free) && (zfs_arc_sys_free != arc_sys_free))
                arc_sys_free = MIN(MAX(zfs_arc_sys_free, 0), allmem);
        WARN_IF_TUNING_IGNORED(zfs_arc_sys_free, arc_sys_free, verbose);
}

static void
arc_state_init(void)
{
        arc_anon = &ARC_anon;
        arc_mru = &ARC_mru;
        arc_mru_ghost = &ARC_mru_ghost;
        arc_mfu = &ARC_mfu;
        arc_mfu_ghost = &ARC_mfu_ghost;
        arc_l2c_only = &ARC_l2c_only;

        arc_mru->arcs_list[ARC_BUFC_METADATA] =
            multilist_create(sizeof (arc_buf_hdr_t),
            offsetof(arc_buf_hdr_t, b_l1hdr.b_arc_node),
            arc_state_multilist_index_func);
        arc_mru->arcs_list[ARC_BUFC_DATA] =
            multilist_create(sizeof (arc_buf_hdr_t),
            offsetof(arc_buf_hdr_t, b_l1hdr.b_arc_node),
            arc_state_multilist_index_func);
        arc_mru_ghost->arcs_list[ARC_BUFC_METADATA] =
            multilist_create(sizeof (arc_buf_hdr_t),
            offsetof(arc_buf_hdr_t, b_l1hdr.b_arc_node),
            arc_state_multilist_index_func);
        arc_mru_ghost->arcs_list[ARC_BUFC_DATA] =
            multilist_create(sizeof (arc_buf_hdr_t),
            offsetof(arc_buf_hdr_t, b_l1hdr.b_arc_node),
            arc_state_multilist_index_func);
        arc_mfu->arcs_list[ARC_BUFC_METADATA] =
            multilist_create(sizeof (arc_buf_hdr_t),
            offsetof(arc_buf_hdr_t, b_l1hdr.b_arc_node),
            arc_state_multilist_index_func);
        arc_mfu->arcs_list[ARC_BUFC_DATA] =
            multilist_create(sizeof (arc_buf_hdr_t),
            offsetof(arc_buf_hdr_t, b_l1hdr.b_arc_node),
            arc_state_multilist_index_func);
        arc_mfu_ghost->arcs_list[ARC_BUFC_METADATA] =
            multilist_create(sizeof (arc_buf_hdr_t),
            offsetof(arc_buf_hdr_t, b_l1hdr.b_arc_node),
            arc_state_multilist_index_func);
        arc_mfu_ghost->arcs_list[ARC_BUFC_DATA] =
            multilist_create(sizeof (arc_buf_hdr_t),
            offsetof(arc_buf_hdr_t, b_l1hdr.b_arc_node),
            arc_state_multilist_index_func);
        arc_l2c_only->arcs_list[ARC_BUFC_METADATA] =
            multilist_create(sizeof (arc_buf_hdr_t),
            offsetof(arc_buf_hdr_t, b_l1hdr.b_arc_node),
            arc_state_multilist_index_func);
        arc_l2c_only->arcs_list[ARC_BUFC_DATA] =
            multilist_create(sizeof (arc_buf_hdr_t),
            offsetof(arc_buf_hdr_t, b_l1hdr.b_arc_node),
            arc_state_multilist_index_func);

        zfs_refcount_create(&arc_anon->arcs_esize[ARC_BUFC_METADATA]);
        zfs_refcount_create(&arc_anon->arcs_esize[ARC_BUFC_DATA]);
        zfs_refcount_create(&arc_mru->arcs_esize[ARC_BUFC_METADATA]);
        zfs_refcount_create(&arc_mru->arcs_esize[ARC_BUFC_DATA]);
        zfs_refcount_create(&arc_mru_ghost->arcs_esize[ARC_BUFC_METADATA]);
        zfs_refcount_create(&arc_mru_ghost->arcs_esize[ARC_BUFC_DATA]);
        zfs_refcount_create(&arc_mfu->arcs_esize[ARC_BUFC_METADATA]);
        zfs_refcount_create(&arc_mfu->arcs_esize[ARC_BUFC_DATA]);
        zfs_refcount_create(&arc_mfu_ghost->arcs_esize[ARC_BUFC_METADATA]);
        zfs_refcount_create(&arc_mfu_ghost->arcs_esize[ARC_BUFC_DATA]);
        zfs_refcount_create(&arc_l2c_only->arcs_esize[ARC_BUFC_METADATA]);
        zfs_refcount_create(&arc_l2c_only->arcs_esize[ARC_BUFC_DATA]);

        zfs_refcount_create(&arc_anon->arcs_size);
        zfs_refcount_create(&arc_mru->arcs_size);
        zfs_refcount_create(&arc_mru_ghost->arcs_size);
        zfs_refcount_create(&arc_mfu->arcs_size);
        zfs_refcount_create(&arc_mfu_ghost->arcs_size);
        zfs_refcount_create(&arc_l2c_only->arcs_size);

        aggsum_init(&arc_meta_used, 0);
        aggsum_init(&arc_size, 0);
        aggsum_init(&astat_data_size, 0);
        aggsum_init(&astat_metadata_size, 0);
        aggsum_init(&astat_hdr_size, 0);
        aggsum_init(&astat_l2_hdr_size, 0);
        aggsum_init(&astat_bonus_size, 0);
        aggsum_init(&astat_dnode_size, 0);
        aggsum_init(&astat_dbuf_size, 0);
        aggsum_init(&astat_abd_chunk_waste_size, 0);

        arc_anon->arcs_state = ARC_STATE_ANON;
        arc_mru->arcs_state = ARC_STATE_MRU;
        arc_mru_ghost->arcs_state = ARC_STATE_MRU_GHOST;
        arc_mfu->arcs_state = ARC_STATE_MFU;
        arc_mfu_ghost->arcs_state = ARC_STATE_MFU_GHOST;
        arc_l2c_only->arcs_state = ARC_STATE_L2C_ONLY;
}

static void
arc_state_fini(void)
{
        zfs_refcount_destroy(&arc_anon->arcs_esize[ARC_BUFC_METADATA]);
        zfs_refcount_destroy(&arc_anon->arcs_esize[ARC_BUFC_DATA]);
        zfs_refcount_destroy(&arc_mru->arcs_esize[ARC_BUFC_METADATA]);
        zfs_refcount_destroy(&arc_mru->arcs_esize[ARC_BUFC_DATA]);
        zfs_refcount_destroy(&arc_mru_ghost->arcs_esize[ARC_BUFC_METADATA]);
        zfs_refcount_destroy(&arc_mru_ghost->arcs_esize[ARC_BUFC_DATA]);
        zfs_refcount_destroy(&arc_mfu->arcs_esize[ARC_BUFC_METADATA]);
        zfs_refcount_destroy(&arc_mfu->arcs_esize[ARC_BUFC_DATA]);
        zfs_refcount_destroy(&arc_mfu_ghost->arcs_esize[ARC_BUFC_METADATA]);
        zfs_refcount_destroy(&arc_mfu_ghost->arcs_esize[ARC_BUFC_DATA]);
        zfs_refcount_destroy(&arc_l2c_only->arcs_esize[ARC_BUFC_METADATA]);
        zfs_refcount_destroy(&arc_l2c_only->arcs_esize[ARC_BUFC_DATA]);

        zfs_refcount_destroy(&arc_anon->arcs_size);
        zfs_refcount_destroy(&arc_mru->arcs_size);
        zfs_refcount_destroy(&arc_mru_ghost->arcs_size);
        zfs_refcount_destroy(&arc_mfu->arcs_size);
        zfs_refcount_destroy(&arc_mfu_ghost->arcs_size);
        zfs_refcount_destroy(&arc_l2c_only->arcs_size);

        multilist_destroy(arc_mru->arcs_list[ARC_BUFC_METADATA]);
        multilist_destroy(arc_mru_ghost->arcs_list[ARC_BUFC_METADATA]);
        multilist_destroy(arc_mfu->arcs_list[ARC_BUFC_METADATA]);
        multilist_destroy(arc_mfu_ghost->arcs_list[ARC_BUFC_METADATA]);
        multilist_destroy(arc_mru->arcs_list[ARC_BUFC_DATA]);
        multilist_destroy(arc_mru_ghost->arcs_list[ARC_BUFC_DATA]);
        multilist_destroy(arc_mfu->arcs_list[ARC_BUFC_DATA]);
        multilist_destroy(arc_mfu_ghost->arcs_list[ARC_BUFC_DATA]);
        multilist_destroy(arc_l2c_only->arcs_list[ARC_BUFC_METADATA]);
        multilist_destroy(arc_l2c_only->arcs_list[ARC_BUFC_DATA]);

        aggsum_fini(&arc_meta_used);
        aggsum_fini(&arc_size);
        aggsum_fini(&astat_data_size);
        aggsum_fini(&astat_metadata_size);
        aggsum_fini(&astat_hdr_size);
        aggsum_fini(&astat_l2_hdr_size);
        aggsum_fini(&astat_bonus_size);
        aggsum_fini(&astat_dnode_size);
        aggsum_fini(&astat_dbuf_size);
        aggsum_fini(&astat_abd_chunk_waste_size);
}

uint64_t
arc_target_bytes(void)
{
        return (arc_c);
}

void
arc_init(void)
{
        uint64_t percent, allmem = arc_all_memory();
        mutex_init(&arc_evict_lock, NULL, MUTEX_DEFAULT, NULL);
        list_create(&arc_evict_waiters, sizeof (arc_evict_waiter_t),
            offsetof(arc_evict_waiter_t, aew_node));

        arc_min_prefetch_ms = 1000;
        arc_min_prescient_prefetch_ms = 6000;

#if defined(_KERNEL)
        arc_lowmem_init();
#endif

        /* Set min cache to 1/32 of all memory, or 32MB, whichever is more. */
        arc_c_min = MAX(allmem / 32, 2ULL << SPA_MAXBLOCKSHIFT);

        /* How to set default max varies by platform. */
        arc_c_max = arc_default_max(arc_c_min, allmem);

#ifndef _KERNEL
        /*
         * In userland, there's only the memory pressure that we artificially
         * create (see arc_available_memory()).  Don't let arc_c get too
         * small, because it can cause transactions to be larger than
         * arc_c, causing arc_tempreserve_space() to fail.
         */
        arc_c_min = MAX(arc_c_max / 2, 2ULL << SPA_MAXBLOCKSHIFT);
#endif

        arc_c = arc_c_min;
        arc_p = (arc_c >> 1);

        /* Set min to 1/2 of arc_c_min */
        arc_meta_min = 1ULL << SPA_MAXBLOCKSHIFT;
        /* Initialize maximum observed usage to zero */
        arc_meta_max = 0;
        /*
         * Set arc_meta_limit to a percent of arc_c_max with a floor of
         * arc_meta_min, and a ceiling of arc_c_max.
         */
        percent = MIN(zfs_arc_meta_limit_percent, 100);
        arc_meta_limit = MAX(arc_meta_min, (percent * arc_c_max) / 100);
        percent = MIN(zfs_arc_dnode_limit_percent, 100);
        arc_dnode_size_limit = (percent * arc_meta_limit) / 100;

        /* Apply user specified tunings */
        arc_tuning_update(B_TRUE);

        /* if kmem_flags are set, lets try to use less memory */
        if (kmem_debugging())
                arc_c = arc_c / 2;
        if (arc_c < arc_c_min)
                arc_c = arc_c_min;

        arc_state_init();

        buf_init();

        list_create(&arc_prune_list, sizeof (arc_prune_t),
            offsetof(arc_prune_t, p_node));
        mutex_init(&arc_prune_mtx, NULL, MUTEX_DEFAULT, NULL);

        arc_prune_taskq = taskq_create("arc_prune", boot_ncpus, defclsyspri,
            boot_ncpus, INT_MAX, TASKQ_PREPOPULATE | TASKQ_DYNAMIC);

        arc_ksp = kstat_create("zfs", 0, "arcstats", "misc", KSTAT_TYPE_NAMED,
            sizeof (arc_stats) / sizeof (kstat_named_t), KSTAT_FLAG_VIRTUAL);

        if (arc_ksp != NULL) {
                arc_ksp->ks_data = &arc_stats;
                arc_ksp->ks_update = arc_kstat_update;
                kstat_install(arc_ksp);
        }

        arc_evict_zthr = zthr_create_timer("arc_evict",
            arc_evict_cb_check, arc_evict_cb, NULL, SEC2NSEC(1));
        arc_reap_zthr = zthr_create_timer("arc_reap",
            arc_reap_cb_check, arc_reap_cb, NULL, SEC2NSEC(1));

        arc_warm = B_FALSE;

        /*
         * Calculate maximum amount of dirty data per pool.
         *
         * If it has been set by a module parameter, take that.
         * Otherwise, use a percentage of physical memory defined by
         * zfs_dirty_data_max_percent (default 10%) with a cap at
         * zfs_dirty_data_max_max (default 4G or 25% of physical memory).
         */
#ifdef __LP64__
        if (zfs_dirty_data_max_max == 0)
                zfs_dirty_data_max_max = MIN(4ULL * 1024 * 1024 * 1024,
                    allmem * zfs_dirty_data_max_max_percent / 100);
#else
        if (zfs_dirty_data_max_max == 0)
                zfs_dirty_data_max_max = MIN(1ULL * 1024 * 1024 * 1024,
                    allmem * zfs_dirty_data_max_max_percent / 100);
#endif

        if (zfs_dirty_data_max == 0) {
                zfs_dirty_data_max = allmem *
                    zfs_dirty_data_max_percent / 100;
                zfs_dirty_data_max = MIN(zfs_dirty_data_max,
                    zfs_dirty_data_max_max);
        }
}

void
arc_fini(void)
{
        arc_prune_t *p;

#ifdef _KERNEL
        arc_lowmem_fini();
#endif /* _KERNEL */

        /* Use B_TRUE to ensure *all* buffers are evicted */
        arc_flush(NULL, B_TRUE);

        if (arc_ksp != NULL) {
                kstat_delete(arc_ksp);
                arc_ksp = NULL;
        }

        taskq_wait(arc_prune_taskq);
        taskq_destroy(arc_prune_taskq);

        mutex_enter(&arc_prune_mtx);
        while ((p = list_head(&arc_prune_list)) != NULL) {
                list_remove(&arc_prune_list, p);
                zfs_refcount_remove(&p->p_refcnt, &arc_prune_list);
                zfs_refcount_destroy(&p->p_refcnt);
                kmem_free(p, sizeof (*p));
        }
        mutex_exit(&arc_prune_mtx);

        list_destroy(&arc_prune_list);
        mutex_destroy(&arc_prune_mtx);

        (void) zthr_cancel(arc_evict_zthr);
        (void) zthr_cancel(arc_reap_zthr);

        mutex_destroy(&arc_evict_lock);
        list_destroy(&arc_evict_waiters);

        /*
         * Free any buffers that were tagged for destruction.  This needs
         * to occur before arc_state_fini() runs and destroys the aggsum
         * values which are updated when freeing scatter ABDs.
         */
        l2arc_do_free_on_write();

        /*
         * buf_fini() must proceed arc_state_fini() because buf_fin() may
         * trigger the release of kmem magazines, which can callback to
         * arc_space_return() which accesses aggsums freed in act_state_fini().
         */
        buf_fini();
        arc_state_fini();

        /*
         * We destroy the zthrs after all the ARC state has been
         * torn down to avoid the case of them receiving any
         * wakeup() signals after they are destroyed.
         */
        zthr_destroy(arc_evict_zthr);
        zthr_destroy(arc_reap_zthr);

        ASSERT0(arc_loaned_bytes);
}

/*
 * Level 2 ARC
 *
 * The level 2 ARC (L2ARC) is a cache layer in-between main memory and disk.
 * It uses dedicated storage devices to hold cached data, which are populated
 * using large infrequent writes.  The main role of this cache is to boost
 * the performance of random read workloads.  The intended L2ARC devices
 * include short-stroked disks, solid state disks, and other media with
 * substantially faster read latency than disk.
 *
 *                 +-----------------------+
 *                 |         ARC           |
 *                 +-----------------------+
 *                    |         ^     ^
 *                    |         |     |
 *      l2arc_feed_thread()    arc_read()
 *                    |         |     |
 *                    |  l2arc read   |
 *                    V         |     |
 *               +---------------+    |
 *               |     L2ARC     |    |
 *               +---------------+    |
 *                   |    ^           |
 *          l2arc_write() |           |
 *                   |    |           |
 *                   V    |           |
 *                 +-------+      +-------+
 *                 | vdev  |      | vdev  |
 *                 | cache |      | cache |
 *                 +-------+      +-------+
 *                 +=========+     .-----.
 *                 :  L2ARC  :    |-_____-|
 *                 : devices :    | Disks |
 *                 +=========+    `-_____-'
 *
 * Read requests are satisfied from the following sources, in order:
 *
 *      1) ARC
 *      2) vdev cache of L2ARC devices
 *      3) L2ARC devices
 *      4) vdev cache of disks
 *      5) disks
 *
 * Some L2ARC device types exhibit extremely slow write performance.
 * To accommodate for this there are some significant differences between
 * the L2ARC and traditional cache design:
 *
 * 1. There is no eviction path from the ARC to the L2ARC.  Evictions from
 * the ARC behave as usual, freeing buffers and placing headers on ghost
 * lists.  The ARC does not send buffers to the L2ARC during eviction as
 * this would add inflated write latencies for all ARC memory pressure.
 *
 * 2. The L2ARC attempts to cache data from the ARC before it is evicted.
 * It does this by periodically scanning buffers from the eviction-end of
 * the MFU and MRU ARC lists, copying them to the L2ARC devices if they are
 * not already there. It scans until a headroom of buffers is satisfied,
 * which itself is a buffer for ARC eviction. If a compressible buffer is
 * found during scanning and selected for writing to an L2ARC device, we
 * temporarily boost scanning headroom during the next scan cycle to make
 * sure we adapt to compression effects (which might significantly reduce
 * the data volume we write to L2ARC). The thread that does this is
 * l2arc_feed_thread(), illustrated below; example sizes are included to
 * provide a better sense of ratio than this diagram:
 *
 *             head -->                        tail
 *              +---------------------+----------+
 *      ARC_mfu |:::::#:::::::::::::::|o#o###o###|-->.   # already on L2ARC
 *              +---------------------+----------+   |   o L2ARC eligible
 *      ARC_mru |:#:::::::::::::::::::|#o#ooo####|-->|   : ARC buffer
 *              +---------------------+----------+   |
 *                   15.9 Gbytes      ^ 32 Mbytes    |
 *                                 headroom          |
 *                                            l2arc_feed_thread()
 *                                                   |
 *                       l2arc write hand <--[oooo]--'
 *                               |           8 Mbyte
 *                               |          write max
 *                               V
 *                +==============================+
 *      L2ARC dev |####|#|###|###|    |####| ... |
 *                +==============================+
 *                           32 Gbytes
 *
 * 3. If an ARC buffer is copied to the L2ARC but then hit instead of
 * evicted, then the L2ARC has cached a buffer much sooner than it probably
 * needed to, potentially wasting L2ARC device bandwidth and storage.  It is
 * safe to say that this is an uncommon case, since buffers at the end of
 * the ARC lists have moved there due to inactivity.
 *
 * 4. If the ARC evicts faster than the L2ARC can maintain a headroom,
 * then the L2ARC simply misses copying some buffers.  This serves as a
 * pressure valve to prevent heavy read workloads from both stalling the ARC
 * with waits and clogging the L2ARC with writes.  This also helps prevent
 * the potential for the L2ARC to churn if it attempts to cache content too
 * quickly, such as during backups of the entire pool.
 *
 * 5. After system boot and before the ARC has filled main memory, there are
 * no evictions from the ARC and so the tails of the ARC_mfu and ARC_mru
 * lists can remain mostly static.  Instead of searching from tail of these
 * lists as pictured, the l2arc_feed_thread() will search from the list heads
 * for eligible buffers, greatly increasing its chance of finding them.
 *
 * The L2ARC device write speed is also boosted during this time so that
 * the L2ARC warms up faster.  Since there have been no ARC evictions yet,
 * there are no L2ARC reads, and no fear of degrading read performance
 * through increased writes.
 *
 * 6. Writes to the L2ARC devices are grouped and sent in-sequence, so that
 * the vdev queue can aggregate them into larger and fewer writes.  Each
 * device is written to in a rotor fashion, sweeping writes through
 * available space then repeating.
 *
 * 7. The L2ARC does not store dirty content.  It never needs to flush
 * write buffers back to disk based storage.
 *
 * 8. If an ARC buffer is written (and dirtied) which also exists in the
 * L2ARC, the now stale L2ARC buffer is immediately dropped.
 *
 * The performance of the L2ARC can be tweaked by a number of tunables, which
 * may be necessary for different workloads:
 *
 *      l2arc_write_max         max write bytes per interval
 *      l2arc_write_boost       extra write bytes during device warmup
 *      l2arc_noprefetch        skip caching prefetched buffers
 *      l2arc_headroom          number of max device writes to precache
 *      l2arc_headroom_boost    when we find compressed buffers during ARC
 *                              scanning, we multiply headroom by this
 *                              percentage factor for the next scan cycle,
 *                              since more compressed buffers are likely to
 *                              be present
 *      l2arc_feed_secs         seconds between L2ARC writing
 *
 * Tunables may be removed or added as future performance improvements are
 * integrated, and also may become zpool properties.
 *
 * There are three key functions that control how the L2ARC warms up:
 *
 *      l2arc_write_eligible()  check if a buffer is eligible to cache
 *      l2arc_write_size()      calculate how much to write
 *      l2arc_write_interval()  calculate sleep delay between writes
 *
 * These three functions determine what to write, how much, and how quickly
 * to send writes.
 *
 * L2ARC persistence:
 *
 * When writing buffers to L2ARC, we periodically add some metadata to
 * make sure we can pick them up after reboot, thus dramatically reducing
 * the impact that any downtime has on the performance of storage systems
 * with large caches.
 *
 * The implementation works fairly simply by integrating the following two
 * modifications:
 *
 * *) When writing to the L2ARC, we occasionally write a "l2arc log block",
 *    which is an additional piece of metadata which describes what's been
 *    written. This allows us to rebuild the arc_buf_hdr_t structures of the
 *    main ARC buffers. There are 2 linked-lists of log blocks headed by
 *    dh_start_lbps[2]. We alternate which chain we append to, so they are
 *    time-wise and offset-wise interleaved, but that is an optimization rather
 *    than for correctness. The log block also includes a pointer to the
 *    previous block in its chain.
 *
 * *) We reserve SPA_MINBLOCKSIZE of space at the start of each L2ARC device
 *    for our header bookkeeping purposes. This contains a device header,
 *    which contains our top-level reference structures. We update it each
 *    time we write a new log block, so that we're able to locate it in the
 *    L2ARC device. If this write results in an inconsistent device header
 *    (e.g. due to power failure), we detect this by verifying the header's
 *    checksum and simply fail to reconstruct the L2ARC after reboot.
 *
 * Implementation diagram:
 *
 * +=== L2ARC device (not to scale) ======================================+
 * |       ___two newest log block pointers__.__________                  |
 * |      /                                   \dh_start_lbps[1]           |
 * |     /                                     \         \dh_start_lbps[0]|
 * |.___/__.                                    V         V               |
 * ||L2 dev|....|lb |bufs |lb |bufs |lb |bufs |lb |bufs |lb |---(empty)---|
 * ||   hdr|      ^         /^       /^        /         /                |
 * |+------+  ...--\-------/  \-----/--\------/         /                 |
 * |                \--------------/    \--------------/                  |
 * +======================================================================+
 *
 * As can be seen on the diagram, rather than using a simple linked list,
 * we use a pair of linked lists with alternating elements. This is a
 * performance enhancement due to the fact that we only find out the
 * address of the next log block access once the current block has been
 * completely read in. Obviously, this hurts performance, because we'd be
 * keeping the device's I/O queue at only a 1 operation deep, thus
 * incurring a large amount of I/O round-trip latency. Having two lists
 * allows us to fetch two log blocks ahead of where we are currently
 * rebuilding L2ARC buffers.
 *
 * On-device data structures:
 *
 * L2ARC device header: l2arc_dev_hdr_phys_t
 * L2ARC log block:     l2arc_log_blk_phys_t
 *
 * L2ARC reconstruction:
 *
 * When writing data, we simply write in the standard rotary fashion,
 * evicting buffers as we go and simply writing new data over them (writing
 * a new log block every now and then). This obviously means that once we
 * loop around the end of the device, we will start cutting into an already
 * committed log block (and its referenced data buffers), like so:
 *
 *    current write head__       __old tail
 *                        \     /
 *                        V    V
 * <--|bufs |lb |bufs |lb |    |bufs |lb |bufs |lb |-->
 *                         ^    ^^^^^^^^^___________________________________
 *                         |                                                \
 *                   <<nextwrite>> may overwrite this blk and/or its bufs --'
 *
 * When importing the pool, we detect this situation and use it to stop
 * our scanning process (see l2arc_rebuild).
 *
 * There is one significant caveat to consider when rebuilding ARC contents
 * from an L2ARC device: what about invalidated buffers? Given the above
 * construction, we cannot update blocks which we've already written to amend
 * them to remove buffers which were invalidated. Thus, during reconstruction,
 * we might be populating the cache with buffers for data that's not on the
 * main pool anymore, or may have been overwritten!
 *
 * As it turns out, this isn't a problem. Every arc_read request includes
 * both the DVA and, crucially, the birth TXG of the BP the caller is
 * looking for. So even if the cache were populated by completely rotten
 * blocks for data that had been long deleted and/or overwritten, we'll
 * never actually return bad data from the cache, since the DVA with the
 * birth TXG uniquely identify a block in space and time - once created,
 * a block is immutable on disk. The worst thing we have done is wasted
 * some time and memory at l2arc rebuild to reconstruct outdated ARC
 * entries that will get dropped from the l2arc as it is being updated
 * with new blocks.
 *
 * L2ARC buffers that have been evicted by l2arc_evict() ahead of the write
 * hand are not restored. This is done by saving the offset (in bytes)
 * l2arc_evict() has evicted to in the L2ARC device header and taking it
 * into account when restoring buffers.
 */

static boolean_t
l2arc_write_eligible(uint64_t spa_guid, arc_buf_hdr_t *hdr)
{
        /*
         * A buffer is *not* eligible for the L2ARC if it:
         * 1. belongs to a different spa.
         * 2. is already cached on the L2ARC.
         * 3. has an I/O in progress (it may be an incomplete read).
         * 4. is flagged not eligible (zfs property).
         */
        if (hdr->b_spa != spa_guid || HDR_HAS_L2HDR(hdr) ||
            HDR_IO_IN_PROGRESS(hdr) || !HDR_L2CACHE(hdr))
                return (B_FALSE);

        return (B_TRUE);
}

static uint64_t
l2arc_write_size(l2arc_dev_t *dev)
{
        uint64_t size, dev_size, tsize;

        /*
         * Make sure our globals have meaningful values in case the user
         * altered them.
         */
        size = l2arc_write_max;
        if (size == 0) {
                cmn_err(CE_NOTE, "Bad value for l2arc_write_max, value must "
                    "be greater than zero, resetting it to the default (%d)",
                    L2ARC_WRITE_SIZE);
                size = l2arc_write_max = L2ARC_WRITE_SIZE;
        }

        if (arc_warm == B_FALSE)
                size += l2arc_write_boost;

        /*
         * Make sure the write size does not exceed the size of the cache
         * device. This is important in l2arc_evict(), otherwise infinite
         * iteration can occur.
         */
        dev_size = dev->l2ad_end - dev->l2ad_start;
        tsize = size + l2arc_log_blk_overhead(size, dev);
        if (dev->l2ad_vdev->vdev_has_trim && l2arc_trim_ahead > 0)
                tsize += MAX(64 * 1024 * 1024,
                    (tsize * l2arc_trim_ahead) / 100);

        if (tsize >= dev_size) {
                cmn_err(CE_NOTE, "l2arc_write_max or l2arc_write_boost "
                    "plus the overhead of log blocks (persistent L2ARC, "
                    "%llu bytes) exceeds the size of the cache device "
                    "(guid %llu), resetting them to the default (%d)",
                    l2arc_log_blk_overhead(size, dev),
                    dev->l2ad_vdev->vdev_guid, L2ARC_WRITE_SIZE);
                size = l2arc_write_max = l2arc_write_boost = L2ARC_WRITE_SIZE;

                if (arc_warm == B_FALSE)
                        size += l2arc_write_boost;
        }

        return (size);

}

static clock_t
l2arc_write_interval(clock_t began, uint64_t wanted, uint64_t wrote)
{
        clock_t interval, next, now;

        /*
         * If the ARC lists are busy, increase our write rate; if the
         * lists are stale, idle back.  This is achieved by checking
         * how much we previously wrote - if it was more than half of
         * what we wanted, schedule the next write much sooner.
         */
        if (l2arc_feed_again && wrote > (wanted / 2))
                interval = (hz * l2arc_feed_min_ms) / 1000;
        else
                interval = hz * l2arc_feed_secs;

        now = ddi_get_lbolt();
        next = MAX(now, MIN(now + interval, began + interval));

        return (next);
}

/*
 * Cycle through L2ARC devices.  This is how L2ARC load balances.
 * If a device is returned, this also returns holding the spa config lock.
 */
static l2arc_dev_t *
l2arc_dev_get_next(void)
{
        l2arc_dev_t *first, *next = NULL;

        /*
         * Lock out the removal of spas (spa_namespace_lock), then removal
         * of cache devices (l2arc_dev_mtx).  Once a device has been selected,
         * both locks will be dropped and a spa config lock held instead.
         */
        mutex_enter(&spa_namespace_lock);
        mutex_enter(&l2arc_dev_mtx);

        /* if there are no vdevs, there is nothing to do */
        if (l2arc_ndev == 0)
                goto out;

        first = NULL;
        next = l2arc_dev_last;
        do {
                /* loop around the list looking for a non-faulted vdev */
                if (next == NULL) {
                        next = list_head(l2arc_dev_list);
                } else {
                        next = list_next(l2arc_dev_list, next);
                        if (next == NULL)
                                next = list_head(l2arc_dev_list);
                }

                /* if we have come back to the start, bail out */
                if (first == NULL)
                        first = next;
                else if (next == first)
                        break;

        } while (vdev_is_dead(next->l2ad_vdev) || next->l2ad_rebuild ||
            next->l2ad_trim_all);

        /* if we were unable to find any usable vdevs, return NULL */
        if (vdev_is_dead(next->l2ad_vdev) || next->l2ad_rebuild ||
            next->l2ad_trim_all)
                next = NULL;

        l2arc_dev_last = next;

out:
        mutex_exit(&l2arc_dev_mtx);

        /*
         * Grab the config lock to prevent the 'next' device from being
         * removed while we are writing to it.
         */
        if (next != NULL)
                spa_config_enter(next->l2ad_spa, SCL_L2ARC, next, RW_READER);
        mutex_exit(&spa_namespace_lock);

        return (next);
}

/*
 * Free buffers that were tagged for destruction.
 */
static void
l2arc_do_free_on_write(void)
{
        list_t *buflist;
        l2arc_data_free_t *df, *df_prev;

        mutex_enter(&l2arc_free_on_write_mtx);
        buflist = l2arc_free_on_write;

        for (df = list_tail(buflist); df; df = df_prev) {
                df_prev = list_prev(buflist, df);
                ASSERT3P(df->l2df_abd, !=, NULL);
                abd_free(df->l2df_abd);
                list_remove(buflist, df);
                kmem_free(df, sizeof (l2arc_data_free_t));
        }

        mutex_exit(&l2arc_free_on_write_mtx);
}

/*
 * A write to a cache device has completed.  Update all headers to allow
 * reads from these buffers to begin.
 */
static void
l2arc_write_done(zio_t *zio)
{
        l2arc_write_callback_t  *cb;
        l2arc_lb_abd_buf_t      *abd_buf;
        l2arc_lb_ptr_buf_t      *lb_ptr_buf;
        l2arc_dev_t             *dev;
        l2arc_dev_hdr_phys_t    *l2dhdr;
        list_t                  *buflist;
        arc_buf_hdr_t           *head, *hdr, *hdr_prev;
        kmutex_t                *hash_lock;
        int64_t                 bytes_dropped = 0;

        cb = zio->io_private;
        ASSERT3P(cb, !=, NULL);
        dev = cb->l2wcb_dev;
        l2dhdr = dev->l2ad_dev_hdr;
        ASSERT3P(dev, !=, NULL);
        head = cb->l2wcb_head;
        ASSERT3P(head, !=, NULL);
        buflist = &dev->l2ad_buflist;
        ASSERT3P(buflist, !=, NULL);
        DTRACE_PROBE2(l2arc__iodone, zio_t *, zio,
            l2arc_write_callback_t *, cb);

        if (zio->io_error != 0)
                ARCSTAT_BUMP(arcstat_l2_writes_error);

        /*
         * All writes completed, or an error was hit.
         */
top:
        mutex_enter(&dev->l2ad_mtx);
        for (hdr = list_prev(buflist, head); hdr; hdr = hdr_prev) {
                hdr_prev = list_prev(buflist, hdr);

                hash_lock = HDR_LOCK(hdr);

                /*
                 * We cannot use mutex_enter or else we can deadlock
                 * with l2arc_write_buffers (due to swapping the order
                 * the hash lock and l2ad_mtx are taken).
                 */
                if (!mutex_tryenter(hash_lock)) {
                        /*
                         * Missed the hash lock. We must retry so we
                         * don't leave the ARC_FLAG_L2_WRITING bit set.
                         */
                        ARCSTAT_BUMP(arcstat_l2_writes_lock_retry);

                        /*
                         * We don't want to rescan the headers we've
                         * already marked as having been written out, so
                         * we reinsert the head node so we can pick up
                         * where we left off.
                         */
                        list_remove(buflist, head);
                        list_insert_after(buflist, hdr, head);

                        mutex_exit(&dev->l2ad_mtx);

                        /*
                         * We wait for the hash lock to become available
                         * to try and prevent busy waiting, and increase
                         * the chance we'll be able to acquire the lock
                         * the next time around.
                         */
                        mutex_enter(hash_lock);
                        mutex_exit(hash_lock);
                        goto top;
                }

                /*
                 * We could not have been moved into the arc_l2c_only
                 * state while in-flight due to our ARC_FLAG_L2_WRITING
                 * bit being set. Let's just ensure that's being enforced.
                 */
                ASSERT(HDR_HAS_L1HDR(hdr));

                /*
                 * Skipped - drop L2ARC entry and mark the header as no
                 * longer L2 eligibile.
                 */
                if (zio->io_error != 0) {
                        /*
                         * Error - drop L2ARC entry.
                         */
                        list_remove(buflist, hdr);
                        arc_hdr_clear_flags(hdr, ARC_FLAG_HAS_L2HDR);

                        uint64_t psize = HDR_GET_PSIZE(hdr);
                        ARCSTAT_INCR(arcstat_l2_psize, -psize);
                        ARCSTAT_INCR(arcstat_l2_lsize, -HDR_GET_LSIZE(hdr));

                        bytes_dropped +=
                            vdev_psize_to_asize(dev->l2ad_vdev, psize);
                        (void) zfs_refcount_remove_many(&dev->l2ad_alloc,
                            arc_hdr_size(hdr), hdr);
                }

                /*
                 * Allow ARC to begin reads and ghost list evictions to
                 * this L2ARC entry.
                 */
                arc_hdr_clear_flags(hdr, ARC_FLAG_L2_WRITING);

                mutex_exit(hash_lock);
        }

        /*
         * Free the allocated abd buffers for writing the log blocks.
         * If the zio failed reclaim the allocated space and remove the
         * pointers to these log blocks from the log block pointer list
         * of the L2ARC device.
         */
        while ((abd_buf = list_remove_tail(&cb->l2wcb_abd_list)) != NULL) {
                abd_free(abd_buf->abd);
                zio_buf_free(abd_buf, sizeof (*abd_buf));
                if (zio->io_error != 0) {
                        lb_ptr_buf = list_remove_head(&dev->l2ad_lbptr_list);
                        /*
                         * L2BLK_GET_PSIZE returns aligned size for log
                         * blocks.
                         */
                        uint64_t asize =
                            L2BLK_GET_PSIZE((lb_ptr_buf->lb_ptr)->lbp_prop);
                        bytes_dropped += asize;
                        ARCSTAT_INCR(arcstat_l2_log_blk_asize, -asize);
                        ARCSTAT_BUMPDOWN(arcstat_l2_log_blk_count);
                        zfs_refcount_remove_many(&dev->l2ad_lb_asize, asize,
                            lb_ptr_buf);
                        zfs_refcount_remove(&dev->l2ad_lb_count, lb_ptr_buf);
                        kmem_free(lb_ptr_buf->lb_ptr,
                            sizeof (l2arc_log_blkptr_t));
                        kmem_free(lb_ptr_buf, sizeof (l2arc_lb_ptr_buf_t));
                }
        }
        list_destroy(&cb->l2wcb_abd_list);

        if (zio->io_error != 0) {
                /*
                 * Restore the lbps array in the header to its previous state.
                 * If the list of log block pointers is empty, zero out the
                 * log block pointers in the device header.
                 */
                lb_ptr_buf = list_head(&dev->l2ad_lbptr_list);
                for (int i = 0; i < 2; i++) {
                        if (lb_ptr_buf == NULL) {
                                /*
                                 * If the list is empty zero out the device
                                 * header. Otherwise zero out the second log
                                 * block pointer in the header.
                                 */
                                if (i == 0) {
                                        bzero(l2dhdr, dev->l2ad_dev_hdr_asize);
                                } else {
                                        bzero(&l2dhdr->dh_start_lbps[i],
                                            sizeof (l2arc_log_blkptr_t));
                                }
                                break;
                        }
                        bcopy(lb_ptr_buf->lb_ptr, &l2dhdr->dh_start_lbps[i],
                            sizeof (l2arc_log_blkptr_t));
                        lb_ptr_buf = list_next(&dev->l2ad_lbptr_list,
                            lb_ptr_buf);
                }
        }

        atomic_inc_64(&l2arc_writes_done);
        list_remove(buflist, head);
        ASSERT(!HDR_HAS_L1HDR(head));
        kmem_cache_free(hdr_l2only_cache, head);
        mutex_exit(&dev->l2ad_mtx);

        ASSERT(dev->l2ad_vdev != NULL);
        vdev_space_update(dev->l2ad_vdev, -bytes_dropped, 0, 0);

        l2arc_do_free_on_write();

        kmem_free(cb, sizeof (l2arc_write_callback_t));
}

static int
l2arc_untransform(zio_t *zio, l2arc_read_callback_t *cb)
{
        int ret;
        spa_t *spa = zio->io_spa;
        arc_buf_hdr_t *hdr = cb->l2rcb_hdr;
        blkptr_t *bp = zio->io_bp;
        uint8_t salt[ZIO_DATA_SALT_LEN];
        uint8_t iv[ZIO_DATA_IV_LEN];
        uint8_t mac[ZIO_DATA_MAC_LEN];
        boolean_t no_crypt = B_FALSE;

        /*
         * ZIL data is never be written to the L2ARC, so we don't need
         * special handling for its unique MAC storage.
         */
        ASSERT3U(BP_GET_TYPE(bp), !=, DMU_OT_INTENT_LOG);
        ASSERT(MUTEX_HELD(HDR_LOCK(hdr)));
        ASSERT3P(hdr->b_l1hdr.b_pabd, !=, NULL);

        /*
         * If the data was encrypted, decrypt it now. Note that
         * we must check the bp here and not the hdr, since the
         * hdr does not have its encryption parameters updated
         * until arc_read_done().
         */
        if (BP_IS_ENCRYPTED(bp)) {
                abd_t *eabd = arc_get_data_abd(hdr, arc_hdr_size(hdr), hdr,
                    B_TRUE);

                zio_crypt_decode_params_bp(bp, salt, iv);
                zio_crypt_decode_mac_bp(bp, mac);

                ret = spa_do_crypt_abd(B_FALSE, spa, &cb->l2rcb_zb,
                    BP_GET_TYPE(bp), BP_GET_DEDUP(bp), BP_SHOULD_BYTESWAP(bp),
                    salt, iv, mac, HDR_GET_PSIZE(hdr), eabd,
                    hdr->b_l1hdr.b_pabd, &no_crypt);
                if (ret != 0) {
                        arc_free_data_abd(hdr, eabd, arc_hdr_size(hdr), hdr);
                        goto error;
                }

                /*
                 * If we actually performed decryption, replace b_pabd
                 * with the decrypted data. Otherwise we can just throw
                 * our decryption buffer away.
                 */
                if (!no_crypt) {
                        arc_free_data_abd(hdr, hdr->b_l1hdr.b_pabd,
                            arc_hdr_size(hdr), hdr);
                        hdr->b_l1hdr.b_pabd = eabd;
                        zio->io_abd = eabd;
                } else {
                        arc_free_data_abd(hdr, eabd, arc_hdr_size(hdr), hdr);
                }
        }

        /*
         * If the L2ARC block was compressed, but ARC compression
         * is disabled we decompress the data into a new buffer and
         * replace the existing data.
         */
        if (HDR_GET_COMPRESS(hdr) != ZIO_COMPRESS_OFF &&
            !HDR_COMPRESSION_ENABLED(hdr)) {
                abd_t *cabd = arc_get_data_abd(hdr, arc_hdr_size(hdr), hdr,
                    B_TRUE);
                void *tmp = abd_borrow_buf(cabd, arc_hdr_size(hdr));

                ret = zio_decompress_data(HDR_GET_COMPRESS(hdr),
                    hdr->b_l1hdr.b_pabd, tmp, HDR_GET_PSIZE(hdr),
                    HDR_GET_LSIZE(hdr), &hdr->b_complevel);
                if (ret != 0) {
                        abd_return_buf_copy(cabd, tmp, arc_hdr_size(hdr));
                        arc_free_data_abd(hdr, cabd, arc_hdr_size(hdr), hdr);
                        goto error;
                }

                abd_return_buf_copy(cabd, tmp, arc_hdr_size(hdr));
                arc_free_data_abd(hdr, hdr->b_l1hdr.b_pabd,
                    arc_hdr_size(hdr), hdr);
                hdr->b_l1hdr.b_pabd = cabd;
                zio->io_abd = cabd;
                zio->io_size = HDR_GET_LSIZE(hdr);
        }

        return (0);

error:
        return (ret);
}


/*
 * A read to a cache device completed.  Validate buffer contents before
 * handing over to the regular ARC routines.
 */
static void
l2arc_read_done(zio_t *zio)
{
        int tfm_error = 0;
        l2arc_read_callback_t *cb = zio->io_private;
        arc_buf_hdr_t *hdr;
        kmutex_t *hash_lock;
        boolean_t valid_cksum;
        boolean_t using_rdata = (BP_IS_ENCRYPTED(&cb->l2rcb_bp) &&
            (cb->l2rcb_flags & ZIO_FLAG_RAW_ENCRYPT));

        ASSERT3P(zio->io_vd, !=, NULL);
        ASSERT(zio->io_flags & ZIO_FLAG_DONT_PROPAGATE);

        spa_config_exit(zio->io_spa, SCL_L2ARC, zio->io_vd);

        ASSERT3P(cb, !=, NULL);
        hdr = cb->l2rcb_hdr;
        ASSERT3P(hdr, !=, NULL);

        hash_lock = HDR_LOCK(hdr);
        mutex_enter(hash_lock);
        ASSERT3P(hash_lock, ==, HDR_LOCK(hdr));

        /*
         * If the data was read into a temporary buffer,
         * move it and free the buffer.
         */
        if (cb->l2rcb_abd != NULL) {
                ASSERT3U(arc_hdr_size(hdr), <, zio->io_size);
                if (zio->io_error == 0) {
                        if (using_rdata) {
                                abd_copy(hdr->b_crypt_hdr.b_rabd,
                                    cb->l2rcb_abd, arc_hdr_size(hdr));
                        } else {
                                abd_copy(hdr->b_l1hdr.b_pabd,
                                    cb->l2rcb_abd, arc_hdr_size(hdr));
                        }
                }

                /*
                 * The following must be done regardless of whether
                 * there was an error:
                 * - free the temporary buffer
                 * - point zio to the real ARC buffer
                 * - set zio size accordingly
                 * These are required because zio is either re-used for
                 * an I/O of the block in the case of the error
                 * or the zio is passed to arc_read_done() and it
                 * needs real data.
                 */
                abd_free(cb->l2rcb_abd);
                zio->io_size = zio->io_orig_size = arc_hdr_size(hdr);

                if (using_rdata) {
                        ASSERT(HDR_HAS_RABD(hdr));
                        zio->io_abd = zio->io_orig_abd =
                            hdr->b_crypt_hdr.b_rabd;
                } else {
                        ASSERT3P(hdr->b_l1hdr.b_pabd, !=, NULL);
                        zio->io_abd = zio->io_orig_abd = hdr->b_l1hdr.b_pabd;
                }
        }

        ASSERT3P(zio->io_abd, !=, NULL);

        /*
         * Check this survived the L2ARC journey.
         */
        ASSERT(zio->io_abd == hdr->b_l1hdr.b_pabd ||
            (HDR_HAS_RABD(hdr) && zio->io_abd == hdr->b_crypt_hdr.b_rabd));
        zio->io_bp_copy = cb->l2rcb_bp; /* XXX fix in L2ARC 2.0 */
        zio->io_bp = &zio->io_bp_copy;  /* XXX fix in L2ARC 2.0 */
        zio->io_prop.zp_complevel = hdr->b_complevel;

        valid_cksum = arc_cksum_is_equal(hdr, zio);

        /*
         * b_rabd will always match the data as it exists on disk if it is
         * being used. Therefore if we are reading into b_rabd we do not
         * attempt to untransform the data.
         */
        if (valid_cksum && !using_rdata)
                tfm_error = l2arc_untransform(zio, cb);

        if (valid_cksum && tfm_error == 0 && zio->io_error == 0 &&
            !HDR_L2_EVICTED(hdr)) {
                mutex_exit(hash_lock);
                zio->io_private = hdr;
                arc_read_done(zio);
        } else {
                /*
                 * Buffer didn't survive caching.  Increment stats and
                 * reissue to the original storage device.
                 */
                if (zio->io_error != 0) {
                        ARCSTAT_BUMP(arcstat_l2_io_error);
                } else {
                        zio->io_error = SET_ERROR(EIO);
                }
                if (!valid_cksum || tfm_error != 0)
                        ARCSTAT_BUMP(arcstat_l2_cksum_bad);

                /*
                 * If there's no waiter, issue an async i/o to the primary
                 * storage now.  If there *is* a waiter, the caller must
                 * issue the i/o in a context where it's OK to block.
                 */
                if (zio->io_waiter == NULL) {
                        zio_t *pio = zio_unique_parent(zio);
                        void *abd = (using_rdata) ?
                            hdr->b_crypt_hdr.b_rabd : hdr->b_l1hdr.b_pabd;

                        ASSERT(!pio || pio->io_child_type == ZIO_CHILD_LOGICAL);

                        zio = zio_read(pio, zio->io_spa, zio->io_bp,
                            abd, zio->io_size, arc_read_done,
                            hdr, zio->io_priority, cb->l2rcb_flags,
                            &cb->l2rcb_zb);

                        /*
                         * Original ZIO will be freed, so we need to update
                         * ARC header with the new ZIO pointer to be used
                         * by zio_change_priority() in arc_read().
                         */
                        for (struct arc_callback *acb = hdr->b_l1hdr.b_acb;
                            acb != NULL; acb = acb->acb_next)
                                acb->acb_zio_head = zio;

                        mutex_exit(hash_lock);
                        zio_nowait(zio);
                } else {
                        mutex_exit(hash_lock);
                }
        }

        kmem_free(cb, sizeof (l2arc_read_callback_t));
}

/*
 * This is the list priority from which the L2ARC will search for pages to
 * cache.  This is used within loops (0..3) to cycle through lists in the
 * desired order.  This order can have a significant effect on cache
 * performance.
 *
 * Currently the metadata lists are hit first, MFU then MRU, followed by
 * the data lists.  This function returns a locked list, and also returns
 * the lock pointer.
 */
static multilist_sublist_t *
l2arc_sublist_lock(int list_num)
{
        multilist_t *ml = NULL;
        unsigned int idx;

        ASSERT(list_num >= 0 && list_num < L2ARC_FEED_TYPES);

        switch (list_num) {
        case 0:
                ml = arc_mfu->arcs_list[ARC_BUFC_METADATA];
                break;
        case 1:
                ml = arc_mru->arcs_list[ARC_BUFC_METADATA];
                break;
        case 2:
                ml = arc_mfu->arcs_list[ARC_BUFC_DATA];
                break;
        case 3:
                ml = arc_mru->arcs_list[ARC_BUFC_DATA];
                break;
        default:
                return (NULL);
        }

        /*
         * Return a randomly-selected sublist. This is acceptable
         * because the caller feeds only a little bit of data for each
         * call (8MB). Subsequent calls will result in different
         * sublists being selected.
         */
        idx = multilist_get_random_index(ml);
        return (multilist_sublist_lock(ml, idx));
}

/*
 * Calculates the maximum overhead of L2ARC metadata log blocks for a given
 * L2ARC write size. l2arc_evict and l2arc_write_size need to include this
 * overhead in processing to make sure there is enough headroom available
 * when writing buffers.
 */
static inline uint64_t
l2arc_log_blk_overhead(uint64_t write_sz, l2arc_dev_t *dev)
{
        if (dev->l2ad_log_entries == 0) {
                return (0);
        } else {
                uint64_t log_entries = write_sz >> SPA_MINBLOCKSHIFT;

                uint64_t log_blocks = (log_entries +
                    dev->l2ad_log_entries - 1) /
                    dev->l2ad_log_entries;

                return (vdev_psize_to_asize(dev->l2ad_vdev,
                    sizeof (l2arc_log_blk_phys_t)) * log_blocks);
        }
}

/*
 * Evict buffers from the device write hand to the distance specified in
 * bytes. This distance may span populated buffers, it may span nothing.
 * This is clearing a region on the L2ARC device ready for writing.
 * If the 'all' boolean is set, every buffer is evicted.
 */
static void
l2arc_evict(l2arc_dev_t *dev, uint64_t distance, boolean_t all)
{
        list_t *buflist;
        arc_buf_hdr_t *hdr, *hdr_prev;
        kmutex_t *hash_lock;
        uint64_t taddr;
        l2arc_lb_ptr_buf_t *lb_ptr_buf, *lb_ptr_buf_prev;
        vdev_t *vd = dev->l2ad_vdev;
        boolean_t rerun;

        buflist = &dev->l2ad_buflist;

        /*
         * We need to add in the worst case scenario of log block overhead.
         */
        distance += l2arc_log_blk_overhead(distance, dev);
        if (vd->vdev_has_trim && l2arc_trim_ahead > 0) {
                /*
                 * Trim ahead of the write size 64MB or (l2arc_trim_ahead/100)
                 * times the write size, whichever is greater.
                 */
                distance += MAX(64 * 1024 * 1024,
                    (distance * l2arc_trim_ahead) / 100);
        }

top:
        rerun = B_FALSE;
        if (dev->l2ad_hand >= (dev->l2ad_end - distance)) {
                /*
                 * When there is no space to accommodate upcoming writes,
                 * evict to the end. Then bump the write and evict hands
                 * to the start and iterate. This iteration does not
                 * happen indefinitely as we make sure in
                 * l2arc_write_size() that when the write hand is reset,
                 * the write size does not exceed the end of the device.
                 */
                rerun = B_TRUE;
                taddr = dev->l2ad_end;
        } else {
                taddr = dev->l2ad_hand + distance;
        }
        DTRACE_PROBE4(l2arc__evict, l2arc_dev_t *, dev, list_t *, buflist,
            uint64_t, taddr, boolean_t, all);

        if (!all) {
                /*
                 * This check has to be placed after deciding whether to
                 * iterate (rerun).
                 */
                if (dev->l2ad_first) {
                        /*
                         * This is the first sweep through the device. There is
                         * nothing to evict. We have already trimmmed the
                         * whole device.
                         */
                        goto out;
                } else {
                        /*
                         * Trim the space to be evicted.
                         */
                        if (vd->vdev_has_trim && dev->l2ad_evict < taddr &&
                            l2arc_trim_ahead > 0) {
                                /*
                                 * We have to drop the spa_config lock because
                                 * vdev_trim_range() will acquire it.
                                 * l2ad_evict already accounts for the label
                                 * size. To prevent vdev_trim_ranges() from
                                 * adding it again, we subtract it from
                                 * l2ad_evict.
                                 */
                                spa_config_exit(dev->l2ad_spa, SCL_L2ARC, dev);
                                vdev_trim_simple(vd,
                                    dev->l2ad_evict - VDEV_LABEL_START_SIZE,
                                    taddr - dev->l2ad_evict);
                                spa_config_enter(dev->l2ad_spa, SCL_L2ARC, dev,
                                    RW_READER);
                        }

                        /*
                         * When rebuilding L2ARC we retrieve the evict hand
                         * from the header of the device. Of note, l2arc_evict()
                         * does not actually delete buffers from the cache
                         * device, but trimming may do so depending on the
                         * hardware implementation. Thus keeping track of the
                         * evict hand is useful.
                         */
                        dev->l2ad_evict = MAX(dev->l2ad_evict, taddr);
                }
        }

retry:
        mutex_enter(&dev->l2ad_mtx);
        /*
         * We have to account for evicted log blocks. Run vdev_space_update()
         * on log blocks whose offset (in bytes) is before the evicted offset
         * (in bytes) by searching in the list of pointers to log blocks
         * present in the L2ARC device.
         */
        for (lb_ptr_buf = list_tail(&dev->l2ad_lbptr_list); lb_ptr_buf;
            lb_ptr_buf = lb_ptr_buf_prev) {

                lb_ptr_buf_prev = list_prev(&dev->l2ad_lbptr_list, lb_ptr_buf);

                /* L2BLK_GET_PSIZE returns aligned size for log blocks */
                uint64_t asize = L2BLK_GET_PSIZE(
                    (lb_ptr_buf->lb_ptr)->lbp_prop);

                /*
                 * We don't worry about log blocks left behind (ie
                 * lbp_payload_start < l2ad_hand) because l2arc_write_buffers()
                 * will never write more than l2arc_evict() evicts.
                 */
                if (!all && l2arc_log_blkptr_valid(dev, lb_ptr_buf->lb_ptr)) {
                        break;
                } else {
                        vdev_space_update(vd, -asize, 0, 0);
                        ARCSTAT_INCR(arcstat_l2_log_blk_asize, -asize);
                        ARCSTAT_BUMPDOWN(arcstat_l2_log_blk_count);
                        zfs_refcount_remove_many(&dev->l2ad_lb_asize, asize,
                            lb_ptr_buf);
                        zfs_refcount_remove(&dev->l2ad_lb_count, lb_ptr_buf);
                        list_remove(&dev->l2ad_lbptr_list, lb_ptr_buf);
                        kmem_free(lb_ptr_buf->lb_ptr,
                            sizeof (l2arc_log_blkptr_t));
                        kmem_free(lb_ptr_buf, sizeof (l2arc_lb_ptr_buf_t));
                }
        }

        for (hdr = list_tail(buflist); hdr; hdr = hdr_prev) {
                hdr_prev = list_prev(buflist, hdr);

                ASSERT(!HDR_EMPTY(hdr));
                hash_lock = HDR_LOCK(hdr);

                /*
                 * We cannot use mutex_enter or else we can deadlock
                 * with l2arc_write_buffers (due to swapping the order
                 * the hash lock and l2ad_mtx are taken).
                 */
                if (!mutex_tryenter(hash_lock)) {
                        /*
                         * Missed the hash lock.  Retry.
                         */
                        ARCSTAT_BUMP(arcstat_l2_evict_lock_retry);
                        mutex_exit(&dev->l2ad_mtx);
                        mutex_enter(hash_lock);
                        mutex_exit(hash_lock);
                        goto retry;
                }

                /*
                 * A header can't be on this list if it doesn't have L2 header.
                 */
                ASSERT(HDR_HAS_L2HDR(hdr));

                /* Ensure this header has finished being written. */
                ASSERT(!HDR_L2_WRITING(hdr));
                ASSERT(!HDR_L2_WRITE_HEAD(hdr));

                if (!all && (hdr->b_l2hdr.b_daddr >= dev->l2ad_evict ||
                    hdr->b_l2hdr.b_daddr < dev->l2ad_hand)) {
                        /*
                         * We've evicted to the target address,
                         * or the end of the device.
                         */
                        mutex_exit(hash_lock);
                        break;
                }

                if (!HDR_HAS_L1HDR(hdr)) {
                        ASSERT(!HDR_L2_READING(hdr));
                        /*
                         * This doesn't exist in the ARC.  Destroy.
                         * arc_hdr_destroy() will call list_remove()
                         * and decrement arcstat_l2_lsize.
                         */
                        arc_change_state(arc_anon, hdr, hash_lock);
                        arc_hdr_destroy(hdr);
                } else {
                        ASSERT(hdr->b_l1hdr.b_state != arc_l2c_only);
                        ARCSTAT_BUMP(arcstat_l2_evict_l1cached);
                        /*
                         * Invalidate issued or about to be issued
                         * reads, since we may be about to write
                         * over this location.
                         */
                        if (HDR_L2_READING(hdr)) {
                                ARCSTAT_BUMP(arcstat_l2_evict_reading);
                                arc_hdr_set_flags(hdr, ARC_FLAG_L2_EVICTED);
                        }

                        arc_hdr_l2hdr_destroy(hdr);
                }
                mutex_exit(hash_lock);
        }
        mutex_exit(&dev->l2ad_mtx);

out:
        /*
         * We need to check if we evict all buffers, otherwise we may iterate
         * unnecessarily.
         */
        if (!all && rerun) {
                /*
                 * Bump device hand to the device start if it is approaching the
                 * end. l2arc_evict() has already evicted ahead for this case.
                 */
                dev->l2ad_hand = dev->l2ad_start;
                dev->l2ad_evict = dev->l2ad_start;
                dev->l2ad_first = B_FALSE;
                goto top;
        }

        ASSERT3U(dev->l2ad_hand + distance, <, dev->l2ad_end);
        if (!dev->l2ad_first)
                ASSERT3U(dev->l2ad_hand, <, dev->l2ad_evict);
}

/*
 * Handle any abd transforms that might be required for writing to the L2ARC.
 * If successful, this function will always return an abd with the data
 * transformed as it is on disk in a new abd of asize bytes.
 */
static int
l2arc_apply_transforms(spa_t *spa, arc_buf_hdr_t *hdr, uint64_t asize,
    abd_t **abd_out)
{
        int ret;
        void *tmp = NULL;
        abd_t *cabd = NULL, *eabd = NULL, *to_write = hdr->b_l1hdr.b_pabd;
        enum zio_compress compress = HDR_GET_COMPRESS(hdr);
        uint64_t psize = HDR_GET_PSIZE(hdr);
        uint64_t size = arc_hdr_size(hdr);
        boolean_t ismd = HDR_ISTYPE_METADATA(hdr);
        boolean_t bswap = (hdr->b_l1hdr.b_byteswap != DMU_BSWAP_NUMFUNCS);
        dsl_crypto_key_t *dck = NULL;
        uint8_t mac[ZIO_DATA_MAC_LEN] = { 0 };
        boolean_t no_crypt = B_FALSE;

        ASSERT((HDR_GET_COMPRESS(hdr) != ZIO_COMPRESS_OFF &&
            !HDR_COMPRESSION_ENABLED(hdr)) ||
            HDR_ENCRYPTED(hdr) || HDR_SHARED_DATA(hdr) || psize != asize);
        ASSERT3U(psize, <=, asize);

        /*
         * If this data simply needs its own buffer, we simply allocate it
         * and copy the data. This may be done to eliminate a dependency on a
         * shared buffer or to reallocate the buffer to match asize.
         */
        if (HDR_HAS_RABD(hdr) && asize != psize) {
                ASSERT3U(asize, >=, psize);
                to_write = abd_alloc_for_io(asize, ismd);
                abd_copy(to_write, hdr->b_crypt_hdr.b_rabd, psize);
                if (psize != asize)
                        abd_zero_off(to_write, psize, asize - psize);
                goto out;
        }

        if ((compress == ZIO_COMPRESS_OFF || HDR_COMPRESSION_ENABLED(hdr)) &&
            !HDR_ENCRYPTED(hdr)) {
                ASSERT3U(size, ==, psize);
                to_write = abd_alloc_for_io(asize, ismd);
                abd_copy(to_write, hdr->b_l1hdr.b_pabd, size);
                if (size != asize)
                        abd_zero_off(to_write, size, asize - size);
                goto out;
        }

        if (compress != ZIO_COMPRESS_OFF && !HDR_COMPRESSION_ENABLED(hdr)) {
                cabd = abd_alloc_for_io(asize, ismd);
                tmp = abd_borrow_buf(cabd, asize);

                psize = zio_compress_data(compress, to_write, tmp, size,
                    hdr->b_complevel);

                if (psize >= size) {
                        abd_return_buf(cabd, tmp, asize);
                        HDR_SET_COMPRESS(hdr, ZIO_COMPRESS_OFF);
                        to_write = cabd;
                        abd_copy(to_write, hdr->b_l1hdr.b_pabd, size);
                        if (size != asize)
                                abd_zero_off(to_write, size, asize - size);
                        goto encrypt;
                }
                ASSERT3U(psize, <=, HDR_GET_PSIZE(hdr));
                if (psize < asize)
                        bzero((char *)tmp + psize, asize - psize);
                psize = HDR_GET_PSIZE(hdr);
                abd_return_buf_copy(cabd, tmp, asize);
                to_write = cabd;
        }

encrypt:
        if (HDR_ENCRYPTED(hdr)) {
                eabd = abd_alloc_for_io(asize, ismd);

                /*
                 * If the dataset was disowned before the buffer
                 * made it to this point, the key to re-encrypt
                 * it won't be available. In this case we simply
                 * won't write the buffer to the L2ARC.
                 */
                ret = spa_keystore_lookup_key(spa, hdr->b_crypt_hdr.b_dsobj,
                    FTAG, &dck);
                if (ret != 0)
                        goto error;

                ret = zio_do_crypt_abd(B_TRUE, &dck->dck_key,
                    hdr->b_crypt_hdr.b_ot, bswap, hdr->b_crypt_hdr.b_salt,
                    hdr->b_crypt_hdr.b_iv, mac, psize, to_write, eabd,
                    &no_crypt);
                if (ret != 0)
                        goto error;

                if (no_crypt)
                        abd_copy(eabd, to_write, psize);

                if (psize != asize)
                        abd_zero_off(eabd, psize, asize - psize);

                /* assert that the MAC we got here matches the one we saved */
                ASSERT0(bcmp(mac, hdr->b_crypt_hdr.b_mac, ZIO_DATA_MAC_LEN));
                spa_keystore_dsl_key_rele(spa, dck, FTAG);

                if (to_write == cabd)
                        abd_free(cabd);

                to_write = eabd;
        }

out:
        ASSERT3P(to_write, !=, hdr->b_l1hdr.b_pabd);
        *abd_out = to_write;
        return (0);

error:
        if (dck != NULL)
                spa_keystore_dsl_key_rele(spa, dck, FTAG);
        if (cabd != NULL)
                abd_free(cabd);
        if (eabd != NULL)
                abd_free(eabd);

        *abd_out = NULL;
        return (ret);
}

static void
l2arc_blk_fetch_done(zio_t *zio)
{
        l2arc_read_callback_t *cb;

        cb = zio->io_private;
        if (cb->l2rcb_abd != NULL)
                abd_put(cb->l2rcb_abd);
        kmem_free(cb, sizeof (l2arc_read_callback_t));
}

/*
 * Find and write ARC buffers to the L2ARC device.
 *
 * An ARC_FLAG_L2_WRITING flag is set so that the L2ARC buffers are not valid
 * for reading until they have completed writing.
 * The headroom_boost is an in-out parameter used to maintain headroom boost
 * state between calls to this function.
 *
 * Returns the number of bytes actually written (which may be smaller than
 * the delta by which the device hand has changed due to alignment and the
 * writing of log blocks).
 */
static uint64_t
l2arc_write_buffers(spa_t *spa, l2arc_dev_t *dev, uint64_t target_sz)
{
        arc_buf_hdr_t           *hdr, *hdr_prev, *head;
        uint64_t                write_asize, write_psize, write_lsize, headroom;
        boolean_t               full;
        l2arc_write_callback_t  *cb = NULL;
        zio_t                   *pio, *wzio;
        uint64_t                guid = spa_load_guid(spa);

        ASSERT3P(dev->l2ad_vdev, !=, NULL);

        pio = NULL;
        write_lsize = write_asize = write_psize = 0;
        full = B_FALSE;
        head = kmem_cache_alloc(hdr_l2only_cache, KM_PUSHPAGE);
        arc_hdr_set_flags(head, ARC_FLAG_L2_WRITE_HEAD | ARC_FLAG_HAS_L2HDR);

        /*
         * Copy buffers for L2ARC writing.
         */
        for (int try = 0; try < L2ARC_FEED_TYPES; try++) {
                /*
                 * If try == 1 or 3, we cache MRU metadata and data
                 * respectively.
                 */
                if (l2arc_mfuonly) {
                        if (try == 1 || try == 3)
                                continue;
                }

                multilist_sublist_t *mls = l2arc_sublist_lock(try);
                uint64_t passed_sz = 0;

                VERIFY3P(mls, !=, NULL);

                /*
                 * L2ARC fast warmup.
                 *
                 * Until the ARC is warm and starts to evict, read from the
                 * head of the ARC lists rather than the tail.
                 */
                if (arc_warm == B_FALSE)
                        hdr = multilist_sublist_head(mls);
                else
                        hdr = multilist_sublist_tail(mls);

                headroom = target_sz * l2arc_headroom;
                if (zfs_compressed_arc_enabled)
                        headroom = (headroom * l2arc_headroom_boost) / 100;

                for (; hdr; hdr = hdr_prev) {
                        kmutex_t *hash_lock;
                        abd_t *to_write = NULL;

                        if (arc_warm == B_FALSE)
                                hdr_prev = multilist_sublist_next(mls, hdr);
                        else
                                hdr_prev = multilist_sublist_prev(mls, hdr);

                        hash_lock = HDR_LOCK(hdr);
                        if (!mutex_tryenter(hash_lock)) {
                                /*
                                 * Skip this buffer rather than waiting.
                                 */
                                continue;
                        }

                        passed_sz += HDR_GET_LSIZE(hdr);
                        if (l2arc_headroom != 0 && passed_sz > headroom) {
                                /*
                                 * Searched too far.
                                 */
                                mutex_exit(hash_lock);
                                break;
                        }

                        if (!l2arc_write_eligible(guid, hdr)) {
                                mutex_exit(hash_lock);
                                continue;
                        }

                        /*
                         * We rely on the L1 portion of the header below, so
                         * it's invalid for this header to have been evicted out
                         * of the ghost cache, prior to being written out. The
                         * ARC_FLAG_L2_WRITING bit ensures this won't happen.
                         */
                        ASSERT(HDR_HAS_L1HDR(hdr));

                        ASSERT3U(HDR_GET_PSIZE(hdr), >, 0);
                        ASSERT3U(arc_hdr_size(hdr), >, 0);
                        ASSERT(hdr->b_l1hdr.b_pabd != NULL ||
                            HDR_HAS_RABD(hdr));
                        uint64_t psize = HDR_GET_PSIZE(hdr);
                        uint64_t asize = vdev_psize_to_asize(dev->l2ad_vdev,
                            psize);

                        if ((write_asize + asize) > target_sz) {
                                full = B_TRUE;
                                mutex_exit(hash_lock);
                                break;
                        }

                        /*
                         * We rely on the L1 portion of the header below, so
                         * it's invalid for this header to have been evicted out
                         * of the ghost cache, prior to being written out. The
                         * ARC_FLAG_L2_WRITING bit ensures this won't happen.
                         */
                        arc_hdr_set_flags(hdr, ARC_FLAG_L2_WRITING);
                        ASSERT(HDR_HAS_L1HDR(hdr));

                        ASSERT3U(HDR_GET_PSIZE(hdr), >, 0);
                        ASSERT(hdr->b_l1hdr.b_pabd != NULL ||
                            HDR_HAS_RABD(hdr));
                        ASSERT3U(arc_hdr_size(hdr), >, 0);

                        /*
                         * If this header has b_rabd, we can use this since it
                         * must always match the data exactly as it exists on
                         * disk. Otherwise, the L2ARC can normally use the
                         * hdr's data, but if we're sharing data between the
                         * hdr and one of its bufs, L2ARC needs its own copy of
                         * the data so that the ZIO below can't race with the
                         * buf consumer. To ensure that this copy will be
                         * available for the lifetime of the ZIO and be cleaned
                         * up afterwards, we add it to the l2arc_free_on_write
                         * queue. If we need to apply any transforms to the
                         * data (compression, encryption) we will also need the
                         * extra buffer.
                         */
                        if (HDR_HAS_RABD(hdr) && psize == asize) {
                                to_write = hdr->b_crypt_hdr.b_rabd;
                        } else if ((HDR_COMPRESSION_ENABLED(hdr) ||
                            HDR_GET_COMPRESS(hdr) == ZIO_COMPRESS_OFF) &&
                            !HDR_ENCRYPTED(hdr) && !HDR_SHARED_DATA(hdr) &&
                            psize == asize) {
                                to_write = hdr->b_l1hdr.b_pabd;
                        } else {
                                int ret;
                                arc_buf_contents_t type = arc_buf_type(hdr);

                                ret = l2arc_apply_transforms(spa, hdr, asize,
                                    &to_write);
                                if (ret != 0) {
                                        arc_hdr_clear_flags(hdr,
                                            ARC_FLAG_L2_WRITING);
                                        mutex_exit(hash_lock);
                                        continue;
                                }

                                l2arc_free_abd_on_write(to_write, asize, type);
                        }

                        if (pio == NULL) {
                                /*
                                 * Insert a dummy header on the buflist so
                                 * l2arc_write_done() can find where the
                                 * write buffers begin without searching.
                                 */
                                mutex_enter(&dev->l2ad_mtx);
                                list_insert_head(&dev->l2ad_buflist, head);
                                mutex_exit(&dev->l2ad_mtx);

                                cb = kmem_alloc(
                                    sizeof (l2arc_write_callback_t), KM_SLEEP);
                                cb->l2wcb_dev = dev;
                                cb->l2wcb_head = head;
                                /*
                                 * Create a list to save allocated abd buffers
                                 * for l2arc_log_blk_commit().
                                 */
                                list_create(&cb->l2wcb_abd_list,
                                    sizeof (l2arc_lb_abd_buf_t),
                                    offsetof(l2arc_lb_abd_buf_t, node));
                                pio = zio_root(spa, l2arc_write_done, cb,
                                    ZIO_FLAG_CANFAIL);
                        }

                        hdr->b_l2hdr.b_dev = dev;
                        hdr->b_l2hdr.b_hits = 0;

                        hdr->b_l2hdr.b_daddr = dev->l2ad_hand;
                        arc_hdr_set_flags(hdr, ARC_FLAG_HAS_L2HDR);

                        mutex_enter(&dev->l2ad_mtx);
                        list_insert_head(&dev->l2ad_buflist, hdr);
                        mutex_exit(&dev->l2ad_mtx);

                        (void) zfs_refcount_add_many(&dev->l2ad_alloc,
                            arc_hdr_size(hdr), hdr);

                        wzio = zio_write_phys(pio, dev->l2ad_vdev,
                            hdr->b_l2hdr.b_daddr, asize, to_write,
                            ZIO_CHECKSUM_OFF, NULL, hdr,
                            ZIO_PRIORITY_ASYNC_WRITE,
                            ZIO_FLAG_CANFAIL, B_FALSE);

                        write_lsize += HDR_GET_LSIZE(hdr);
                        DTRACE_PROBE2(l2arc__write, vdev_t *, dev->l2ad_vdev,
                            zio_t *, wzio);

                        write_psize += psize;
                        write_asize += asize;
                        dev->l2ad_hand += asize;
                        vdev_space_update(dev->l2ad_vdev, asize, 0, 0);

                        mutex_exit(hash_lock);

                        /*
                         * Append buf info to current log and commit if full.
                         * arcstat_l2_{size,asize} kstats are updated
                         * internally.
                         */
                        if (l2arc_log_blk_insert(dev, hdr))
                                l2arc_log_blk_commit(dev, pio, cb);

                        zio_nowait(wzio);
                }

                multilist_sublist_unlock(mls);

                if (full == B_TRUE)
                        break;
        }

        /* No buffers selected for writing? */
        if (pio == NULL) {
                ASSERT0(write_lsize);
                ASSERT(!HDR_HAS_L1HDR(head));
                kmem_cache_free(hdr_l2only_cache, head);

                /*
                 * Although we did not write any buffers l2ad_evict may
                 * have advanced.
                 */
                l2arc_dev_hdr_update(dev);

                return (0);
        }

        if (!dev->l2ad_first)
                ASSERT3U(dev->l2ad_hand, <=, dev->l2ad_evict);

        ASSERT3U(write_asize, <=, target_sz);
        ARCSTAT_BUMP(arcstat_l2_writes_sent);
        ARCSTAT_INCR(arcstat_l2_write_bytes, write_psize);
        ARCSTAT_INCR(arcstat_l2_lsize, write_lsize);
        ARCSTAT_INCR(arcstat_l2_psize, write_psize);

        dev->l2ad_writing = B_TRUE;
        (void) zio_wait(pio);
        dev->l2ad_writing = B_FALSE;

        /*
         * Update the device header after the zio completes as
         * l2arc_write_done() may have updated the memory holding the log block
         * pointers in the device header.
         */
        l2arc_dev_hdr_update(dev);

        return (write_asize);
}

static boolean_t
l2arc_hdr_limit_reached(void)
{
        int64_t s = aggsum_upper_bound(&astat_l2_hdr_size);

        return (arc_reclaim_needed() || (s > arc_meta_limit * 3 / 4) ||
            (s > (arc_warm ? arc_c : arc_c_max) * l2arc_meta_percent / 100));
}

/*
 * This thread feeds the L2ARC at regular intervals.  This is the beating
 * heart of the L2ARC.
 */
/* ARGSUSED */
static void
l2arc_feed_thread(void *unused)
{
        callb_cpr_t cpr;
        l2arc_dev_t *dev;
        spa_t *spa;
        uint64_t size, wrote;
        clock_t begin, next = ddi_get_lbolt();
        fstrans_cookie_t cookie;

        CALLB_CPR_INIT(&cpr, &l2arc_feed_thr_lock, callb_generic_cpr, FTAG);

        mutex_enter(&l2arc_feed_thr_lock);

        cookie = spl_fstrans_mark();
        while (l2arc_thread_exit == 0) {
                CALLB_CPR_SAFE_BEGIN(&cpr);
                (void) cv_timedwait_idle(&l2arc_feed_thr_cv,
                    &l2arc_feed_thr_lock, next);
                CALLB_CPR_SAFE_END(&cpr, &l2arc_feed_thr_lock);
                next = ddi_get_lbolt() + hz;

                /*
                 * Quick check for L2ARC devices.
                 */
                mutex_enter(&l2arc_dev_mtx);
                if (l2arc_ndev == 0) {
                        mutex_exit(&l2arc_dev_mtx);
                        continue;
                }
                mutex_exit(&l2arc_dev_mtx);
                begin = ddi_get_lbolt();

                /*
                 * This selects the next l2arc device to write to, and in
                 * doing so the next spa to feed from: dev->l2ad_spa.   This
                 * will return NULL if there are now no l2arc devices or if
                 * they are all faulted.
                 *
                 * If a device is returned, its spa's config lock is also
                 * held to prevent device removal.  l2arc_dev_get_next()
                 * will grab and release l2arc_dev_mtx.
                 */
                if ((dev = l2arc_dev_get_next()) == NULL)
                        continue;

                spa = dev->l2ad_spa;
                ASSERT3P(spa, !=, NULL);

                /*
                 * If the pool is read-only then force the feed thread to
                 * sleep a little longer.
                 */
                if (!spa_writeable(spa)) {
                        next = ddi_get_lbolt() + 5 * l2arc_feed_secs * hz;
                        spa_config_exit(spa, SCL_L2ARC, dev);
                        continue;
                }

                /*
                 * Avoid contributing to memory pressure.
                 */
                if (l2arc_hdr_limit_reached()) {
                        ARCSTAT_BUMP(arcstat_l2_abort_lowmem);
                        spa_config_exit(spa, SCL_L2ARC, dev);
                        continue;
                }

                ARCSTAT_BUMP(arcstat_l2_feeds);

                size = l2arc_write_size(dev);

                /*
                 * Evict L2ARC buffers that will be overwritten.
                 */
                l2arc_evict(dev, size, B_FALSE);

                /*
                 * Write ARC buffers.
                 */
                wrote = l2arc_write_buffers(spa, dev, size);

                /*
                 * Calculate interval between writes.
                 */
                next = l2arc_write_interval(begin, size, wrote);
                spa_config_exit(spa, SCL_L2ARC, dev);
        }
        spl_fstrans_unmark(cookie);

        l2arc_thread_exit = 0;
        cv_broadcast(&l2arc_feed_thr_cv);
        CALLB_CPR_EXIT(&cpr);           /* drops l2arc_feed_thr_lock */
        thread_exit();
}

boolean_t
l2arc_vdev_present(vdev_t *vd)
{
        return (l2arc_vdev_get(vd) != NULL);
}

/*
 * Returns the l2arc_dev_t associated with a particular vdev_t or NULL if
 * the vdev_t isn't an L2ARC device.
 */
l2arc_dev_t *
l2arc_vdev_get(vdev_t *vd)
{
        l2arc_dev_t     *dev;

        mutex_enter(&l2arc_dev_mtx);
        for (dev = list_head(l2arc_dev_list); dev != NULL;
            dev = list_next(l2arc_dev_list, dev)) {
                if (dev->l2ad_vdev == vd)
                        break;
        }
        mutex_exit(&l2arc_dev_mtx);

        return (dev);
}

/*
 * Add a vdev for use by the L2ARC.  By this point the spa has already
 * validated the vdev and opened it.
 */
void
l2arc_add_vdev(spa_t *spa, vdev_t *vd)
{
        l2arc_dev_t             *adddev;
        uint64_t                l2dhdr_asize;

        ASSERT(!l2arc_vdev_present(vd));

        /*
         * Create a new l2arc device entry.
         */
        adddev = vmem_zalloc(sizeof (l2arc_dev_t), KM_SLEEP);
        adddev->l2ad_spa = spa;
        adddev->l2ad_vdev = vd;
        /* leave extra size for an l2arc device header */
        l2dhdr_asize = adddev->l2ad_dev_hdr_asize =
            MAX(sizeof (*adddev->l2ad_dev_hdr), 1 << vd->vdev_ashift);
        adddev->l2ad_start = VDEV_LABEL_START_SIZE + l2dhdr_asize;
        adddev->l2ad_end = VDEV_LABEL_START_SIZE + vdev_get_min_asize(vd);
        ASSERT3U(adddev->l2ad_start, <, adddev->l2ad_end);
        adddev->l2ad_hand = adddev->l2ad_start;
        adddev->l2ad_evict = adddev->l2ad_start;
        adddev->l2ad_first = B_TRUE;
        adddev->l2ad_writing = B_FALSE;
        adddev->l2ad_trim_all = B_FALSE;
        list_link_init(&adddev->l2ad_node);
        adddev->l2ad_dev_hdr = kmem_zalloc(l2dhdr_asize, KM_SLEEP);

        mutex_init(&adddev->l2ad_mtx, NULL, MUTEX_DEFAULT, NULL);
        /*
         * This is a list of all ARC buffers that are still valid on the
         * device.
         */
        list_create(&adddev->l2ad_buflist, sizeof (arc_buf_hdr_t),
            offsetof(arc_buf_hdr_t, b_l2hdr.b_l2node));

        /*
         * This is a list of pointers to log blocks that are still present
         * on the device.
         */
        list_create(&adddev->l2ad_lbptr_list, sizeof (l2arc_lb_ptr_buf_t),
            offsetof(l2arc_lb_ptr_buf_t, node));

        vdev_space_update(vd, 0, 0, adddev->l2ad_end - adddev->l2ad_hand);
        zfs_refcount_create(&adddev->l2ad_alloc);
        zfs_refcount_create(&adddev->l2ad_lb_asize);
        zfs_refcount_create(&adddev->l2ad_lb_count);

        /*
         * Add device to global list
         */
        mutex_enter(&l2arc_dev_mtx);
        list_insert_head(l2arc_dev_list, adddev);
        atomic_inc_64(&l2arc_ndev);
        mutex_exit(&l2arc_dev_mtx);

        /*
         * Decide if vdev is eligible for L2ARC rebuild
         */
        l2arc_rebuild_vdev(adddev->l2ad_vdev, B_FALSE);
}

void
l2arc_rebuild_vdev(vdev_t *vd, boolean_t reopen)
{
        l2arc_dev_t             *dev = NULL;
        l2arc_dev_hdr_phys_t    *l2dhdr;
        uint64_t                l2dhdr_asize;
        spa_t                   *spa;
        int                     err;
        boolean_t               l2dhdr_valid = B_TRUE;

        dev = l2arc_vdev_get(vd);
        ASSERT3P(dev, !=, NULL);
        spa = dev->l2ad_spa;
        l2dhdr = dev->l2ad_dev_hdr;
        l2dhdr_asize = dev->l2ad_dev_hdr_asize;

        /*
         * The L2ARC has to hold at least the payload of one log block for
         * them to be restored (persistent L2ARC). The payload of a log block
         * depends on the amount of its log entries. We always write log blocks
         * with 1022 entries. How many of them are committed or restored depends
         * on the size of the L2ARC device. Thus the maximum payload of
         * one log block is 1022 * SPA_MAXBLOCKSIZE = 16GB. If the L2ARC device
         * is less than that, we reduce the amount of committed and restored
         * log entries per block so as to enable persistence.
         */
        if (dev->l2ad_end < l2arc_rebuild_blocks_min_l2size) {
                dev->l2ad_log_entries = 0;
        } else {
                dev->l2ad_log_entries = MIN((dev->l2ad_end -
                    dev->l2ad_start) >> SPA_MAXBLOCKSHIFT,
                    L2ARC_LOG_BLK_MAX_ENTRIES);
        }

        /*
         * Read the device header, if an error is returned do not rebuild L2ARC.
         */
        if ((err = l2arc_dev_hdr_read(dev)) != 0)
                l2dhdr_valid = B_FALSE;

        if (l2dhdr_valid && dev->l2ad_log_entries > 0) {
                /*
                 * If we are onlining a cache device (vdev_reopen) that was
                 * still present (l2arc_vdev_present()) and rebuild is enabled,
                 * we should evict all ARC buffers and pointers to log blocks
                 * and reclaim their space before restoring its contents to
                 * L2ARC.
                 */
                if (reopen) {
                        if (!l2arc_rebuild_enabled) {
                                return;
                        } else {
                                l2arc_evict(dev, 0, B_TRUE);
                                /* start a new log block */
                                dev->l2ad_log_ent_idx = 0;
                                dev->l2ad_log_blk_payload_asize = 0;
                                dev->l2ad_log_blk_payload_start = 0;
                        }
                }
                /*
                 * Just mark the device as pending for a rebuild. We won't
                 * be starting a rebuild in line here as it would block pool
                 * import. Instead spa_load_impl will hand that off to an
                 * async task which will call l2arc_spa_rebuild_start.
                 */
                dev->l2ad_rebuild = B_TRUE;
        } else if (spa_writeable(spa)) {
                /*
                 * In this case TRIM the whole device if l2arc_trim_ahead > 0,
                 * otherwise create a new header. We zero out the memory holding
                 * the header to reset dh_start_lbps. If we TRIM the whole
                 * device the new header will be written by
                 * vdev_trim_l2arc_thread() at the end of the TRIM to update the
                 * trim_state in the header too. When reading the header, if
                 * trim_state is not VDEV_TRIM_COMPLETE and l2arc_trim_ahead > 0
                 * we opt to TRIM the whole device again.
                 */
                if (l2arc_trim_ahead > 0) {
                        dev->l2ad_trim_all = B_TRUE;
                } else {
                        bzero(l2dhdr, l2dhdr_asize);
                        l2arc_dev_hdr_update(dev);
                }
        }
}

/*
 * Remove a vdev from the L2ARC.
 */
void
l2arc_remove_vdev(vdev_t *vd)
{
        l2arc_dev_t *remdev = NULL;

        /*
         * Find the device by vdev
         */
        remdev = l2arc_vdev_get(vd);
        ASSERT3P(remdev, !=, NULL);

        /*
         * Cancel any ongoing or scheduled rebuild.
         */
        mutex_enter(&l2arc_rebuild_thr_lock);
        if (remdev->l2ad_rebuild_began == B_TRUE) {
                remdev->l2ad_rebuild_cancel = B_TRUE;
                while (remdev->l2ad_rebuild == B_TRUE)
                        cv_wait(&l2arc_rebuild_thr_cv, &l2arc_rebuild_thr_lock);
        }
        mutex_exit(&l2arc_rebuild_thr_lock);

        /*
         * Remove device from global list
         */
        mutex_enter(&l2arc_dev_mtx);
        list_remove(l2arc_dev_list, remdev);
        l2arc_dev_last = NULL;          /* may have been invalidated */
        atomic_dec_64(&l2arc_ndev);
        mutex_exit(&l2arc_dev_mtx);

        /*
         * Clear all buflists and ARC references.  L2ARC device flush.
         */
        l2arc_evict(remdev, 0, B_TRUE);
        list_destroy(&remdev->l2ad_buflist);
        ASSERT(list_is_empty(&remdev->l2ad_lbptr_list));
        list_destroy(&remdev->l2ad_lbptr_list);
        mutex_destroy(&remdev->l2ad_mtx);
        zfs_refcount_destroy(&remdev->l2ad_alloc);
        zfs_refcount_destroy(&remdev->l2ad_lb_asize);
        zfs_refcount_destroy(&remdev->l2ad_lb_count);
        kmem_free(remdev->l2ad_dev_hdr, remdev->l2ad_dev_hdr_asize);
        vmem_free(remdev, sizeof (l2arc_dev_t));
}

void
l2arc_init(void)
{
        l2arc_thread_exit = 0;
        l2arc_ndev = 0;
        l2arc_writes_sent = 0;
        l2arc_writes_done = 0;

        mutex_init(&l2arc_feed_thr_lock, NULL, MUTEX_DEFAULT, NULL);
        cv_init(&l2arc_feed_thr_cv, NULL, CV_DEFAULT, NULL);
        mutex_init(&l2arc_rebuild_thr_lock, NULL, MUTEX_DEFAULT, NULL);
        cv_init(&l2arc_rebuild_thr_cv, NULL, CV_DEFAULT, NULL);
        mutex_init(&l2arc_dev_mtx, NULL, MUTEX_DEFAULT, NULL);
        mutex_init(&l2arc_free_on_write_mtx, NULL, MUTEX_DEFAULT, NULL);

        l2arc_dev_list = &L2ARC_dev_list;
        l2arc_free_on_write = &L2ARC_free_on_write;
        list_create(l2arc_dev_list, sizeof (l2arc_dev_t),
            offsetof(l2arc_dev_t, l2ad_node));
        list_create(l2arc_free_on_write, sizeof (l2arc_data_free_t),
            offsetof(l2arc_data_free_t, l2df_list_node));
}

void
l2arc_fini(void)
{
        mutex_destroy(&l2arc_feed_thr_lock);
        cv_destroy(&l2arc_feed_thr_cv);
        mutex_destroy(&l2arc_rebuild_thr_lock);
        cv_destroy(&l2arc_rebuild_thr_cv);
        mutex_destroy(&l2arc_dev_mtx);
        mutex_destroy(&l2arc_free_on_write_mtx);

        list_destroy(l2arc_dev_list);
        list_destroy(l2arc_free_on_write);
}

void
l2arc_start(void)
{
        if (!(spa_mode_global & SPA_MODE_WRITE))
                return;

        (void) thread_create(NULL, 0, l2arc_feed_thread, NULL, 0, &p0,
            TS_RUN, defclsyspri);
}

void
l2arc_stop(void)
{
        if (!(spa_mode_global & SPA_MODE_WRITE))
                return;

        mutex_enter(&l2arc_feed_thr_lock);
        cv_signal(&l2arc_feed_thr_cv);  /* kick thread out of startup */
        l2arc_thread_exit = 1;
        while (l2arc_thread_exit != 0)
                cv_wait(&l2arc_feed_thr_cv, &l2arc_feed_thr_lock);
        mutex_exit(&l2arc_feed_thr_lock);
}

/*
 * Punches out rebuild threads for the L2ARC devices in a spa. This should
 * be called after pool import from the spa async thread, since starting
 * these threads directly from spa_import() will make them part of the
 * "zpool import" context and delay process exit (and thus pool import).
 */
void
l2arc_spa_rebuild_start(spa_t *spa)
{
        ASSERT(MUTEX_HELD(&spa_namespace_lock));

        /*
         * Locate the spa's l2arc devices and kick off rebuild threads.
         */
        for (int i = 0; i < spa->spa_l2cache.sav_count; i++) {
                l2arc_dev_t *dev =
                    l2arc_vdev_get(spa->spa_l2cache.sav_vdevs[i]);
                if (dev == NULL) {
                        /* Don't attempt a rebuild if the vdev is UNAVAIL */
                        continue;
                }
                mutex_enter(&l2arc_rebuild_thr_lock);
                if (dev->l2ad_rebuild && !dev->l2ad_rebuild_cancel) {
                        dev->l2ad_rebuild_began = B_TRUE;
                        (void) thread_create(NULL, 0, l2arc_dev_rebuild_thread,
                            dev, 0, &p0, TS_RUN, minclsyspri);
                }
                mutex_exit(&l2arc_rebuild_thr_lock);
        }
}

/*
 * Main entry point for L2ARC rebuilding.
 */
static void
l2arc_dev_rebuild_thread(void *arg)
{
        l2arc_dev_t *dev = arg;

        VERIFY(!dev->l2ad_rebuild_cancel);
        VERIFY(dev->l2ad_rebuild);
        (void) l2arc_rebuild(dev);
        mutex_enter(&l2arc_rebuild_thr_lock);
        dev->l2ad_rebuild_began = B_FALSE;
        dev->l2ad_rebuild = B_FALSE;
        mutex_exit(&l2arc_rebuild_thr_lock);

        thread_exit();
}

/*
 * This function implements the actual L2ARC metadata rebuild. It:
 * starts reading the log block chain and restores each block's contents
 * to memory (reconstructing arc_buf_hdr_t's).
 *
 * Operation stops under any of the following conditions:
 *
 * 1) We reach the end of the log block chain.
 * 2) We encounter *any* error condition (cksum errors, io errors)
 */
static int
l2arc_rebuild(l2arc_dev_t *dev)
{
        vdev_t                  *vd = dev->l2ad_vdev;
        spa_t                   *spa = vd->vdev_spa;
        int                     err = 0;
        l2arc_dev_hdr_phys_t    *l2dhdr = dev->l2ad_dev_hdr;
        l2arc_log_blk_phys_t    *this_lb, *next_lb;
        zio_t                   *this_io = NULL, *next_io = NULL;
        l2arc_log_blkptr_t      lbps[2];
        l2arc_lb_ptr_buf_t      *lb_ptr_buf;
        boolean_t               lock_held;

        this_lb = vmem_zalloc(sizeof (*this_lb), KM_SLEEP);
        next_lb = vmem_zalloc(sizeof (*next_lb), KM_SLEEP);

        /*
         * We prevent device removal while issuing reads to the device,
         * then during the rebuilding phases we drop this lock again so
         * that a spa_unload or device remove can be initiated - this is
         * safe, because the spa will signal us to stop before removing
         * our device and wait for us to stop.
         */
        spa_config_enter(spa, SCL_L2ARC, vd, RW_READER);
        lock_held = B_TRUE;

        /*
         * Retrieve the persistent L2ARC device state.
         * L2BLK_GET_PSIZE returns aligned size for log blocks.
         */
        dev->l2ad_evict = MAX(l2dhdr->dh_evict, dev->l2ad_start);
        dev->l2ad_hand = MAX(l2dhdr->dh_start_lbps[0].lbp_daddr +
            L2BLK_GET_PSIZE((&l2dhdr->dh_start_lbps[0])->lbp_prop),
            dev->l2ad_start);
        dev->l2ad_first = !!(l2dhdr->dh_flags & L2ARC_DEV_HDR_EVICT_FIRST);

        vd->vdev_trim_action_time = l2dhdr->dh_trim_action_time;
        vd->vdev_trim_state = l2dhdr->dh_trim_state;

        /*
         * In case the zfs module parameter l2arc_rebuild_enabled is false
         * we do not start the rebuild process.
         */
        if (!l2arc_rebuild_enabled)
                goto out;

        /* Prepare the rebuild process */
        bcopy(l2dhdr->dh_start_lbps, lbps, sizeof (lbps));

        /* Start the rebuild process */
        for (;;) {
                if (!l2arc_log_blkptr_valid(dev, &lbps[0]))
                        break;

                if ((err = l2arc_log_blk_read(dev, &lbps[0], &lbps[1],
                    this_lb, next_lb, this_io, &next_io)) != 0)
                        goto out;

                /*
                 * Our memory pressure valve. If the system is running low
                 * on memory, rather than swamping memory with new ARC buf
                 * hdrs, we opt not to rebuild the L2ARC. At this point,
                 * however, we have already set up our L2ARC dev to chain in
                 * new metadata log blocks, so the user may choose to offline/
                 * online the L2ARC dev at a later time (or re-import the pool)
                 * to reconstruct it (when there's less memory pressure).
                 */
                if (l2arc_hdr_limit_reached()) {
                        ARCSTAT_BUMP(arcstat_l2_rebuild_abort_lowmem);
                        cmn_err(CE_NOTE, "System running low on memory, "
                            "aborting L2ARC rebuild.");
                        err = SET_ERROR(ENOMEM);
                        goto out;
                }

                spa_config_exit(spa, SCL_L2ARC, vd);
                lock_held = B_FALSE;

                /*
                 * Now that we know that the next_lb checks out alright, we
                 * can start reconstruction from this log block.
                 * L2BLK_GET_PSIZE returns aligned size for log blocks.
                 */
                uint64_t asize = L2BLK_GET_PSIZE((&lbps[0])->lbp_prop);
                l2arc_log_blk_restore(dev, this_lb, asize, lbps[0].lbp_daddr);

                /*
                 * log block restored, include its pointer in the list of
                 * pointers to log blocks present in the L2ARC device.
                 */
                lb_ptr_buf = kmem_zalloc(sizeof (l2arc_lb_ptr_buf_t), KM_SLEEP);
                lb_ptr_buf->lb_ptr = kmem_zalloc(sizeof (l2arc_log_blkptr_t),
                    KM_SLEEP);
                bcopy(&lbps[0], lb_ptr_buf->lb_ptr,
                    sizeof (l2arc_log_blkptr_t));
                mutex_enter(&dev->l2ad_mtx);
                list_insert_tail(&dev->l2ad_lbptr_list, lb_ptr_buf);
                ARCSTAT_INCR(arcstat_l2_log_blk_asize, asize);
                ARCSTAT_BUMP(arcstat_l2_log_blk_count);
                zfs_refcount_add_many(&dev->l2ad_lb_asize, asize, lb_ptr_buf);
                zfs_refcount_add(&dev->l2ad_lb_count, lb_ptr_buf);
                mutex_exit(&dev->l2ad_mtx);
                vdev_space_update(vd, asize, 0, 0);

                /*
                 * Protection against loops of log blocks:
                 *
                 *                                     l2ad_hand  l2ad_evict
                 *                                         V          V
                 * l2ad_start |=======================================| l2ad_end
                 *             -----|||----|||---|||----|||
                 *                  (3)    (2)   (1)    (0)
                 *             ---|||---|||----|||---|||
                 *                (7)   (6)    (5)   (4)
                 *
                 * In this situation the pointer of log block (4) passes
                 * l2arc_log_blkptr_valid() but the log block should not be
                 * restored as it is overwritten by the payload of log block
                 * (0). Only log blocks (0)-(3) should be restored. We check
                 * whether l2ad_evict lies in between the payload starting
                 * offset of the next log block (lbps[1].lbp_payload_start)
                 * and the payload starting offset of the present log block
                 * (lbps[0].lbp_payload_start). If true and this isn't the
                 * first pass, we are looping from the beginning and we should
                 * stop.
                 */
                if (l2arc_range_check_overlap(lbps[1].lbp_payload_start,
                    lbps[0].lbp_payload_start, dev->l2ad_evict) &&
                    !dev->l2ad_first)
                        goto out;

                for (;;) {
                        mutex_enter(&l2arc_rebuild_thr_lock);
                        if (dev->l2ad_rebuild_cancel) {
                                dev->l2ad_rebuild = B_FALSE;
                                cv_signal(&l2arc_rebuild_thr_cv);
                                mutex_exit(&l2arc_rebuild_thr_lock);
                                err = SET_ERROR(ECANCELED);
                                goto out;
                        }
                        mutex_exit(&l2arc_rebuild_thr_lock);
                        if (spa_config_tryenter(spa, SCL_L2ARC, vd,
                            RW_READER)) {
                                lock_held = B_TRUE;
                                break;
                        }
                        /*
                         * L2ARC config lock held by somebody in writer,
                         * possibly due to them trying to remove us. They'll
                         * likely to want us to shut down, so after a little
                         * delay, we check l2ad_rebuild_cancel and retry
                         * the lock again.
                         */
                        delay(1);
                }

                /*
                 * Continue with the next log block.
                 */
                lbps[0] = lbps[1];
                lbps[1] = this_lb->lb_prev_lbp;
                PTR_SWAP(this_lb, next_lb);
                this_io = next_io;
                next_io = NULL;
                }

        if (this_io != NULL)
                l2arc_log_blk_fetch_abort(this_io);
out:
        if (next_io != NULL)
                l2arc_log_blk_fetch_abort(next_io);
        vmem_free(this_lb, sizeof (*this_lb));
        vmem_free(next_lb, sizeof (*next_lb));

        if (!l2arc_rebuild_enabled) {
                spa_history_log_internal(spa, "L2ARC rebuild", NULL,
                    "disabled");
        } else if (err == 0 && zfs_refcount_count(&dev->l2ad_lb_count) > 0) {
                ARCSTAT_BUMP(arcstat_l2_rebuild_success);
                spa_history_log_internal(spa, "L2ARC rebuild", NULL,
                    "successful, restored %llu blocks",
                    (u_longlong_t)zfs_refcount_count(&dev->l2ad_lb_count));
        } else if (err == 0 && zfs_refcount_count(&dev->l2ad_lb_count) == 0) {
                /*
                 * No error but also nothing restored, meaning the lbps array
                 * in the device header points to invalid/non-present log
                 * blocks. Reset the header.
                 */
                spa_history_log_internal(spa, "L2ARC rebuild", NULL,
                    "no valid log blocks");
                bzero(l2dhdr, dev->l2ad_dev_hdr_asize);
                l2arc_dev_hdr_update(dev);
        } else if (err == ECANCELED) {
                /*
                 * In case the rebuild was canceled do not log to spa history
                 * log as the pool may be in the process of being removed.
                 */
                zfs_dbgmsg("L2ARC rebuild aborted, restored %llu blocks",
                    zfs_refcount_count(&dev->l2ad_lb_count));
        } else if (err != 0) {
                spa_history_log_internal(spa, "L2ARC rebuild", NULL,
                    "aborted, restored %llu blocks",
                    (u_longlong_t)zfs_refcount_count(&dev->l2ad_lb_count));
        }

        if (lock_held)
                spa_config_exit(spa, SCL_L2ARC, vd);

        return (err);
}

/*
 * Attempts to read the device header on the provided L2ARC device and writes
 * it to `hdr'. On success, this function returns 0, otherwise the appropriate
 * error code is returned.
 */
static int
l2arc_dev_hdr_read(l2arc_dev_t *dev)
{
        int                     err;
        uint64_t                guid;
        l2arc_dev_hdr_phys_t    *l2dhdr = dev->l2ad_dev_hdr;
        const uint64_t          l2dhdr_asize = dev->l2ad_dev_hdr_asize;
        abd_t                   *abd;

        guid = spa_guid(dev->l2ad_vdev->vdev_spa);

        abd = abd_get_from_buf(l2dhdr, l2dhdr_asize);

        err = zio_wait(zio_read_phys(NULL, dev->l2ad_vdev,
            VDEV_LABEL_START_SIZE, l2dhdr_asize, abd,
            ZIO_CHECKSUM_LABEL, NULL, NULL, ZIO_PRIORITY_ASYNC_READ,
            ZIO_FLAG_DONT_CACHE | ZIO_FLAG_CANFAIL |
            ZIO_FLAG_DONT_PROPAGATE | ZIO_FLAG_DONT_RETRY |
            ZIO_FLAG_SPECULATIVE, B_FALSE));

        abd_put(abd);

        if (err != 0) {
                ARCSTAT_BUMP(arcstat_l2_rebuild_abort_dh_errors);
                zfs_dbgmsg("L2ARC IO error (%d) while reading device header, "
                    "vdev guid: %llu", err, dev->l2ad_vdev->vdev_guid);
                return (err);
        }

        if (l2dhdr->dh_magic == BSWAP_64(L2ARC_DEV_HDR_MAGIC))
                byteswap_uint64_array(l2dhdr, sizeof (*l2dhdr));

        if (l2dhdr->dh_magic != L2ARC_DEV_HDR_MAGIC ||
            l2dhdr->dh_spa_guid != guid ||
            l2dhdr->dh_vdev_guid != dev->l2ad_vdev->vdev_guid ||
            l2dhdr->dh_version != L2ARC_PERSISTENT_VERSION ||
            l2dhdr->dh_log_entries != dev->l2ad_log_entries ||
            l2dhdr->dh_end != dev->l2ad_end ||
            !l2arc_range_check_overlap(dev->l2ad_start, dev->l2ad_end,
            l2dhdr->dh_evict) ||
            (l2dhdr->dh_trim_state != VDEV_TRIM_COMPLETE &&
            l2arc_trim_ahead > 0)) {
                /*
                 * Attempt to rebuild a device containing no actual dev hdr
                 * or containing a header from some other pool or from another
                 * version of persistent L2ARC.
                 */
                ARCSTAT_BUMP(arcstat_l2_rebuild_abort_unsupported);
                return (SET_ERROR(ENOTSUP));
        }

        return (0);
}

/*
 * Reads L2ARC log blocks from storage and validates their contents.
 *
 * This function implements a simple fetcher to make sure that while
 * we're processing one buffer the L2ARC is already fetching the next
 * one in the chain.
 *
 * The arguments this_lp and next_lp point to the current and next log block
 * address in the block chain. Similarly, this_lb and next_lb hold the
 * l2arc_log_blk_phys_t's of the current and next L2ARC blk.
 *
 * The `this_io' and `next_io' arguments are used for block fetching.
 * When issuing the first blk IO during rebuild, you should pass NULL for
 * `this_io'. This function will then issue a sync IO to read the block and
 * also issue an async IO to fetch the next block in the block chain. The
 * fetched IO is returned in `next_io'. On subsequent calls to this
 * function, pass the value returned in `next_io' from the previous call
 * as `this_io' and a fresh `next_io' pointer to hold the next fetch IO.
 * Prior to the call, you should initialize your `next_io' pointer to be
 * NULL. If no fetch IO was issued, the pointer is left set at NULL.
 *
 * On success, this function returns 0, otherwise it returns an appropriate
 * error code. On error the fetching IO is aborted and cleared before
 * returning from this function. Therefore, if we return `success', the
 * caller can assume that we have taken care of cleanup of fetch IOs.
 */
static int
l2arc_log_blk_read(l2arc_dev_t *dev,
    const l2arc_log_blkptr_t *this_lbp, const l2arc_log_blkptr_t *next_lbp,
    l2arc_log_blk_phys_t *this_lb, l2arc_log_blk_phys_t *next_lb,
    zio_t *this_io, zio_t **next_io)
{
        int             err = 0;
        zio_cksum_t     cksum;
        abd_t           *abd = NULL;
        uint64_t        asize;

        ASSERT(this_lbp != NULL && next_lbp != NULL);
        ASSERT(this_lb != NULL && next_lb != NULL);
        ASSERT(next_io != NULL && *next_io == NULL);
        ASSERT(l2arc_log_blkptr_valid(dev, this_lbp));

        /*
         * Check to see if we have issued the IO for this log block in a
         * previous run. If not, this is the first call, so issue it now.
         */
        if (this_io == NULL) {
                this_io = l2arc_log_blk_fetch(dev->l2ad_vdev, this_lbp,
                    this_lb);
        }

        /*
         * Peek to see if we can start issuing the next IO immediately.
         */
        if (l2arc_log_blkptr_valid(dev, next_lbp)) {
                /*
                 * Start issuing IO for the next log block early - this
                 * should help keep the L2ARC device busy while we
                 * decompress and restore this log block.
                 */
                *next_io = l2arc_log_blk_fetch(dev->l2ad_vdev, next_lbp,
                    next_lb);
        }

        /* Wait for the IO to read this log block to complete */
        if ((err = zio_wait(this_io)) != 0) {
                ARCSTAT_BUMP(arcstat_l2_rebuild_abort_io_errors);
                zfs_dbgmsg("L2ARC IO error (%d) while reading log block, "
                    "offset: %llu, vdev guid: %llu", err, this_lbp->lbp_daddr,
                    dev->l2ad_vdev->vdev_guid);
                goto cleanup;
        }

        /*
         * Make sure the buffer checks out.
         * L2BLK_GET_PSIZE returns aligned size for log blocks.
         */
        asize = L2BLK_GET_PSIZE((this_lbp)->lbp_prop);
        fletcher_4_native(this_lb, asize, NULL, &cksum);
        if (!ZIO_CHECKSUM_EQUAL(cksum, this_lbp->lbp_cksum)) {
                ARCSTAT_BUMP(arcstat_l2_rebuild_abort_cksum_lb_errors);
                zfs_dbgmsg("L2ARC log block cksum failed, offset: %llu, "
                    "vdev guid: %llu, l2ad_hand: %llu, l2ad_evict: %llu",
                    this_lbp->lbp_daddr, dev->l2ad_vdev->vdev_guid,
                    dev->l2ad_hand, dev->l2ad_evict);
                err = SET_ERROR(ECKSUM);
                goto cleanup;
        }

        /* Now we can take our time decoding this buffer */
        switch (L2BLK_GET_COMPRESS((this_lbp)->lbp_prop)) {
        case ZIO_COMPRESS_OFF:
                break;
        case ZIO_COMPRESS_LZ4:
                abd = abd_alloc_for_io(asize, B_TRUE);
                abd_copy_from_buf_off(abd, this_lb, 0, asize);
                if ((err = zio_decompress_data(
                    L2BLK_GET_COMPRESS((this_lbp)->lbp_prop),
                    abd, this_lb, asize, sizeof (*this_lb), NULL)) != 0) {
                        err = SET_ERROR(EINVAL);
                        goto cleanup;
                }
                break;
        default:
                err = SET_ERROR(EINVAL);
                goto cleanup;
        }
        if (this_lb->lb_magic == BSWAP_64(L2ARC_LOG_BLK_MAGIC))
                byteswap_uint64_array(this_lb, sizeof (*this_lb));
        if (this_lb->lb_magic != L2ARC_LOG_BLK_MAGIC) {
                err = SET_ERROR(EINVAL);
                goto cleanup;
        }
cleanup:
        /* Abort an in-flight fetch I/O in case of error */
        if (err != 0 && *next_io != NULL) {
                l2arc_log_blk_fetch_abort(*next_io);
                *next_io = NULL;
        }
        if (abd != NULL)
                abd_free(abd);
        return (err);
}

/*
 * Restores the payload of a log block to ARC. This creates empty ARC hdr
 * entries which only contain an l2arc hdr, essentially restoring the
 * buffers to their L2ARC evicted state. This function also updates space
 * usage on the L2ARC vdev to make sure it tracks restored buffers.
 */
static void
l2arc_log_blk_restore(l2arc_dev_t *dev, const l2arc_log_blk_phys_t *lb,
    uint64_t lb_asize, uint64_t lb_daddr)
{
        uint64_t        size = 0, asize = 0;
        uint64_t        log_entries = dev->l2ad_log_entries;

        /*
         * Usually arc_adapt() is called only for data, not headers, but
         * since we may allocate significant amount of memory here, let ARC
         * grow its arc_c.
         */
        arc_adapt(log_entries * HDR_L2ONLY_SIZE, arc_l2c_only);

        for (int i = log_entries - 1; i >= 0; i--) {
                /*
                 * Restore goes in the reverse temporal direction to preserve
                 * correct temporal ordering of buffers in the l2ad_buflist.
                 * l2arc_hdr_restore also does a list_insert_tail instead of
                 * list_insert_head on the l2ad_buflist:
                 *
                 *              LIST    l2ad_buflist            LIST
                 *              HEAD  <------ (time) ------     TAIL
                 * direction    +-----+-----+-----+-----+-----+    direction
                 * of l2arc <== | buf | buf | buf | buf | buf | ===> of rebuild
                 * fill         +-----+-----+-----+-----+-----+
                 *              ^                               ^
                 *              |                               |
                 *              |                               |
                 *      l2arc_feed_thread               l2arc_rebuild
                 *      will place new bufs here        restores bufs here
                 *
                 * During l2arc_rebuild() the device is not used by
                 * l2arc_feed_thread() as dev->l2ad_rebuild is set to true.
                 */
                size += L2BLK_GET_LSIZE((&lb->lb_entries[i])->le_prop);
                asize += vdev_psize_to_asize(dev->l2ad_vdev,
                    L2BLK_GET_PSIZE((&lb->lb_entries[i])->le_prop));
                l2arc_hdr_restore(&lb->lb_entries[i], dev);
        }

        /*
         * Record rebuild stats:
         *      size            Logical size of restored buffers in the L2ARC
         *      asize           Aligned size of restored buffers in the L2ARC
         */
        ARCSTAT_INCR(arcstat_l2_rebuild_size, size);
        ARCSTAT_INCR(arcstat_l2_rebuild_asize, asize);
        ARCSTAT_INCR(arcstat_l2_rebuild_bufs, log_entries);
        ARCSTAT_F_AVG(arcstat_l2_log_blk_avg_asize, lb_asize);
        ARCSTAT_F_AVG(arcstat_l2_data_to_meta_ratio, asize / lb_asize);
        ARCSTAT_BUMP(arcstat_l2_rebuild_log_blks);
}

/*
 * Restores a single ARC buf hdr from a log entry. The ARC buffer is put
 * into a state indicating that it has been evicted to L2ARC.
 */
static void
l2arc_hdr_restore(const l2arc_log_ent_phys_t *le, l2arc_dev_t *dev)
{
        arc_buf_hdr_t           *hdr, *exists;
        kmutex_t                *hash_lock;
        arc_buf_contents_t      type = L2BLK_GET_TYPE((le)->le_prop);
        uint64_t                asize;

        /*
         * Do all the allocation before grabbing any locks, this lets us
         * sleep if memory is full and we don't have to deal with failed
         * allocations.
         */
        hdr = arc_buf_alloc_l2only(L2BLK_GET_LSIZE((le)->le_prop), type,
            dev, le->le_dva, le->le_daddr,
            L2BLK_GET_PSIZE((le)->le_prop), le->le_birth,
            L2BLK_GET_COMPRESS((le)->le_prop), le->le_complevel,
            L2BLK_GET_PROTECTED((le)->le_prop),
            L2BLK_GET_PREFETCH((le)->le_prop));
        asize = vdev_psize_to_asize(dev->l2ad_vdev,
            L2BLK_GET_PSIZE((le)->le_prop));

        /*
         * vdev_space_update() has to be called before arc_hdr_destroy() to
         * avoid underflow since the latter also calls the former.
         */
        vdev_space_update(dev->l2ad_vdev, asize, 0, 0);

        ARCSTAT_INCR(arcstat_l2_lsize, HDR_GET_LSIZE(hdr));
        ARCSTAT_INCR(arcstat_l2_psize, HDR_GET_PSIZE(hdr));

        mutex_enter(&dev->l2ad_mtx);
        list_insert_tail(&dev->l2ad_buflist, hdr);
        (void) zfs_refcount_add_many(&dev->l2ad_alloc, arc_hdr_size(hdr), hdr);
        mutex_exit(&dev->l2ad_mtx);

        exists = buf_hash_insert(hdr, &hash_lock);
        if (exists) {
                /* Buffer was already cached, no need to restore it. */
                arc_hdr_destroy(hdr);
                /*
                 * If the buffer is already cached, check whether it has
                 * L2ARC metadata. If not, enter them and update the flag.
                 * This is important is case of onlining a cache device, since
                 * we previously evicted all L2ARC metadata from ARC.
                 */
                if (!HDR_HAS_L2HDR(exists)) {
                        arc_hdr_set_flags(exists, ARC_FLAG_HAS_L2HDR);
                        exists->b_l2hdr.b_dev = dev;
                        exists->b_l2hdr.b_daddr = le->le_daddr;
                        mutex_enter(&dev->l2ad_mtx);
                        list_insert_tail(&dev->l2ad_buflist, exists);
                        (void) zfs_refcount_add_many(&dev->l2ad_alloc,
                            arc_hdr_size(exists), exists);
                        mutex_exit(&dev->l2ad_mtx);
                        vdev_space_update(dev->l2ad_vdev, asize, 0, 0);
                        ARCSTAT_INCR(arcstat_l2_lsize, HDR_GET_LSIZE(exists));
                        ARCSTAT_INCR(arcstat_l2_psize, HDR_GET_PSIZE(exists));
                }
                ARCSTAT_BUMP(arcstat_l2_rebuild_bufs_precached);
        }

        mutex_exit(hash_lock);
}

/*
 * Starts an asynchronous read IO to read a log block. This is used in log
 * block reconstruction to start reading the next block before we are done
 * decoding and reconstructing the current block, to keep the l2arc device
 * nice and hot with read IO to process.
 * The returned zio will contain a newly allocated memory buffers for the IO
 * data which should then be freed by the caller once the zio is no longer
 * needed (i.e. due to it having completed). If you wish to abort this
 * zio, you should do so using l2arc_log_blk_fetch_abort, which takes
 * care of disposing of the allocated buffers correctly.
 */
static zio_t *
l2arc_log_blk_fetch(vdev_t *vd, const l2arc_log_blkptr_t *lbp,
    l2arc_log_blk_phys_t *lb)
{
        uint32_t                asize;
        zio_t                   *pio;
        l2arc_read_callback_t   *cb;

        /* L2BLK_GET_PSIZE returns aligned size for log blocks */
        asize = L2BLK_GET_PSIZE((lbp)->lbp_prop);
        ASSERT(asize <= sizeof (l2arc_log_blk_phys_t));

        cb = kmem_zalloc(sizeof (l2arc_read_callback_t), KM_SLEEP);
        cb->l2rcb_abd = abd_get_from_buf(lb, asize);
        pio = zio_root(vd->vdev_spa, l2arc_blk_fetch_done, cb,
            ZIO_FLAG_DONT_CACHE | ZIO_FLAG_CANFAIL | ZIO_FLAG_DONT_PROPAGATE |
            ZIO_FLAG_DONT_RETRY);
        (void) zio_nowait(zio_read_phys(pio, vd, lbp->lbp_daddr, asize,
            cb->l2rcb_abd, ZIO_CHECKSUM_OFF, NULL, NULL,
            ZIO_PRIORITY_ASYNC_READ, ZIO_FLAG_DONT_CACHE | ZIO_FLAG_CANFAIL |
            ZIO_FLAG_DONT_PROPAGATE | ZIO_FLAG_DONT_RETRY, B_FALSE));

        return (pio);
}

/*
 * Aborts a zio returned from l2arc_log_blk_fetch and frees the data
 * buffers allocated for it.
 */
static void
l2arc_log_blk_fetch_abort(zio_t *zio)
{
        (void) zio_wait(zio);
}

/*
 * Creates a zio to update the device header on an l2arc device.
 */
void
l2arc_dev_hdr_update(l2arc_dev_t *dev)
{
        l2arc_dev_hdr_phys_t    *l2dhdr = dev->l2ad_dev_hdr;
        const uint64_t          l2dhdr_asize = dev->l2ad_dev_hdr_asize;
        abd_t                   *abd;
        int                     err;

        VERIFY(spa_config_held(dev->l2ad_spa, SCL_STATE_ALL, RW_READER));

        l2dhdr->dh_magic = L2ARC_DEV_HDR_MAGIC;
        l2dhdr->dh_version = L2ARC_PERSISTENT_VERSION;
        l2dhdr->dh_spa_guid = spa_guid(dev->l2ad_vdev->vdev_spa);
        l2dhdr->dh_vdev_guid = dev->l2ad_vdev->vdev_guid;
        l2dhdr->dh_log_entries = dev->l2ad_log_entries;
        l2dhdr->dh_evict = dev->l2ad_evict;
        l2dhdr->dh_start = dev->l2ad_start;
        l2dhdr->dh_end = dev->l2ad_end;
        l2dhdr->dh_lb_asize = zfs_refcount_count(&dev->l2ad_lb_asize);
        l2dhdr->dh_lb_count = zfs_refcount_count(&dev->l2ad_lb_count);
        l2dhdr->dh_flags = 0;
        l2dhdr->dh_trim_action_time = dev->l2ad_vdev->vdev_trim_action_time;
        l2dhdr->dh_trim_state = dev->l2ad_vdev->vdev_trim_state;
        if (dev->l2ad_first)
                l2dhdr->dh_flags |= L2ARC_DEV_HDR_EVICT_FIRST;

        abd = abd_get_from_buf(l2dhdr, l2dhdr_asize);

        err = zio_wait(zio_write_phys(NULL, dev->l2ad_vdev,
            VDEV_LABEL_START_SIZE, l2dhdr_asize, abd, ZIO_CHECKSUM_LABEL, NULL,
            NULL, ZIO_PRIORITY_ASYNC_WRITE, ZIO_FLAG_CANFAIL, B_FALSE));

        abd_put(abd);

        if (err != 0) {
                zfs_dbgmsg("L2ARC IO error (%d) while writing device header, "
                    "vdev guid: %llu", err, dev->l2ad_vdev->vdev_guid);
        }
}

/*
 * Commits a log block to the L2ARC device. This routine is invoked from
 * l2arc_write_buffers when the log block fills up.
 * This function allocates some memory to temporarily hold the serialized
 * buffer to be written. This is then released in l2arc_write_done.
 */
static void
l2arc_log_blk_commit(l2arc_dev_t *dev, zio_t *pio, l2arc_write_callback_t *cb)
{
        l2arc_log_blk_phys_t    *lb = &dev->l2ad_log_blk;
        l2arc_dev_hdr_phys_t    *l2dhdr = dev->l2ad_dev_hdr;
        uint64_t                psize, asize;
        zio_t                   *wzio;
        l2arc_lb_abd_buf_t      *abd_buf;
        uint8_t                 *tmpbuf;
        l2arc_lb_ptr_buf_t      *lb_ptr_buf;

        VERIFY3S(dev->l2ad_log_ent_idx, ==, dev->l2ad_log_entries);

        tmpbuf = zio_buf_alloc(sizeof (*lb));
        abd_buf = zio_buf_alloc(sizeof (*abd_buf));
        abd_buf->abd = abd_get_from_buf(lb, sizeof (*lb));
        lb_ptr_buf = kmem_zalloc(sizeof (l2arc_lb_ptr_buf_t), KM_SLEEP);
        lb_ptr_buf->lb_ptr = kmem_zalloc(sizeof (l2arc_log_blkptr_t), KM_SLEEP);

        /* link the buffer into the block chain */
        lb->lb_prev_lbp = l2dhdr->dh_start_lbps[1];
        lb->lb_magic = L2ARC_LOG_BLK_MAGIC;

        /*
         * l2arc_log_blk_commit() may be called multiple times during a single
         * l2arc_write_buffers() call. Save the allocated abd buffers in a list
         * so we can free them in l2arc_write_done() later on.
         */
        list_insert_tail(&cb->l2wcb_abd_list, abd_buf);

        /* try to compress the buffer */
        psize = zio_compress_data(ZIO_COMPRESS_LZ4,
            abd_buf->abd, tmpbuf, sizeof (*lb), 0);

        /* a log block is never entirely zero */
        ASSERT(psize != 0);
        asize = vdev_psize_to_asize(dev->l2ad_vdev, psize);
        ASSERT(asize <= sizeof (*lb));

        /*
         * Update the start log block pointer in the device header to point
         * to the log block we're about to write.
         */
        l2dhdr->dh_start_lbps[1] = l2dhdr->dh_start_lbps[0];
        l2dhdr->dh_start_lbps[0].lbp_daddr = dev->l2ad_hand;
        l2dhdr->dh_start_lbps[0].lbp_payload_asize =
            dev->l2ad_log_blk_payload_asize;
        l2dhdr->dh_start_lbps[0].lbp_payload_start =
            dev->l2ad_log_blk_payload_start;
        _NOTE(CONSTCOND)
        L2BLK_SET_LSIZE(
            (&l2dhdr->dh_start_lbps[0])->lbp_prop, sizeof (*lb));
        L2BLK_SET_PSIZE(
            (&l2dhdr->dh_start_lbps[0])->lbp_prop, asize);
        L2BLK_SET_CHECKSUM(
            (&l2dhdr->dh_start_lbps[0])->lbp_prop,
            ZIO_CHECKSUM_FLETCHER_4);
        if (asize < sizeof (*lb)) {
                /* compression succeeded */
                bzero(tmpbuf + psize, asize - psize);
                L2BLK_SET_COMPRESS(
                    (&l2dhdr->dh_start_lbps[0])->lbp_prop,
                    ZIO_COMPRESS_LZ4);
        } else {
                /* compression failed */
                bcopy(lb, tmpbuf, sizeof (*lb));
                L2BLK_SET_COMPRESS(
                    (&l2dhdr->dh_start_lbps[0])->lbp_prop,
                    ZIO_COMPRESS_OFF);
        }

        /* checksum what we're about to write */
        fletcher_4_native(tmpbuf, asize, NULL,
            &l2dhdr->dh_start_lbps[0].lbp_cksum);

        abd_put(abd_buf->abd);

        /* perform the write itself */
        abd_buf->abd = abd_get_from_buf(tmpbuf, sizeof (*lb));
        abd_take_ownership_of_buf(abd_buf->abd, B_TRUE);
        wzio = zio_write_phys(pio, dev->l2ad_vdev, dev->l2ad_hand,
            asize, abd_buf->abd, ZIO_CHECKSUM_OFF, NULL, NULL,
            ZIO_PRIORITY_ASYNC_WRITE, ZIO_FLAG_CANFAIL, B_FALSE);
        DTRACE_PROBE2(l2arc__write, vdev_t *, dev->l2ad_vdev, zio_t *, wzio);
        (void) zio_nowait(wzio);

        dev->l2ad_hand += asize;
        /*
         * Include the committed log block's pointer  in the list of pointers
         * to log blocks present in the L2ARC device.
         */
        bcopy(&l2dhdr->dh_start_lbps[0], lb_ptr_buf->lb_ptr,
            sizeof (l2arc_log_blkptr_t));
        mutex_enter(&dev->l2ad_mtx);
        list_insert_head(&dev->l2ad_lbptr_list, lb_ptr_buf);
        ARCSTAT_INCR(arcstat_l2_log_blk_asize, asize);
        ARCSTAT_BUMP(arcstat_l2_log_blk_count);
        zfs_refcount_add_many(&dev->l2ad_lb_asize, asize, lb_ptr_buf);
        zfs_refcount_add(&dev->l2ad_lb_count, lb_ptr_buf);
        mutex_exit(&dev->l2ad_mtx);
        vdev_space_update(dev->l2ad_vdev, asize, 0, 0);

        /* bump the kstats */
        ARCSTAT_INCR(arcstat_l2_write_bytes, asize);
        ARCSTAT_BUMP(arcstat_l2_log_blk_writes);
        ARCSTAT_F_AVG(arcstat_l2_log_blk_avg_asize, asize);
        ARCSTAT_F_AVG(arcstat_l2_data_to_meta_ratio,
            dev->l2ad_log_blk_payload_asize / asize);

        /* start a new log block */
        dev->l2ad_log_ent_idx = 0;
        dev->l2ad_log_blk_payload_asize = 0;
        dev->l2ad_log_blk_payload_start = 0;
}

/*
 * Validates an L2ARC log block address to make sure that it can be read
 * from the provided L2ARC device.
 */
boolean_t
l2arc_log_blkptr_valid(l2arc_dev_t *dev, const l2arc_log_blkptr_t *lbp)
{
        /* L2BLK_GET_PSIZE returns aligned size for log blocks */
        uint64_t asize = L2BLK_GET_PSIZE((lbp)->lbp_prop);
        uint64_t end = lbp->lbp_daddr + asize - 1;
        uint64_t start = lbp->lbp_payload_start;
        boolean_t evicted = B_FALSE;

        /*
         * A log block is valid if all of the following conditions are true:
         * - it fits entirely (including its payload) between l2ad_start and
         *   l2ad_end
         * - it has a valid size
         * - neither the log block itself nor part of its payload was evicted
         *   by l2arc_evict():
         *
         *              l2ad_hand          l2ad_evict
         *              |                        |      lbp_daddr
         *              |     start              |      |  end
         *              |     |                  |      |  |
         *              V     V                  V      V  V
         *   l2ad_start ============================================ l2ad_end
         *                    --------------------------||||
         *                              ^                ^
         *                              |               log block
         *                              payload
         */

        evicted =
            l2arc_range_check_overlap(start, end, dev->l2ad_hand) ||
            l2arc_range_check_overlap(start, end, dev->l2ad_evict) ||
            l2arc_range_check_overlap(dev->l2ad_hand, dev->l2ad_evict, start) ||
            l2arc_range_check_overlap(dev->l2ad_hand, dev->l2ad_evict, end);

        return (start >= dev->l2ad_start && end <= dev->l2ad_end &&
            asize > 0 && asize <= sizeof (l2arc_log_blk_phys_t) &&
            (!evicted || dev->l2ad_first));
}

/*
 * Inserts ARC buffer header `hdr' into the current L2ARC log block on
 * the device. The buffer being inserted must be present in L2ARC.
 * Returns B_TRUE if the L2ARC log block is full and needs to be committed
 * to L2ARC, or B_FALSE if it still has room for more ARC buffers.
 */
static boolean_t
l2arc_log_blk_insert(l2arc_dev_t *dev, const arc_buf_hdr_t *hdr)
{
        l2arc_log_blk_phys_t    *lb = &dev->l2ad_log_blk;
        l2arc_log_ent_phys_t    *le;

        if (dev->l2ad_log_entries == 0)
                return (B_FALSE);

        int index = dev->l2ad_log_ent_idx++;

        ASSERT3S(index, <, dev->l2ad_log_entries);
        ASSERT(HDR_HAS_L2HDR(hdr));

        le = &lb->lb_entries[index];
        bzero(le, sizeof (*le));
        le->le_dva = hdr->b_dva;
        le->le_birth = hdr->b_birth;
        le->le_daddr = hdr->b_l2hdr.b_daddr;
        if (index == 0)
                dev->l2ad_log_blk_payload_start = le->le_daddr;
        L2BLK_SET_LSIZE((le)->le_prop, HDR_GET_LSIZE(hdr));
        L2BLK_SET_PSIZE((le)->le_prop, HDR_GET_PSIZE(hdr));
        L2BLK_SET_COMPRESS((le)->le_prop, HDR_GET_COMPRESS(hdr));
        le->le_complevel = hdr->b_complevel;
        L2BLK_SET_TYPE((le)->le_prop, hdr->b_type);
        L2BLK_SET_PROTECTED((le)->le_prop, !!(HDR_PROTECTED(hdr)));
        L2BLK_SET_PREFETCH((le)->le_prop, !!(HDR_PREFETCH(hdr)));

        dev->l2ad_log_blk_payload_asize += vdev_psize_to_asize(dev->l2ad_vdev,
            HDR_GET_PSIZE(hdr));

        return (dev->l2ad_log_ent_idx == dev->l2ad_log_entries);
}

/*
 * Checks whether a given L2ARC device address sits in a time-sequential
 * range. The trick here is that the L2ARC is a rotary buffer, so we can't
 * just do a range comparison, we need to handle the situation in which the
 * range wraps around the end of the L2ARC device. Arguments:
 *      bottom -- Lower end of the range to check (written to earlier).
 *      top    -- Upper end of the range to check (written to later).
 *      check  -- The address for which we want to determine if it sits in
 *                between the top and bottom.
 *
 * The 3-way conditional below represents the following cases:
 *
 *      bottom < top : Sequentially ordered case:
 *        <check>--------+-------------------+
 *                       |  (overlap here?)  |
 *       L2ARC dev       V                   V
 *       |---------------<bottom>============<top>--------------|
 *
 *      bottom > top: Looped-around case:
 *                            <check>--------+------------------+
 *                                           |  (overlap here?) |
 *       L2ARC dev                           V                  V
 *       |===============<top>---------------<bottom>===========|
 *       ^               ^
 *       |  (or here?)   |
 *       +---------------+---------<check>
 *
 *      top == bottom : Just a single address comparison.
 */
boolean_t
l2arc_range_check_overlap(uint64_t bottom, uint64_t top, uint64_t check)
{
        if (bottom < top)
                return (bottom <= check && check <= top);
        else if (bottom > top)
                return (check <= top || bottom <= check);
        else
                return (check == top);
}

EXPORT_SYMBOL(arc_buf_size);
EXPORT_SYMBOL(arc_write);
EXPORT_SYMBOL(arc_read);
EXPORT_SYMBOL(arc_buf_info);
EXPORT_SYMBOL(arc_getbuf_func);
EXPORT_SYMBOL(arc_add_prune_callback);
EXPORT_SYMBOL(arc_remove_prune_callback);

/* BEGIN CSTYLED */
ZFS_MODULE_PARAM_CALL(zfs_arc, zfs_arc_, min, param_set_arc_long,
        param_get_long, ZMOD_RW, "Min arc size");

ZFS_MODULE_PARAM_CALL(zfs_arc, zfs_arc_, max, param_set_arc_long,
        param_get_long, ZMOD_RW, "Max arc size");

ZFS_MODULE_PARAM_CALL(zfs_arc, zfs_arc_, meta_limit, param_set_arc_long,
        param_get_long, ZMOD_RW, "Metadata limit for arc size");

ZFS_MODULE_PARAM_CALL(zfs_arc, zfs_arc_, meta_limit_percent,
        param_set_arc_long, param_get_long, ZMOD_RW,
        "Percent of arc size for arc meta limit");

ZFS_MODULE_PARAM_CALL(zfs_arc, zfs_arc_, meta_min, param_set_arc_long,
        param_get_long, ZMOD_RW, "Min arc metadata");

ZFS_MODULE_PARAM(zfs_arc, zfs_arc_, meta_prune, INT, ZMOD_RW,
        "Meta objects to scan for prune");

ZFS_MODULE_PARAM(zfs_arc, zfs_arc_, meta_adjust_restarts, INT, ZMOD_RW,
        "Limit number of restarts in arc_evict_meta");

ZFS_MODULE_PARAM(zfs_arc, zfs_arc_, meta_strategy, INT, ZMOD_RW,
        "Meta reclaim strategy");

ZFS_MODULE_PARAM_CALL(zfs_arc, zfs_arc_, grow_retry, param_set_arc_int,
        param_get_int, ZMOD_RW, "Seconds before growing arc size");

ZFS_MODULE_PARAM(zfs_arc, zfs_arc_, p_dampener_disable, INT, ZMOD_RW,
        "Disable arc_p adapt dampener");

ZFS_MODULE_PARAM_CALL(zfs_arc, zfs_arc_, shrink_shift, param_set_arc_int,
        param_get_int, ZMOD_RW, "log2(fraction of arc to reclaim)");

ZFS_MODULE_PARAM(zfs_arc, zfs_arc_, pc_percent, UINT, ZMOD_RW,
        "Percent of pagecache to reclaim arc to");

ZFS_MODULE_PARAM_CALL(zfs_arc, zfs_arc_, p_min_shift, param_set_arc_int,
        param_get_int, ZMOD_RW, "arc_c shift to calc min/max arc_p");

ZFS_MODULE_PARAM(zfs_arc, zfs_arc_, average_blocksize, INT, ZMOD_RD,
        "Target average block size");

ZFS_MODULE_PARAM(zfs, zfs_, compressed_arc_enabled, INT, ZMOD_RW,
        "Disable compressed arc buffers");

ZFS_MODULE_PARAM_CALL(zfs_arc, zfs_arc_, min_prefetch_ms, param_set_arc_int,
        param_get_int, ZMOD_RW, "Min life of prefetch block in ms");

ZFS_MODULE_PARAM_CALL(zfs_arc, zfs_arc_, min_prescient_prefetch_ms,
        param_set_arc_int, param_get_int, ZMOD_RW,
        "Min life of prescient prefetched block in ms");

ZFS_MODULE_PARAM(zfs_l2arc, l2arc_, write_max, ULONG, ZMOD_RW,
        "Max write bytes per interval");

ZFS_MODULE_PARAM(zfs_l2arc, l2arc_, write_boost, ULONG, ZMOD_RW,
        "Extra write bytes during device warmup");

ZFS_MODULE_PARAM(zfs_l2arc, l2arc_, headroom, ULONG, ZMOD_RW,
        "Number of max device writes to precache");

ZFS_MODULE_PARAM(zfs_l2arc, l2arc_, headroom_boost, ULONG, ZMOD_RW,
        "Compressed l2arc_headroom multiplier");

ZFS_MODULE_PARAM(zfs_l2arc, l2arc_, trim_ahead, ULONG, ZMOD_RW,
        "TRIM ahead L2ARC write size multiplier");

ZFS_MODULE_PARAM(zfs_l2arc, l2arc_, feed_secs, ULONG, ZMOD_RW,
        "Seconds between L2ARC writing");

ZFS_MODULE_PARAM(zfs_l2arc, l2arc_, feed_min_ms, ULONG, ZMOD_RW,
        "Min feed interval in milliseconds");

ZFS_MODULE_PARAM(zfs_l2arc, l2arc_, noprefetch, INT, ZMOD_RW,
        "Skip caching prefetched buffers");

ZFS_MODULE_PARAM(zfs_l2arc, l2arc_, feed_again, INT, ZMOD_RW,
        "Turbo L2ARC warmup");

ZFS_MODULE_PARAM(zfs_l2arc, l2arc_, norw, INT, ZMOD_RW,
        "No reads during writes");

ZFS_MODULE_PARAM(zfs_l2arc, l2arc_, meta_percent, INT, ZMOD_RW,
        "Percent of ARC size allowed for L2ARC-only headers");

ZFS_MODULE_PARAM(zfs_l2arc, l2arc_, rebuild_enabled, INT, ZMOD_RW,
        "Rebuild the L2ARC when importing a pool");

ZFS_MODULE_PARAM(zfs_l2arc, l2arc_, rebuild_blocks_min_l2size, ULONG, ZMOD_RW,
        "Min size in bytes to write rebuild log blocks in L2ARC");

ZFS_MODULE_PARAM(zfs_l2arc, l2arc_, mfuonly, INT, ZMOD_RW,
        "Cache only MFU data from ARC into L2ARC");

ZFS_MODULE_PARAM_CALL(zfs_arc, zfs_arc_, lotsfree_percent, param_set_arc_int,
        param_get_int, ZMOD_RW, "System free memory I/O throttle in bytes");

ZFS_MODULE_PARAM_CALL(zfs_arc, zfs_arc_, sys_free, param_set_arc_long,
        param_get_long, ZMOD_RW, "System free memory target size in bytes");

ZFS_MODULE_PARAM_CALL(zfs_arc, zfs_arc_, dnode_limit, param_set_arc_long,
        param_get_long, ZMOD_RW, "Minimum bytes of dnodes in arc");

ZFS_MODULE_PARAM_CALL(zfs_arc, zfs_arc_, dnode_limit_percent,
        param_set_arc_long, param_get_long, ZMOD_RW,
        "Percent of ARC meta buffers for dnodes");

ZFS_MODULE_PARAM(zfs_arc, zfs_arc_, dnode_reduce_percent, ULONG, ZMOD_RW,
        "Percentage of excess dnodes to try to unpin");

ZFS_MODULE_PARAM(zfs_arc, zfs_arc_, eviction_pct, INT, ZMOD_RW,
       "When full, ARC allocation waits for eviction of this % of alloc size");
/* END CSTYLED */