MFV r336958: 9337 zfs get all is slow due to uncached metadata

[FreeBSD/FreeBSD.git] / sys / cddl / contrib / opensolaris / uts / common / fs / zfs / dbuf.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 2011 Nexenta Systems, Inc.  All rights reserved.
 * Copyright (c) 2012, 2017 by Delphix. All rights reserved.
 * Copyright (c) 2013 by Saso Kiselkov. All rights reserved.
 * Copyright (c) 2013, Joyent, Inc. All rights reserved.
 * Copyright (c) 2014 Spectra Logic Corporation, All rights reserved.
 * Copyright (c) 2014 Integros [integros.com]
 */

#include <sys/zfs_context.h>
#include <sys/dmu.h>
#include <sys/dmu_send.h>
#include <sys/dmu_impl.h>
#include <sys/dbuf.h>
#include <sys/dmu_objset.h>
#include <sys/dsl_dataset.h>
#include <sys/dsl_dir.h>
#include <sys/dmu_tx.h>
#include <sys/spa.h>
#include <sys/zio.h>
#include <sys/dmu_zfetch.h>
#include <sys/sa.h>
#include <sys/sa_impl.h>
#include <sys/zfeature.h>
#include <sys/blkptr.h>
#include <sys/range_tree.h>
#include <sys/callb.h>
#include <sys/abd.h>
#include <sys/vdev.h>
#include <sys/cityhash.h>
#include <sys/spa_impl.h>

uint_t zfs_dbuf_evict_key;

static boolean_t dbuf_undirty(dmu_buf_impl_t *db, dmu_tx_t *tx);
static void dbuf_write(dbuf_dirty_record_t *dr, arc_buf_t *data, dmu_tx_t *tx);

#ifndef __lint
extern inline void dmu_buf_init_user(dmu_buf_user_t *dbu,
    dmu_buf_evict_func_t *evict_func_sync,
    dmu_buf_evict_func_t *evict_func_async,
    dmu_buf_t **clear_on_evict_dbufp);
#endif /* ! __lint */

/*
 * Global data structures and functions for the dbuf cache.
 */
static kmem_cache_t *dbuf_kmem_cache;
static taskq_t *dbu_evict_taskq;

static kthread_t *dbuf_cache_evict_thread;
static kmutex_t dbuf_evict_lock;
static kcondvar_t dbuf_evict_cv;
static boolean_t dbuf_evict_thread_exit;

/*
 * There are two dbuf caches; each dbuf can only be in one of them at a time.
 *
 * 1. Cache of metadata dbufs, to help make read-heavy administrative commands
 *    from /sbin/zfs run faster. The "metadata cache" specifically stores dbufs
 *    that represent the metadata that describes filesystems/snapshots/
 *    bookmarks/properties/etc. We only evict from this cache when we export a
 *    pool, to short-circuit as much I/O as possible for all administrative
 *    commands that need the metadata. There is no eviction policy for this
 *    cache, because we try to only include types in it which would occupy a
 *    very small amount of space per object but create a large impact on the
 *    performance of these commands. Instead, after it reaches a maximum size
 *    (which should only happen on very small memory systems with a very large
 *    number of filesystem objects), we stop taking new dbufs into the
 *    metadata cache, instead putting them in the normal dbuf cache.
 *
 * 2. LRU cache of dbufs. The "dbuf cache" maintains a list of dbufs that
 *    are not currently held but have been recently released. These dbufs
 *    are not eligible for arc eviction until they are aged out of the cache.
 *    Dbufs that are aged out of the cache will be immediately destroyed and
 *    become eligible for arc eviction.
 *
 * Dbufs are added to these caches once the last hold is released. If a dbuf is
 * later accessed and still exists in the dbuf cache, then it will be removed
 * from the cache and later re-added to the head of the cache.
 *
 * If a given dbuf meets the requirements for the metadata cache, it will go
 * there, otherwise it will be considered for the generic LRU dbuf cache. The
 * caches and the refcounts tracking their sizes are stored in an array indexed
 * by those caches' matching enum values (from dbuf_cached_state_t).
 */
typedef struct dbuf_cache {
        multilist_t *cache;
        refcount_t size;
} dbuf_cache_t;
dbuf_cache_t dbuf_caches[DB_CACHE_MAX];

/* Size limits for the caches */
uint64_t dbuf_cache_max_bytes = 0;
uint64_t dbuf_metadata_cache_max_bytes = 0;
/* Set the default sizes of the caches to log2 fraction of arc size */
int dbuf_cache_shift = 5;
int dbuf_metadata_cache_shift = 6;

/*
 * For diagnostic purposes, this is incremented whenever we can't add
 * something to the metadata cache because it's full, and instead put
 * the data in the regular dbuf cache.
 */
uint64_t dbuf_metadata_cache_overflow;

/*
 * The LRU dbuf cache uses a three-stage eviction policy:
 *      - A low water marker designates when the dbuf eviction thread
 *      should stop evicting from the dbuf cache.
 *      - When we reach the maximum size (aka mid water mark), we
 *      signal the eviction thread to run.
 *      - The high water mark indicates when the eviction thread
 *      is unable to keep up with the incoming load and eviction must
 *      happen in the context of the calling thread.
 *
 * The dbuf cache:
 *                                                 (max size)
 *                                      low water   mid water   hi water
 * +----------------------------------------+----------+----------+
 * |                                        |          |          |
 * |                                        |          |          |
 * |                                        |          |          |
 * |                                        |          |          |
 * +----------------------------------------+----------+----------+
 *                                        stop        signal     evict
 *                                      evicting     eviction   directly
 *                                                    thread
 *
 * The high and low water marks indicate the operating range for the eviction
 * thread. The low water mark is, by default, 90% of the total size of the
 * cache and the high water mark is at 110% (both of these percentages can be
 * changed by setting dbuf_cache_lowater_pct and dbuf_cache_hiwater_pct,
 * respectively). The eviction thread will try to ensure that the cache remains
 * within this range by waking up every second and checking if the cache is
 * above the low water mark. The thread can also be woken up by callers adding
 * elements into the cache if the cache is larger than the mid water (i.e max
 * cache size). Once the eviction thread is woken up and eviction is required,
 * it will continue evicting buffers until it's able to reduce the cache size
 * to the low water mark. If the cache size continues to grow and hits the high
 * water mark, then callers adding elments to the cache will begin to evict
 * directly from the cache until the cache is no longer above the high water
 * mark.
 */

/*
 * The percentage above and below the maximum cache size.
 */
uint_t dbuf_cache_hiwater_pct = 10;
uint_t dbuf_cache_lowater_pct = 10;

SYSCTL_DECL(_vfs_zfs);
SYSCTL_QUAD(_vfs_zfs, OID_AUTO, dbuf_cache_max_bytes, CTLFLAG_RWTUN,
    &dbuf_cache_max_bytes, 0, "dbuf cache size in bytes");
SYSCTL_INT(_vfs_zfs, OID_AUTO, dbuf_cache_shift, CTLFLAG_RDTUN,
    &dbuf_cache_shift, 0, "dbuf cache size as log2 fraction of ARC");
SYSCTL_UINT(_vfs_zfs, OID_AUTO, dbuf_cache_hiwater_pct, CTLFLAG_RWTUN,
    &dbuf_cache_hiwater_pct, 0, "max percents above the dbuf cache size");
SYSCTL_UINT(_vfs_zfs, OID_AUTO, dbuf_cache_lowater_pct, CTLFLAG_RWTUN,
    &dbuf_cache_lowater_pct, 0, "max percents below the dbuf cache size");

/* ARGSUSED */
static int
dbuf_cons(void *vdb, void *unused, int kmflag)
{
        dmu_buf_impl_t *db = vdb;
        bzero(db, sizeof (dmu_buf_impl_t));

        mutex_init(&db->db_mtx, NULL, MUTEX_DEFAULT, NULL);
        cv_init(&db->db_changed, NULL, CV_DEFAULT, NULL);
        multilist_link_init(&db->db_cache_link);
        refcount_create(&db->db_holds);

        return (0);
}

/* ARGSUSED */
static void
dbuf_dest(void *vdb, void *unused)
{
        dmu_buf_impl_t *db = vdb;
        mutex_destroy(&db->db_mtx);
        cv_destroy(&db->db_changed);
        ASSERT(!multilist_link_active(&db->db_cache_link));
        refcount_destroy(&db->db_holds);
}

/*
 * dbuf hash table routines
 */
static dbuf_hash_table_t dbuf_hash_table;

static uint64_t dbuf_hash_count;

/*
 * We use Cityhash for this. It's fast, and has good hash properties without
 * requiring any large static buffers.
 */
static uint64_t
dbuf_hash(void *os, uint64_t obj, uint8_t lvl, uint64_t blkid)
{
        return (cityhash4((uintptr_t)os, obj, (uint64_t)lvl, blkid));
}

#define DBUF_EQUAL(dbuf, os, obj, level, blkid)         \
        ((dbuf)->db.db_object == (obj) &&               \
        (dbuf)->db_objset == (os) &&                    \
        (dbuf)->db_level == (level) &&                  \
        (dbuf)->db_blkid == (blkid))

dmu_buf_impl_t *
dbuf_find(objset_t *os, uint64_t obj, uint8_t level, uint64_t blkid)
{
        dbuf_hash_table_t *h = &dbuf_hash_table;
        uint64_t hv = dbuf_hash(os, obj, level, blkid);
        uint64_t idx = hv & h->hash_table_mask;
        dmu_buf_impl_t *db;

        mutex_enter(DBUF_HASH_MUTEX(h, idx));
        for (db = h->hash_table[idx]; db != NULL; db = db->db_hash_next) {
                if (DBUF_EQUAL(db, os, obj, level, blkid)) {
                        mutex_enter(&db->db_mtx);
                        if (db->db_state != DB_EVICTING) {
                                mutex_exit(DBUF_HASH_MUTEX(h, idx));
                                return (db);
                        }
                        mutex_exit(&db->db_mtx);
                }
        }
        mutex_exit(DBUF_HASH_MUTEX(h, idx));
        return (NULL);
}

static dmu_buf_impl_t *
dbuf_find_bonus(objset_t *os, uint64_t object)
{
        dnode_t *dn;
        dmu_buf_impl_t *db = NULL;

        if (dnode_hold(os, object, FTAG, &dn) == 0) {
                rw_enter(&dn->dn_struct_rwlock, RW_READER);
                if (dn->dn_bonus != NULL) {
                        db = dn->dn_bonus;
                        mutex_enter(&db->db_mtx);
                }
                rw_exit(&dn->dn_struct_rwlock);
                dnode_rele(dn, FTAG);
        }
        return (db);
}

/*
 * 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.
 */
static dmu_buf_impl_t *
dbuf_hash_insert(dmu_buf_impl_t *db)
{
        dbuf_hash_table_t *h = &dbuf_hash_table;
        objset_t *os = db->db_objset;
        uint64_t obj = db->db.db_object;
        int level = db->db_level;
        uint64_t blkid = db->db_blkid;
        uint64_t hv = dbuf_hash(os, obj, level, blkid);
        uint64_t idx = hv & h->hash_table_mask;
        dmu_buf_impl_t *dbf;

        mutex_enter(DBUF_HASH_MUTEX(h, idx));
        for (dbf = h->hash_table[idx]; dbf != NULL; dbf = dbf->db_hash_next) {
                if (DBUF_EQUAL(dbf, os, obj, level, blkid)) {
                        mutex_enter(&dbf->db_mtx);
                        if (dbf->db_state != DB_EVICTING) {
                                mutex_exit(DBUF_HASH_MUTEX(h, idx));
                                return (dbf);
                        }
                        mutex_exit(&dbf->db_mtx);
                }
        }

        mutex_enter(&db->db_mtx);
        db->db_hash_next = h->hash_table[idx];
        h->hash_table[idx] = db;
        mutex_exit(DBUF_HASH_MUTEX(h, idx));
        atomic_inc_64(&dbuf_hash_count);

        return (NULL);
}

/*
 * Remove an entry from the hash table.  It must be in the EVICTING state.
 */
static void
dbuf_hash_remove(dmu_buf_impl_t *db)
{
        dbuf_hash_table_t *h = &dbuf_hash_table;
        uint64_t hv = dbuf_hash(db->db_objset, db->db.db_object,
            db->db_level, db->db_blkid);
        uint64_t idx = hv & h->hash_table_mask;
        dmu_buf_impl_t *dbf, **dbp;

        /*
         * We musn't hold db_mtx to maintain lock ordering:
         * DBUF_HASH_MUTEX > db_mtx.
         */
        ASSERT(refcount_is_zero(&db->db_holds));
        ASSERT(db->db_state == DB_EVICTING);
        ASSERT(!MUTEX_HELD(&db->db_mtx));

        mutex_enter(DBUF_HASH_MUTEX(h, idx));
        dbp = &h->hash_table[idx];
        while ((dbf = *dbp) != db) {
                dbp = &dbf->db_hash_next;
                ASSERT(dbf != NULL);
        }
        *dbp = db->db_hash_next;
        db->db_hash_next = NULL;
        mutex_exit(DBUF_HASH_MUTEX(h, idx));
        atomic_dec_64(&dbuf_hash_count);
}

typedef enum {
        DBVU_EVICTING,
        DBVU_NOT_EVICTING
} dbvu_verify_type_t;

static void
dbuf_verify_user(dmu_buf_impl_t *db, dbvu_verify_type_t verify_type)
{
#ifdef ZFS_DEBUG
        int64_t holds;

        if (db->db_user == NULL)
                return;

        /* Only data blocks support the attachment of user data. */
        ASSERT(db->db_level == 0);

        /* Clients must resolve a dbuf before attaching user data. */
        ASSERT(db->db.db_data != NULL);
        ASSERT3U(db->db_state, ==, DB_CACHED);

        holds = refcount_count(&db->db_holds);
        if (verify_type == DBVU_EVICTING) {
                /*
                 * Immediate eviction occurs when holds == dirtycnt.
                 * For normal eviction buffers, holds is zero on
                 * eviction, except when dbuf_fix_old_data() calls
                 * dbuf_clear_data().  However, the hold count can grow
                 * during eviction even though db_mtx is held (see
                 * dmu_bonus_hold() for an example), so we can only
                 * test the generic invariant that holds >= dirtycnt.
                 */
                ASSERT3U(holds, >=, db->db_dirtycnt);
        } else {
                if (db->db_user_immediate_evict == TRUE)
                        ASSERT3U(holds, >=, db->db_dirtycnt);
                else
                        ASSERT3U(holds, >, 0);
        }
#endif
}

static void
dbuf_evict_user(dmu_buf_impl_t *db)
{
        dmu_buf_user_t *dbu = db->db_user;

        ASSERT(MUTEX_HELD(&db->db_mtx));

        if (dbu == NULL)
                return;

        dbuf_verify_user(db, DBVU_EVICTING);
        db->db_user = NULL;

#ifdef ZFS_DEBUG
        if (dbu->dbu_clear_on_evict_dbufp != NULL)
                *dbu->dbu_clear_on_evict_dbufp = NULL;
#endif

        /*
         * There are two eviction callbacks - one that we call synchronously
         * and one that we invoke via a taskq.  The async one is useful for
         * avoiding lock order reversals and limiting stack depth.
         *
         * Note that if we have a sync callback but no async callback,
         * it's likely that the sync callback will free the structure
         * containing the dbu.  In that case we need to take care to not
         * dereference dbu after calling the sync evict func.
         */
        boolean_t has_async = (dbu->dbu_evict_func_async != NULL);

        if (dbu->dbu_evict_func_sync != NULL)
                dbu->dbu_evict_func_sync(dbu);

        if (has_async) {
                taskq_dispatch_ent(dbu_evict_taskq, dbu->dbu_evict_func_async,
                    dbu, 0, &dbu->dbu_tqent);
        }
}

boolean_t
dbuf_is_metadata(dmu_buf_impl_t *db)
{
        if (db->db_level > 0) {
                return (B_TRUE);
        } else {
                boolean_t is_metadata;

                DB_DNODE_ENTER(db);
                is_metadata = DMU_OT_IS_METADATA(DB_DNODE(db)->dn_type);
                DB_DNODE_EXIT(db);

                return (is_metadata);
        }
}

/*
 * This returns whether this dbuf should be stored in the metadata cache, which
 * is based on whether it's from one of the dnode types that store data related
 * to traversing dataset hierarchies.
 */
static boolean_t
dbuf_include_in_metadata_cache(dmu_buf_impl_t *db)
{
        DB_DNODE_ENTER(db);
        dmu_object_type_t type = DB_DNODE(db)->dn_type;
        DB_DNODE_EXIT(db);

        /* Check if this dbuf is one of the types we care about */
        if (DMU_OT_IS_METADATA_CACHED(type)) {
                /* If we hit this, then we set something up wrong in dmu_ot */
                ASSERT(DMU_OT_IS_METADATA(type));

                /*
                 * Sanity check for small-memory systems: don't allocate too
                 * much memory for this purpose.
                 */
                if (refcount_count(&dbuf_caches[DB_DBUF_METADATA_CACHE].size) >
                    dbuf_metadata_cache_max_bytes) {
                        dbuf_metadata_cache_overflow++;
                        DTRACE_PROBE1(dbuf__metadata__cache__overflow,
                            dmu_buf_impl_t *, db);
                        return (B_FALSE);
                }

                return (B_TRUE);
        }

        return (B_FALSE);
}

/*
 * This function *must* return indices evenly distributed between all
 * sublists of the multilist. This is needed due to how the dbuf eviction
 * code is laid out; dbuf_evict_thread() assumes dbufs are evenly
 * distributed between all sublists and uses this assumption when
 * deciding which sublist to evict from and how much to evict from it.
 */
unsigned int
dbuf_cache_multilist_index_func(multilist_t *ml, void *obj)
{
        dmu_buf_impl_t *db = obj;

        /*
         * The assumption here, is the hash value for a given
         * dmu_buf_impl_t will remain constant throughout it's lifetime
         * (i.e. it's objset, object, level and blkid fields don't change).
         * Thus, we don't need to store the dbuf'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 (dbuf_hash(db->db_objset, db->db.db_object,
            db->db_level, db->db_blkid) %
            multilist_get_num_sublists(ml));
}

static inline boolean_t
dbuf_cache_above_hiwater(void)
{
        uint64_t dbuf_cache_hiwater_bytes =
            (dbuf_cache_max_bytes * dbuf_cache_hiwater_pct) / 100;

        return (refcount_count(&dbuf_caches[DB_DBUF_CACHE].size) >
            dbuf_cache_max_bytes + dbuf_cache_hiwater_bytes);
}

static inline boolean_t
dbuf_cache_above_lowater(void)
{
        uint64_t dbuf_cache_lowater_bytes =
            (dbuf_cache_max_bytes * dbuf_cache_lowater_pct) / 100;

        return (refcount_count(&dbuf_caches[DB_DBUF_CACHE].size) >
            dbuf_cache_max_bytes - dbuf_cache_lowater_bytes);
}

/*
 * Evict the oldest eligible dbuf from the dbuf cache.
 */
static void
dbuf_evict_one(void)
{
        int idx = multilist_get_random_index(dbuf_caches[DB_DBUF_CACHE].cache);
        multilist_sublist_t *mls = multilist_sublist_lock(
            dbuf_caches[DB_DBUF_CACHE].cache, idx);

        ASSERT(!MUTEX_HELD(&dbuf_evict_lock));

        /*
         * Set the thread's tsd to indicate that it's processing evictions.
         * Once a thread stops evicting from the dbuf cache it will
         * reset its tsd to NULL.
         */
        ASSERT3P(tsd_get(zfs_dbuf_evict_key), ==, NULL);
        (void) tsd_set(zfs_dbuf_evict_key, (void *)B_TRUE);

        dmu_buf_impl_t *db = multilist_sublist_tail(mls);
        while (db != NULL && mutex_tryenter(&db->db_mtx) == 0) {
                db = multilist_sublist_prev(mls, db);
        }

        DTRACE_PROBE2(dbuf__evict__one, dmu_buf_impl_t *, db,
            multilist_sublist_t *, mls);

        if (db != NULL) {
                multilist_sublist_remove(mls, db);
                multilist_sublist_unlock(mls);
                (void) refcount_remove_many(&dbuf_caches[DB_DBUF_CACHE].size,
                    db->db.db_size, db);
                ASSERT3U(db->db_caching_status, ==, DB_DBUF_CACHE);
                db->db_caching_status = DB_NO_CACHE;
                dbuf_destroy(db);
        } else {
                multilist_sublist_unlock(mls);
        }
        (void) tsd_set(zfs_dbuf_evict_key, NULL);
}

/*
 * The dbuf evict thread is responsible for aging out dbufs from the
 * cache. Once the cache has reached it's maximum size, dbufs are removed
 * and destroyed. The eviction thread will continue running until the size
 * of the dbuf cache is at or below the maximum size. Once the dbuf is aged
 * out of the cache it is destroyed and becomes eligible for arc eviction.
 */
/* ARGSUSED */
static void
dbuf_evict_thread(void *unused __unused)
{
        callb_cpr_t cpr;

        CALLB_CPR_INIT(&cpr, &dbuf_evict_lock, callb_generic_cpr, FTAG);

        mutex_enter(&dbuf_evict_lock);
        while (!dbuf_evict_thread_exit) {
                while (!dbuf_cache_above_lowater() && !dbuf_evict_thread_exit) {
                        CALLB_CPR_SAFE_BEGIN(&cpr);
                        (void) cv_timedwait_hires(&dbuf_evict_cv,
                            &dbuf_evict_lock, SEC2NSEC(1), MSEC2NSEC(1), 0);
                        CALLB_CPR_SAFE_END(&cpr, &dbuf_evict_lock);
                }
                mutex_exit(&dbuf_evict_lock);

                /*
                 * Keep evicting as long as we're above the low water mark
                 * for the cache. We do this without holding the locks to
                 * minimize lock contention.
                 */
                while (dbuf_cache_above_lowater() && !dbuf_evict_thread_exit) {
                        dbuf_evict_one();
                }

                mutex_enter(&dbuf_evict_lock);
        }

        dbuf_evict_thread_exit = B_FALSE;
        cv_broadcast(&dbuf_evict_cv);
        CALLB_CPR_EXIT(&cpr);   /* drops dbuf_evict_lock */
        thread_exit();
}

/*
 * Wake up the dbuf eviction thread if the dbuf cache is at its max size.
 * If the dbuf cache is at its high water mark, then evict a dbuf from the
 * dbuf cache using the callers context.
 */
static void
dbuf_evict_notify(void)
{

        /*
         * We use thread specific data to track when a thread has
         * started processing evictions. This allows us to avoid deeply
         * nested stacks that would have a call flow similar to this:
         *
         * dbuf_rele()-->dbuf_rele_and_unlock()-->dbuf_evict_notify()
         *      ^                                               |
         *      |                                               |
         *      +-----dbuf_destroy()<--dbuf_evict_one()<--------+
         *
         * The dbuf_eviction_thread will always have its tsd set until
         * that thread exits. All other threads will only set their tsd
         * if they are participating in the eviction process. This only
         * happens if the eviction thread is unable to process evictions
         * fast enough. To keep the dbuf cache size in check, other threads
         * can evict from the dbuf cache directly. Those threads will set
         * their tsd values so that we ensure that they only evict one dbuf
         * from the dbuf cache.
         */
        if (tsd_get(zfs_dbuf_evict_key) != NULL)
                return;

        /*
         * We check if we should evict without holding the dbuf_evict_lock,
         * because it's OK to occasionally make the wrong decision here,
         * and grabbing the lock results in massive lock contention.
         */
        if (refcount_count(&dbuf_caches[DB_DBUF_CACHE].size) >
            dbuf_cache_max_bytes) {
                if (dbuf_cache_above_hiwater())
                        dbuf_evict_one();
                cv_signal(&dbuf_evict_cv);
        }
}

void
dbuf_init(void)
{
        uint64_t hsize = 1ULL << 16;
        dbuf_hash_table_t *h = &dbuf_hash_table;
        int i;

        /*
         * The hash table is big enough to fill all of physical memory
         * with an average 4K block size.  The table will take up
         * totalmem*sizeof(void*)/4K (i.e. 2MB/GB with 8-byte pointers).
         */
        while (hsize * 4096 < (uint64_t)physmem * PAGESIZE)
                hsize <<= 1;

retry:
        h->hash_table_mask = hsize - 1;
        h->hash_table = kmem_zalloc(hsize * sizeof (void *), KM_NOSLEEP);
        if (h->hash_table == NULL) {
                /* XXX - we should really return an error instead of assert */
                ASSERT(hsize > (1ULL << 10));
                hsize >>= 1;
                goto retry;
        }

        dbuf_kmem_cache = kmem_cache_create("dmu_buf_impl_t",
            sizeof (dmu_buf_impl_t),
            0, dbuf_cons, dbuf_dest, NULL, NULL, NULL, 0);

        for (i = 0; i < DBUF_MUTEXES; i++)
                mutex_init(&h->hash_mutexes[i], NULL, MUTEX_DEFAULT, NULL);

        /*
         * Setup the parameters for the dbuf caches. We set the sizes of the
         * dbuf cache and the metadata cache to 1/32nd and 1/16th (default)
         * of the size of the ARC, respectively. If the values are set in
         * /etc/system and they're not greater than the size of the ARC, then
         * we honor that value.
         */
        if (dbuf_cache_max_bytes == 0 ||
            dbuf_cache_max_bytes >= arc_max_bytes())  {
                dbuf_cache_max_bytes = arc_max_bytes() >> dbuf_cache_shift;
        }
        if (dbuf_metadata_cache_max_bytes == 0 ||
            dbuf_metadata_cache_max_bytes >= arc_max_bytes()) {
                dbuf_metadata_cache_max_bytes =
                    arc_max_bytes() >> dbuf_metadata_cache_shift;
        }

        /*
         * All entries are queued via taskq_dispatch_ent(), so min/maxalloc
         * configuration is not required.
         */
        dbu_evict_taskq = taskq_create("dbu_evict", 1, minclsyspri, 0, 0, 0);

        for (dbuf_cached_state_t dcs = 0; dcs < DB_CACHE_MAX; dcs++) {
                dbuf_caches[dcs].cache =
                    multilist_create(sizeof (dmu_buf_impl_t),
                    offsetof(dmu_buf_impl_t, db_cache_link),
                    dbuf_cache_multilist_index_func);
                refcount_create(&dbuf_caches[dcs].size);
        }

        tsd_create(&zfs_dbuf_evict_key, NULL);
        dbuf_evict_thread_exit = B_FALSE;
        mutex_init(&dbuf_evict_lock, NULL, MUTEX_DEFAULT, NULL);
        cv_init(&dbuf_evict_cv, NULL, CV_DEFAULT, NULL);
        dbuf_cache_evict_thread = thread_create(NULL, 0, dbuf_evict_thread,
            NULL, 0, &p0, TS_RUN, minclsyspri);
}

void
dbuf_fini(void)
{
        dbuf_hash_table_t *h = &dbuf_hash_table;
        int i;

        for (i = 0; i < DBUF_MUTEXES; i++)
                mutex_destroy(&h->hash_mutexes[i]);
        kmem_free(h->hash_table, (h->hash_table_mask + 1) * sizeof (void *));
        kmem_cache_destroy(dbuf_kmem_cache);
        taskq_destroy(dbu_evict_taskq);

        mutex_enter(&dbuf_evict_lock);
        dbuf_evict_thread_exit = B_TRUE;
        while (dbuf_evict_thread_exit) {
                cv_signal(&dbuf_evict_cv);
                cv_wait(&dbuf_evict_cv, &dbuf_evict_lock);
        }
        mutex_exit(&dbuf_evict_lock);
        tsd_destroy(&zfs_dbuf_evict_key);

        mutex_destroy(&dbuf_evict_lock);
        cv_destroy(&dbuf_evict_cv);

        for (dbuf_cached_state_t dcs = 0; dcs < DB_CACHE_MAX; dcs++) {
                refcount_destroy(&dbuf_caches[dcs].size);
                multilist_destroy(dbuf_caches[dcs].cache);
        }
}

/*
 * Other stuff.
 */

#ifdef ZFS_DEBUG
static void
dbuf_verify(dmu_buf_impl_t *db)
{
        dnode_t *dn;
        dbuf_dirty_record_t *dr;

        ASSERT(MUTEX_HELD(&db->db_mtx));

        if (!(zfs_flags & ZFS_DEBUG_DBUF_VERIFY))
                return;

        ASSERT(db->db_objset != NULL);
        DB_DNODE_ENTER(db);
        dn = DB_DNODE(db);
        if (dn == NULL) {
                ASSERT(db->db_parent == NULL);
                ASSERT(db->db_blkptr == NULL);
        } else {
                ASSERT3U(db->db.db_object, ==, dn->dn_object);
                ASSERT3P(db->db_objset, ==, dn->dn_objset);
                ASSERT3U(db->db_level, <, dn->dn_nlevels);
                ASSERT(db->db_blkid == DMU_BONUS_BLKID ||
                    db->db_blkid == DMU_SPILL_BLKID ||
                    !avl_is_empty(&dn->dn_dbufs));
        }
        if (db->db_blkid == DMU_BONUS_BLKID) {
                ASSERT(dn != NULL);
                ASSERT3U(db->db.db_size, >=, dn->dn_bonuslen);
                ASSERT3U(db->db.db_offset, ==, DMU_BONUS_BLKID);
        } else if (db->db_blkid == DMU_SPILL_BLKID) {
                ASSERT(dn != NULL);
                ASSERT3U(db->db.db_size, >=, dn->dn_bonuslen);
                ASSERT0(db->db.db_offset);
        } else {
                ASSERT3U(db->db.db_offset, ==, db->db_blkid * db->db.db_size);
        }

        for (dr = db->db_data_pending; dr != NULL; dr = dr->dr_next)
                ASSERT(dr->dr_dbuf == db);

        for (dr = db->db_last_dirty; dr != NULL; dr = dr->dr_next)
                ASSERT(dr->dr_dbuf == db);

        /*
         * We can't assert that db_size matches dn_datablksz because it
         * can be momentarily different when another thread is doing
         * dnode_set_blksz().
         */
        if (db->db_level == 0 && db->db.db_object == DMU_META_DNODE_OBJECT) {
                dr = db->db_data_pending;
                /*
                 * It should only be modified in syncing context, so
                 * make sure we only have one copy of the data.
                 */
                ASSERT(dr == NULL || dr->dt.dl.dr_data == db->db_buf);
        }

        /* verify db->db_blkptr */
        if (db->db_blkptr) {
                if (db->db_parent == dn->dn_dbuf) {
                        /* db is pointed to by the dnode */
                        /* ASSERT3U(db->db_blkid, <, dn->dn_nblkptr); */
                        if (DMU_OBJECT_IS_SPECIAL(db->db.db_object))
                                ASSERT(db->db_parent == NULL);
                        else
                                ASSERT(db->db_parent != NULL);
                        if (db->db_blkid != DMU_SPILL_BLKID)
                                ASSERT3P(db->db_blkptr, ==,
                                    &dn->dn_phys->dn_blkptr[db->db_blkid]);
                } else {
                        /* db is pointed to by an indirect block */
                        int epb = db->db_parent->db.db_size >> SPA_BLKPTRSHIFT;
                        ASSERT3U(db->db_parent->db_level, ==, db->db_level+1);
                        ASSERT3U(db->db_parent->db.db_object, ==,
                            db->db.db_object);
                        /*
                         * dnode_grow_indblksz() can make this fail if we don't
                         * have the struct_rwlock.  XXX indblksz no longer
                         * grows.  safe to do this now?
                         */
                        if (RW_WRITE_HELD(&dn->dn_struct_rwlock)) {
                                ASSERT3P(db->db_blkptr, ==,
                                    ((blkptr_t *)db->db_parent->db.db_data +
                                    db->db_blkid % epb));
                        }
                }
        }
        if ((db->db_blkptr == NULL || BP_IS_HOLE(db->db_blkptr)) &&
            (db->db_buf == NULL || db->db_buf->b_data) &&
            db->db.db_data && db->db_blkid != DMU_BONUS_BLKID &&
            db->db_state != DB_FILL && !dn->dn_free_txg) {
                /*
                 * If the blkptr isn't set but they have nonzero data,
                 * it had better be dirty, otherwise we'll lose that
                 * data when we evict this buffer.
                 *
                 * There is an exception to this rule for indirect blocks; in
                 * this case, if the indirect block is a hole, we fill in a few
                 * fields on each of the child blocks (importantly, birth time)
                 * to prevent hole birth times from being lost when you
                 * partially fill in a hole.
                 */
                if (db->db_dirtycnt == 0) {
                        if (db->db_level == 0) {
                                uint64_t *buf = db->db.db_data;
                                int i;

                                for (i = 0; i < db->db.db_size >> 3; i++) {
                                        ASSERT(buf[i] == 0);
                                }
                        } else {
                                blkptr_t *bps = db->db.db_data;
                                ASSERT3U(1 << DB_DNODE(db)->dn_indblkshift, ==,
                                    db->db.db_size);
                                /*
                                 * We want to verify that all the blkptrs in the
                                 * indirect block are holes, but we may have
                                 * automatically set up a few fields for them.
                                 * We iterate through each blkptr and verify
                                 * they only have those fields set.
                                 */
                                for (int i = 0;
                                    i < db->db.db_size / sizeof (blkptr_t);
                                    i++) {
                                        blkptr_t *bp = &bps[i];
                                        ASSERT(ZIO_CHECKSUM_IS_ZERO(
                                            &bp->blk_cksum));
                                        ASSERT(
                                            DVA_IS_EMPTY(&bp->blk_dva[0]) &&
                                            DVA_IS_EMPTY(&bp->blk_dva[1]) &&
                                            DVA_IS_EMPTY(&bp->blk_dva[2]));
                                        ASSERT0(bp->blk_fill);
                                        ASSERT0(bp->blk_pad[0]);
                                        ASSERT0(bp->blk_pad[1]);
                                        ASSERT(!BP_IS_EMBEDDED(bp));
                                        ASSERT(BP_IS_HOLE(bp));
                                        ASSERT0(bp->blk_phys_birth);
                                }
                        }
                }
        }
        DB_DNODE_EXIT(db);
}
#endif

static void
dbuf_clear_data(dmu_buf_impl_t *db)
{
        ASSERT(MUTEX_HELD(&db->db_mtx));
        dbuf_evict_user(db);
        ASSERT3P(db->db_buf, ==, NULL);
        db->db.db_data = NULL;
        if (db->db_state != DB_NOFILL)
                db->db_state = DB_UNCACHED;
}

static void
dbuf_set_data(dmu_buf_impl_t *db, arc_buf_t *buf)
{
        ASSERT(MUTEX_HELD(&db->db_mtx));
        ASSERT(buf != NULL);

        db->db_buf = buf;
        ASSERT(buf->b_data != NULL);
        db->db.db_data = buf->b_data;
}

/*
 * Loan out an arc_buf for read.  Return the loaned arc_buf.
 */
arc_buf_t *
dbuf_loan_arcbuf(dmu_buf_impl_t *db)
{
        arc_buf_t *abuf;

        ASSERT(db->db_blkid != DMU_BONUS_BLKID);
        mutex_enter(&db->db_mtx);
        if (arc_released(db->db_buf) || refcount_count(&db->db_holds) > 1) {
                int blksz = db->db.db_size;
                spa_t *spa = db->db_objset->os_spa;

                mutex_exit(&db->db_mtx);
                abuf = arc_loan_buf(spa, B_FALSE, blksz);
                bcopy(db->db.db_data, abuf->b_data, blksz);
        } else {
                abuf = db->db_buf;
                arc_loan_inuse_buf(abuf, db);
                db->db_buf = NULL;
                dbuf_clear_data(db);
                mutex_exit(&db->db_mtx);
        }
        return (abuf);
}

/*
 * Calculate which level n block references the data at the level 0 offset
 * provided.
 */
uint64_t
dbuf_whichblock(dnode_t *dn, int64_t level, uint64_t offset)
{
        if (dn->dn_datablkshift != 0 && dn->dn_indblkshift != 0) {
                /*
                 * The level n blkid is equal to the level 0 blkid divided by
                 * the number of level 0s in a level n block.
                 *
                 * The level 0 blkid is offset >> datablkshift =
                 * offset / 2^datablkshift.
                 *
                 * The number of level 0s in a level n is the number of block
                 * pointers in an indirect block, raised to the power of level.
                 * This is 2^(indblkshift - SPA_BLKPTRSHIFT)^level =
                 * 2^(level*(indblkshift - SPA_BLKPTRSHIFT)).
                 *
                 * Thus, the level n blkid is: offset /
                 * ((2^datablkshift)*(2^(level*(indblkshift - SPA_BLKPTRSHIFT)))
                 * = offset / 2^(datablkshift + level *
                 *   (indblkshift - SPA_BLKPTRSHIFT))
                 * = offset >> (datablkshift + level *
                 *   (indblkshift - SPA_BLKPTRSHIFT))
                 */
                return (offset >> (dn->dn_datablkshift + level *
                    (dn->dn_indblkshift - SPA_BLKPTRSHIFT)));
        } else {
                ASSERT3U(offset, <, dn->dn_datablksz);
                return (0);
        }
}

static void
dbuf_read_done(zio_t *zio, const zbookmark_phys_t *zb, const blkptr_t *bp,
    arc_buf_t *buf, void *vdb)
{
        dmu_buf_impl_t *db = vdb;

        mutex_enter(&db->db_mtx);
        ASSERT3U(db->db_state, ==, DB_READ);
        /*
         * All reads are synchronous, so we must have a hold on the dbuf
         */
        ASSERT(refcount_count(&db->db_holds) > 0);
        ASSERT(db->db_buf == NULL);
        ASSERT(db->db.db_data == NULL);
        if (db->db_level == 0 && db->db_freed_in_flight) {
                /* we were freed in flight; disregard any error */
                if (buf == NULL) {
                        buf = arc_alloc_buf(db->db_objset->os_spa,
                             db, DBUF_GET_BUFC_TYPE(db), db->db.db_size);
                }
                arc_release(buf, db);
                bzero(buf->b_data, db->db.db_size);
                arc_buf_freeze(buf);
                db->db_freed_in_flight = FALSE;
                dbuf_set_data(db, buf);
                db->db_state = DB_CACHED;
        } else if (buf != NULL) {
                dbuf_set_data(db, buf);
                db->db_state = DB_CACHED;
        } else {
                ASSERT(db->db_blkid != DMU_BONUS_BLKID);
                ASSERT3P(db->db_buf, ==, NULL);
                db->db_state = DB_UNCACHED;
        }
        cv_broadcast(&db->db_changed);
        dbuf_rele_and_unlock(db, NULL);
}

static void
dbuf_read_impl(dmu_buf_impl_t *db, zio_t *zio, uint32_t flags)
{
        dnode_t *dn;
        zbookmark_phys_t zb;
        arc_flags_t aflags = ARC_FLAG_NOWAIT;

        DB_DNODE_ENTER(db);
        dn = DB_DNODE(db);
        ASSERT(!refcount_is_zero(&db->db_holds));
        /* We need the struct_rwlock to prevent db_blkptr from changing. */
        ASSERT(RW_LOCK_HELD(&dn->dn_struct_rwlock));
        ASSERT(MUTEX_HELD(&db->db_mtx));
        ASSERT(db->db_state == DB_UNCACHED);
        ASSERT(db->db_buf == NULL);

        if (db->db_blkid == DMU_BONUS_BLKID) {
                int bonuslen = MIN(dn->dn_bonuslen, dn->dn_phys->dn_bonuslen);

                ASSERT3U(bonuslen, <=, db->db.db_size);
                db->db.db_data = zio_buf_alloc(DN_MAX_BONUSLEN);
                arc_space_consume(DN_MAX_BONUSLEN, ARC_SPACE_OTHER);
                if (bonuslen < DN_MAX_BONUSLEN)
                        bzero(db->db.db_data, DN_MAX_BONUSLEN);
                if (bonuslen)
                        bcopy(DN_BONUS(dn->dn_phys), db->db.db_data, bonuslen);
                DB_DNODE_EXIT(db);
                db->db_state = DB_CACHED;
                mutex_exit(&db->db_mtx);
                return;
        }

        /*
         * Recheck BP_IS_HOLE() after dnode_block_freed() in case dnode_sync()
         * processes the delete record and clears the bp while we are waiting
         * for the dn_mtx (resulting in a "no" from block_freed).
         */
        if (db->db_blkptr == NULL || BP_IS_HOLE(db->db_blkptr) ||
            (db->db_level == 0 && (dnode_block_freed(dn, db->db_blkid) ||
            BP_IS_HOLE(db->db_blkptr)))) {
                arc_buf_contents_t type = DBUF_GET_BUFC_TYPE(db);

                dbuf_set_data(db, arc_alloc_buf(db->db_objset->os_spa, db, type,
                    db->db.db_size));
                bzero(db->db.db_data, db->db.db_size);

                if (db->db_blkptr != NULL && db->db_level > 0 &&
                    BP_IS_HOLE(db->db_blkptr) &&
                    db->db_blkptr->blk_birth != 0) {
                        blkptr_t *bps = db->db.db_data;
                        for (int i = 0; i < ((1 <<
                            DB_DNODE(db)->dn_indblkshift) / sizeof (blkptr_t));
                            i++) {
                                blkptr_t *bp = &bps[i];
                                ASSERT3U(BP_GET_LSIZE(db->db_blkptr), ==,
                                    1 << dn->dn_indblkshift);
                                BP_SET_LSIZE(bp,
                                    BP_GET_LEVEL(db->db_blkptr) == 1 ?
                                    dn->dn_datablksz :
                                    BP_GET_LSIZE(db->db_blkptr));
                                BP_SET_TYPE(bp, BP_GET_TYPE(db->db_blkptr));
                                BP_SET_LEVEL(bp,
                                    BP_GET_LEVEL(db->db_blkptr) - 1);
                                BP_SET_BIRTH(bp, db->db_blkptr->blk_birth, 0);
                        }
                }
                DB_DNODE_EXIT(db);
                db->db_state = DB_CACHED;
                mutex_exit(&db->db_mtx);
                return;
        }

        DB_DNODE_EXIT(db);

        db->db_state = DB_READ;
        mutex_exit(&db->db_mtx);

        if (DBUF_IS_L2CACHEABLE(db))
                aflags |= ARC_FLAG_L2CACHE;

        SET_BOOKMARK(&zb, db->db_objset->os_dsl_dataset ?
            db->db_objset->os_dsl_dataset->ds_object : DMU_META_OBJSET,
            db->db.db_object, db->db_level, db->db_blkid);

        dbuf_add_ref(db, NULL);

        (void) arc_read(zio, db->db_objset->os_spa, db->db_blkptr,
            dbuf_read_done, db, ZIO_PRIORITY_SYNC_READ,
            (flags & DB_RF_CANFAIL) ? ZIO_FLAG_CANFAIL : ZIO_FLAG_MUSTSUCCEED,
            &aflags, &zb);
}

/*
 * This is our just-in-time copy function.  It makes a copy of buffers that
 * have been modified in a previous transaction group before we access them in
 * the current active group.
 *
 * This function is used in three places: when we are dirtying a buffer for the
 * first time in a txg, when we are freeing a range in a dnode that includes
 * this buffer, and when we are accessing a buffer which was received compressed
 * and later referenced in a WRITE_BYREF record.
 *
 * Note that when we are called from dbuf_free_range() we do not put a hold on
 * the buffer, we just traverse the active dbuf list for the dnode.
 */
static void
dbuf_fix_old_data(dmu_buf_impl_t *db, uint64_t txg)
{
        dbuf_dirty_record_t *dr = db->db_last_dirty;

        ASSERT(MUTEX_HELD(&db->db_mtx));
        ASSERT(db->db.db_data != NULL);
        ASSERT(db->db_level == 0);
        ASSERT(db->db.db_object != DMU_META_DNODE_OBJECT);

        if (dr == NULL ||
            (dr->dt.dl.dr_data !=
            ((db->db_blkid  == DMU_BONUS_BLKID) ? db->db.db_data : db->db_buf)))
                return;

        /*
         * If the last dirty record for this dbuf has not yet synced
         * and its referencing the dbuf data, either:
         *      reset the reference to point to a new copy,
         * or (if there a no active holders)
         *      just null out the current db_data pointer.
         */
        ASSERT(dr->dr_txg >= txg - 2);
        if (db->db_blkid == DMU_BONUS_BLKID) {
                /* Note that the data bufs here are zio_bufs */
                dr->dt.dl.dr_data = zio_buf_alloc(DN_MAX_BONUSLEN);
                arc_space_consume(DN_MAX_BONUSLEN, ARC_SPACE_OTHER);
                bcopy(db->db.db_data, dr->dt.dl.dr_data, DN_MAX_BONUSLEN);
        } else if (refcount_count(&db->db_holds) > db->db_dirtycnt) {
                int size = arc_buf_size(db->db_buf);
                arc_buf_contents_t type = DBUF_GET_BUFC_TYPE(db);
                spa_t *spa = db->db_objset->os_spa;
                enum zio_compress compress_type =
                    arc_get_compression(db->db_buf);

                if (compress_type == ZIO_COMPRESS_OFF) {
                        dr->dt.dl.dr_data = arc_alloc_buf(spa, db, type, size);
                } else {
                        ASSERT3U(type, ==, ARC_BUFC_DATA);
                        dr->dt.dl.dr_data = arc_alloc_compressed_buf(spa, db,
                            size, arc_buf_lsize(db->db_buf), compress_type);
                }
                bcopy(db->db.db_data, dr->dt.dl.dr_data->b_data, size);
        } else {
                db->db_buf = NULL;
                dbuf_clear_data(db);
        }
}

int
dbuf_read(dmu_buf_impl_t *db, zio_t *zio, uint32_t flags)
{
        int err = 0;
        boolean_t prefetch;
        dnode_t *dn;

        /*
         * We don't have to hold the mutex to check db_state because it
         * can't be freed while we have a hold on the buffer.
         */
        ASSERT(!refcount_is_zero(&db->db_holds));

        if (db->db_state == DB_NOFILL)
                return (SET_ERROR(EIO));

        DB_DNODE_ENTER(db);
        dn = DB_DNODE(db);
        if ((flags & DB_RF_HAVESTRUCT) == 0)
                rw_enter(&dn->dn_struct_rwlock, RW_READER);

        prefetch = db->db_level == 0 && db->db_blkid != DMU_BONUS_BLKID &&
            (flags & DB_RF_NOPREFETCH) == 0 && dn != NULL &&
            DBUF_IS_CACHEABLE(db);

        mutex_enter(&db->db_mtx);
        if (db->db_state == DB_CACHED) {
                /*
                 * If the arc buf is compressed, we need to decompress it to
                 * read the data. This could happen during the "zfs receive" of
                 * a stream which is compressed and deduplicated.
                 */
                if (db->db_buf != NULL &&
                    arc_get_compression(db->db_buf) != ZIO_COMPRESS_OFF) {
                        dbuf_fix_old_data(db,
                            spa_syncing_txg(dmu_objset_spa(db->db_objset)));
                        err = arc_decompress(db->db_buf);
                        dbuf_set_data(db, db->db_buf);
                }
                mutex_exit(&db->db_mtx);
                if (prefetch)
                        dmu_zfetch(&dn->dn_zfetch, db->db_blkid, 1, B_TRUE);
                if ((flags & DB_RF_HAVESTRUCT) == 0)
                        rw_exit(&dn->dn_struct_rwlock);
                DB_DNODE_EXIT(db);
        } else if (db->db_state == DB_UNCACHED) {
                spa_t *spa = dn->dn_objset->os_spa;
                boolean_t need_wait = B_FALSE;

                if (zio == NULL &&
                    db->db_blkptr != NULL && !BP_IS_HOLE(db->db_blkptr)) {
                        zio = zio_root(spa, NULL, NULL, ZIO_FLAG_CANFAIL);
                        need_wait = B_TRUE;
                }
                dbuf_read_impl(db, zio, flags);

                /* dbuf_read_impl has dropped db_mtx for us */

                if (prefetch)
                        dmu_zfetch(&dn->dn_zfetch, db->db_blkid, 1, B_TRUE);

                if ((flags & DB_RF_HAVESTRUCT) == 0)
                        rw_exit(&dn->dn_struct_rwlock);
                DB_DNODE_EXIT(db);

                if (need_wait)
                        err = zio_wait(zio);
        } else {
                /*
                 * Another reader came in while the dbuf was in flight
                 * between UNCACHED and CACHED.  Either a writer will finish
                 * writing the buffer (sending the dbuf to CACHED) or the
                 * first reader's request will reach the read_done callback
                 * and send the dbuf to CACHED.  Otherwise, a failure
                 * occurred and the dbuf went to UNCACHED.
                 */
                mutex_exit(&db->db_mtx);
                if (prefetch)
                        dmu_zfetch(&dn->dn_zfetch, db->db_blkid, 1, B_TRUE);
                if ((flags & DB_RF_HAVESTRUCT) == 0)
                        rw_exit(&dn->dn_struct_rwlock);
                DB_DNODE_EXIT(db);

                /* Skip the wait per the caller's request. */
                mutex_enter(&db->db_mtx);
                if ((flags & DB_RF_NEVERWAIT) == 0) {
                        while (db->db_state == DB_READ ||
                            db->db_state == DB_FILL) {
                                ASSERT(db->db_state == DB_READ ||
                                    (flags & DB_RF_HAVESTRUCT) == 0);
                                DTRACE_PROBE2(blocked__read, dmu_buf_impl_t *,
                                    db, zio_t *, zio);
                                cv_wait(&db->db_changed, &db->db_mtx);
                        }
                        if (db->db_state == DB_UNCACHED)
                                err = SET_ERROR(EIO);
                }
                mutex_exit(&db->db_mtx);
        }

        return (err);
}

static void
dbuf_noread(dmu_buf_impl_t *db)
{
        ASSERT(!refcount_is_zero(&db->db_holds));
        ASSERT(db->db_blkid != DMU_BONUS_BLKID);
        mutex_enter(&db->db_mtx);
        while (db->db_state == DB_READ || db->db_state == DB_FILL)
                cv_wait(&db->db_changed, &db->db_mtx);
        if (db->db_state == DB_UNCACHED) {
                arc_buf_contents_t type = DBUF_GET_BUFC_TYPE(db);
                spa_t *spa = db->db_objset->os_spa;

                ASSERT(db->db_buf == NULL);
                ASSERT(db->db.db_data == NULL);
                dbuf_set_data(db, arc_alloc_buf(spa, db, type, db->db.db_size));
                db->db_state = DB_FILL;
        } else if (db->db_state == DB_NOFILL) {
                dbuf_clear_data(db);
        } else {
                ASSERT3U(db->db_state, ==, DB_CACHED);
        }
        mutex_exit(&db->db_mtx);
}

void
dbuf_unoverride(dbuf_dirty_record_t *dr)
{
        dmu_buf_impl_t *db = dr->dr_dbuf;
        blkptr_t *bp = &dr->dt.dl.dr_overridden_by;
        uint64_t txg = dr->dr_txg;

        ASSERT(MUTEX_HELD(&db->db_mtx));
        /*
         * This assert is valid because dmu_sync() expects to be called by
         * a zilog's get_data while holding a range lock.  This call only
         * comes from dbuf_dirty() callers who must also hold a range lock.
         */
        ASSERT(dr->dt.dl.dr_override_state != DR_IN_DMU_SYNC);
        ASSERT(db->db_level == 0);

        if (db->db_blkid == DMU_BONUS_BLKID ||
            dr->dt.dl.dr_override_state == DR_NOT_OVERRIDDEN)
                return;

        ASSERT(db->db_data_pending != dr);

        /* free this block */
        if (!BP_IS_HOLE(bp) && !dr->dt.dl.dr_nopwrite)
                zio_free(db->db_objset->os_spa, txg, bp);

        dr->dt.dl.dr_override_state = DR_NOT_OVERRIDDEN;
        dr->dt.dl.dr_nopwrite = B_FALSE;

        /*
         * Release the already-written buffer, so we leave it in
         * a consistent dirty state.  Note that all callers are
         * modifying the buffer, so they will immediately do
         * another (redundant) arc_release().  Therefore, leave
         * the buf thawed to save the effort of freezing &
         * immediately re-thawing it.
         */
        arc_release(dr->dt.dl.dr_data, db);
}

/*
 * Evict (if its unreferenced) or clear (if its referenced) any level-0
 * data blocks in the free range, so that any future readers will find
 * empty blocks.
 */
void
dbuf_free_range(dnode_t *dn, uint64_t start_blkid, uint64_t end_blkid,
    dmu_tx_t *tx)
{
        dmu_buf_impl_t db_search;
        dmu_buf_impl_t *db, *db_next;
        uint64_t txg = tx->tx_txg;
        avl_index_t where;

        if (end_blkid > dn->dn_maxblkid &&
            !(start_blkid == DMU_SPILL_BLKID || end_blkid == DMU_SPILL_BLKID))
                end_blkid = dn->dn_maxblkid;
        dprintf_dnode(dn, "start=%llu end=%llu\n", start_blkid, end_blkid);

        db_search.db_level = 0;
        db_search.db_blkid = start_blkid;
        db_search.db_state = DB_SEARCH;

        mutex_enter(&dn->dn_dbufs_mtx);
        db = avl_find(&dn->dn_dbufs, &db_search, &where);
        ASSERT3P(db, ==, NULL);

        db = avl_nearest(&dn->dn_dbufs, where, AVL_AFTER);

        for (; db != NULL; db = db_next) {
                db_next = AVL_NEXT(&dn->dn_dbufs, db);
                ASSERT(db->db_blkid != DMU_BONUS_BLKID);

                if (db->db_level != 0 || db->db_blkid > end_blkid) {
                        break;
                }
                ASSERT3U(db->db_blkid, >=, start_blkid);

                /* found a level 0 buffer in the range */
                mutex_enter(&db->db_mtx);
                if (dbuf_undirty(db, tx)) {
                        /* mutex has been dropped and dbuf destroyed */
                        continue;
                }

                if (db->db_state == DB_UNCACHED ||
                    db->db_state == DB_NOFILL ||
                    db->db_state == DB_EVICTING) {
                        ASSERT(db->db.db_data == NULL);
                        mutex_exit(&db->db_mtx);
                        continue;
                }
                if (db->db_state == DB_READ || db->db_state == DB_FILL) {
                        /* will be handled in dbuf_read_done or dbuf_rele */
                        db->db_freed_in_flight = TRUE;
                        mutex_exit(&db->db_mtx);
                        continue;
                }
                if (refcount_count(&db->db_holds) == 0) {
                        ASSERT(db->db_buf);
                        dbuf_destroy(db);
                        continue;
                }
                /* The dbuf is referenced */

                if (db->db_last_dirty != NULL) {
                        dbuf_dirty_record_t *dr = db->db_last_dirty;

                        if (dr->dr_txg == txg) {
                                /*
                                 * This buffer is "in-use", re-adjust the file
                                 * size to reflect that this buffer may
                                 * contain new data when we sync.
                                 */
                                if (db->db_blkid != DMU_SPILL_BLKID &&
                                    db->db_blkid > dn->dn_maxblkid)
                                        dn->dn_maxblkid = db->db_blkid;
                                dbuf_unoverride(dr);
                        } else {
                                /*
                                 * This dbuf is not dirty in the open context.
                                 * Either uncache it (if its not referenced in
                                 * the open context) or reset its contents to
                                 * empty.
                                 */
                                dbuf_fix_old_data(db, txg);
                        }
                }
                /* clear the contents if its cached */
                if (db->db_state == DB_CACHED) {
                        ASSERT(db->db.db_data != NULL);
                        arc_release(db->db_buf, db);
                        bzero(db->db.db_data, db->db.db_size);
                        arc_buf_freeze(db->db_buf);
                }

                mutex_exit(&db->db_mtx);
        }
        mutex_exit(&dn->dn_dbufs_mtx);
}

void
dbuf_new_size(dmu_buf_impl_t *db, int size, dmu_tx_t *tx)
{
        arc_buf_t *buf, *obuf;
        int osize = db->db.db_size;
        arc_buf_contents_t type = DBUF_GET_BUFC_TYPE(db);
        dnode_t *dn;

        ASSERT(db->db_blkid != DMU_BONUS_BLKID);

        DB_DNODE_ENTER(db);
        dn = DB_DNODE(db);

        /* XXX does *this* func really need the lock? */
        ASSERT(RW_WRITE_HELD(&dn->dn_struct_rwlock));

        /*
         * This call to dmu_buf_will_dirty() with the dn_struct_rwlock held
         * is OK, because there can be no other references to the db
         * when we are changing its size, so no concurrent DB_FILL can
         * be happening.
         */
        /*
         * XXX we should be doing a dbuf_read, checking the return
         * value and returning that up to our callers
         */
        dmu_buf_will_dirty(&db->db, tx);

        /* create the data buffer for the new block */
        buf = arc_alloc_buf(dn->dn_objset->os_spa, db, type, size);

        /* copy old block data to the new block */
        obuf = db->db_buf;
        bcopy(obuf->b_data, buf->b_data, MIN(osize, size));
        /* zero the remainder */
        if (size > osize)
                bzero((uint8_t *)buf->b_data + osize, size - osize);

        mutex_enter(&db->db_mtx);
        dbuf_set_data(db, buf);
        arc_buf_destroy(obuf, db);
        db->db.db_size = size;

        if (db->db_level == 0) {
                ASSERT3U(db->db_last_dirty->dr_txg, ==, tx->tx_txg);
                db->db_last_dirty->dt.dl.dr_data = buf;
        }
        mutex_exit(&db->db_mtx);

        dmu_objset_willuse_space(dn->dn_objset, size - osize, tx);
        DB_DNODE_EXIT(db);
}

void
dbuf_release_bp(dmu_buf_impl_t *db)
{
        objset_t *os = db->db_objset;

        ASSERT(dsl_pool_sync_context(dmu_objset_pool(os)));
        ASSERT(arc_released(os->os_phys_buf) ||
            list_link_active(&os->os_dsl_dataset->ds_synced_link));
        ASSERT(db->db_parent == NULL || arc_released(db->db_parent->db_buf));

        (void) arc_release(db->db_buf, db);
}

/*
 * We already have a dirty record for this TXG, and we are being
 * dirtied again.
 */
static void
dbuf_redirty(dbuf_dirty_record_t *dr)
{
        dmu_buf_impl_t *db = dr->dr_dbuf;

        ASSERT(MUTEX_HELD(&db->db_mtx));

        if (db->db_level == 0 && db->db_blkid != DMU_BONUS_BLKID) {
                /*
                 * If this buffer has already been written out,
                 * we now need to reset its state.
                 */
                dbuf_unoverride(dr);
                if (db->db.db_object != DMU_META_DNODE_OBJECT &&
                    db->db_state != DB_NOFILL) {
                        /* Already released on initial dirty, so just thaw. */
                        ASSERT(arc_released(db->db_buf));
                        arc_buf_thaw(db->db_buf);
                }
        }
}

dbuf_dirty_record_t *
dbuf_dirty(dmu_buf_impl_t *db, dmu_tx_t *tx)
{
        dnode_t *dn;
        objset_t *os;
        dbuf_dirty_record_t **drp, *dr;
        int drop_struct_lock = FALSE;
        int txgoff = tx->tx_txg & TXG_MASK;

        ASSERT(tx->tx_txg != 0);
        ASSERT(!refcount_is_zero(&db->db_holds));
        DMU_TX_DIRTY_BUF(tx, db);

        DB_DNODE_ENTER(db);
        dn = DB_DNODE(db);
        /*
         * Shouldn't dirty a regular buffer in syncing context.  Private
         * objects may be dirtied in syncing context, but only if they
         * were already pre-dirtied in open context.
         */
#ifdef DEBUG
        if (dn->dn_objset->os_dsl_dataset != NULL) {
                rrw_enter(&dn->dn_objset->os_dsl_dataset->ds_bp_rwlock,
                    RW_READER, FTAG);
        }
        ASSERT(!dmu_tx_is_syncing(tx) ||
            BP_IS_HOLE(dn->dn_objset->os_rootbp) ||
            DMU_OBJECT_IS_SPECIAL(dn->dn_object) ||
            dn->dn_objset->os_dsl_dataset == NULL);
        if (dn->dn_objset->os_dsl_dataset != NULL)
                rrw_exit(&dn->dn_objset->os_dsl_dataset->ds_bp_rwlock, FTAG);
#endif
        /*
         * We make this assert for private objects as well, but after we
         * check if we're already dirty.  They are allowed to re-dirty
         * in syncing context.
         */
        ASSERT(dn->dn_object == DMU_META_DNODE_OBJECT ||
            dn->dn_dirtyctx == DN_UNDIRTIED || dn->dn_dirtyctx ==
            (dmu_tx_is_syncing(tx) ? DN_DIRTY_SYNC : DN_DIRTY_OPEN));

        mutex_enter(&db->db_mtx);
        /*
         * XXX make this true for indirects too?  The problem is that
         * transactions created with dmu_tx_create_assigned() from
         * syncing context don't bother holding ahead.
         */
        ASSERT(db->db_level != 0 ||
            db->db_state == DB_CACHED || db->db_state == DB_FILL ||
            db->db_state == DB_NOFILL);

        mutex_enter(&dn->dn_mtx);
        /*
         * Don't set dirtyctx to SYNC if we're just modifying this as we
         * initialize the objset.
         */
        if (dn->dn_dirtyctx == DN_UNDIRTIED) {
                if (dn->dn_objset->os_dsl_dataset != NULL) {
                        rrw_enter(&dn->dn_objset->os_dsl_dataset->ds_bp_rwlock,
                            RW_READER, FTAG);
                }
                if (!BP_IS_HOLE(dn->dn_objset->os_rootbp)) {
                        dn->dn_dirtyctx = (dmu_tx_is_syncing(tx) ?
                            DN_DIRTY_SYNC : DN_DIRTY_OPEN);
                        ASSERT(dn->dn_dirtyctx_firstset == NULL);
                        dn->dn_dirtyctx_firstset = kmem_alloc(1, KM_SLEEP);
                }
                if (dn->dn_objset->os_dsl_dataset != NULL) {
                        rrw_exit(&dn->dn_objset->os_dsl_dataset->ds_bp_rwlock,
                            FTAG);
                }
        }
        mutex_exit(&dn->dn_mtx);

        if (db->db_blkid == DMU_SPILL_BLKID)
                dn->dn_have_spill = B_TRUE;

        /*
         * If this buffer is already dirty, we're done.
         */
        drp = &db->db_last_dirty;
        ASSERT(*drp == NULL || (*drp)->dr_txg <= tx->tx_txg ||
            db->db.db_object == DMU_META_DNODE_OBJECT);
        while ((dr = *drp) != NULL && dr->dr_txg > tx->tx_txg)
                drp = &dr->dr_next;
        if (dr && dr->dr_txg == tx->tx_txg) {
                DB_DNODE_EXIT(db);

                dbuf_redirty(dr);
                mutex_exit(&db->db_mtx);
                return (dr);
        }

        /*
         * Only valid if not already dirty.
         */
        ASSERT(dn->dn_object == 0 ||
            dn->dn_dirtyctx == DN_UNDIRTIED || dn->dn_dirtyctx ==
            (dmu_tx_is_syncing(tx) ? DN_DIRTY_SYNC : DN_DIRTY_OPEN));

        ASSERT3U(dn->dn_nlevels, >, db->db_level);

        /*
         * We should only be dirtying in syncing context if it's the
         * mos or we're initializing the os or it's a special object.
         * However, we are allowed to dirty in syncing context provided
         * we already dirtied it in open context.  Hence we must make
         * this assertion only if we're not already dirty.
         */
        os = dn->dn_objset;
        VERIFY3U(tx->tx_txg, <=, spa_final_dirty_txg(os->os_spa));
#ifdef DEBUG
        if (dn->dn_objset->os_dsl_dataset != NULL)
                rrw_enter(&os->os_dsl_dataset->ds_bp_rwlock, RW_READER, FTAG);
        ASSERT(!dmu_tx_is_syncing(tx) || DMU_OBJECT_IS_SPECIAL(dn->dn_object) ||
            os->os_dsl_dataset == NULL || BP_IS_HOLE(os->os_rootbp));
        if (dn->dn_objset->os_dsl_dataset != NULL)
                rrw_exit(&os->os_dsl_dataset->ds_bp_rwlock, FTAG);
#endif
        ASSERT(db->db.db_size != 0);

        dprintf_dbuf(db, "size=%llx\n", (u_longlong_t)db->db.db_size);

        if (db->db_blkid != DMU_BONUS_BLKID) {
                dmu_objset_willuse_space(os, db->db.db_size, tx);
        }

        /*
         * If this buffer is dirty in an old transaction group we need
         * to make a copy of it so that the changes we make in this
         * transaction group won't leak out when we sync the older txg.
         */
        dr = kmem_zalloc(sizeof (dbuf_dirty_record_t), KM_SLEEP);
        if (db->db_level == 0) {
                void *data_old = db->db_buf;

                if (db->db_state != DB_NOFILL) {
                        if (db->db_blkid == DMU_BONUS_BLKID) {
                                dbuf_fix_old_data(db, tx->tx_txg);
                                data_old = db->db.db_data;
                        } else if (db->db.db_object != DMU_META_DNODE_OBJECT) {
                                /*
                                 * Release the data buffer from the cache so
                                 * that we can modify it without impacting
                                 * possible other users of this cached data
                                 * block.  Note that indirect blocks and
                                 * private objects are not released until the
                                 * syncing state (since they are only modified
                                 * then).
                                 */
                                arc_release(db->db_buf, db);
                                dbuf_fix_old_data(db, tx->tx_txg);
                                data_old = db->db_buf;
                        }
                        ASSERT(data_old != NULL);
                }
                dr->dt.dl.dr_data = data_old;
        } else {
                mutex_init(&dr->dt.di.dr_mtx, NULL, MUTEX_DEFAULT, NULL);
                list_create(&dr->dt.di.dr_children,
                    sizeof (dbuf_dirty_record_t),
                    offsetof(dbuf_dirty_record_t, dr_dirty_node));
        }
        if (db->db_blkid != DMU_BONUS_BLKID && os->os_dsl_dataset != NULL)
                dr->dr_accounted = db->db.db_size;
        dr->dr_dbuf = db;
        dr->dr_txg = tx->tx_txg;
        dr->dr_next = *drp;
        *drp = dr;

        /*
         * We could have been freed_in_flight between the dbuf_noread
         * and dbuf_dirty.  We win, as though the dbuf_noread() had
         * happened after the free.
         */
        if (db->db_level == 0 && db->db_blkid != DMU_BONUS_BLKID &&
            db->db_blkid != DMU_SPILL_BLKID) {
                mutex_enter(&dn->dn_mtx);
                if (dn->dn_free_ranges[txgoff] != NULL) {
                        range_tree_clear(dn->dn_free_ranges[txgoff],
                            db->db_blkid, 1);
                }
                mutex_exit(&dn->dn_mtx);
                db->db_freed_in_flight = FALSE;
        }

        /*
         * This buffer is now part of this txg
         */
        dbuf_add_ref(db, (void *)(uintptr_t)tx->tx_txg);
        db->db_dirtycnt += 1;
        ASSERT3U(db->db_dirtycnt, <=, 3);

        mutex_exit(&db->db_mtx);

        if (db->db_blkid == DMU_BONUS_BLKID ||
            db->db_blkid == DMU_SPILL_BLKID) {
                mutex_enter(&dn->dn_mtx);
                ASSERT(!list_link_active(&dr->dr_dirty_node));
                list_insert_tail(&dn->dn_dirty_records[txgoff], dr);
                mutex_exit(&dn->dn_mtx);
                dnode_setdirty(dn, tx);
                DB_DNODE_EXIT(db);
                return (dr);
        }

        /*
         * The dn_struct_rwlock prevents db_blkptr from changing
         * due to a write from syncing context completing
         * while we are running, so we want to acquire it before
         * looking at db_blkptr.
         */
        if (!RW_WRITE_HELD(&dn->dn_struct_rwlock)) {
                rw_enter(&dn->dn_struct_rwlock, RW_READER);
                drop_struct_lock = TRUE;
        }

        /*
         * We need to hold the dn_struct_rwlock to make this assertion,
         * because it protects dn_phys / dn_next_nlevels from changing.
         */
        ASSERT((dn->dn_phys->dn_nlevels == 0 && db->db_level == 0) ||
            dn->dn_phys->dn_nlevels > db->db_level ||
            dn->dn_next_nlevels[txgoff] > db->db_level ||
            dn->dn_next_nlevels[(tx->tx_txg-1) & TXG_MASK] > db->db_level ||
            dn->dn_next_nlevels[(tx->tx_txg-2) & TXG_MASK] > db->db_level);

        /*
         * If we are overwriting a dedup BP, then unless it is snapshotted,
         * when we get to syncing context we will need to decrement its
         * refcount in the DDT.  Prefetch the relevant DDT block so that
         * syncing context won't have to wait for the i/o.
         */
        ddt_prefetch(os->os_spa, db->db_blkptr);

        if (db->db_level == 0) {
                dnode_new_blkid(dn, db->db_blkid, tx, drop_struct_lock);
                ASSERT(dn->dn_maxblkid >= db->db_blkid);
        }

        if (db->db_level+1 < dn->dn_nlevels) {
                dmu_buf_impl_t *parent = db->db_parent;
                dbuf_dirty_record_t *di;
                int parent_held = FALSE;

                if (db->db_parent == NULL || db->db_parent == dn->dn_dbuf) {
                        int epbs = dn->dn_indblkshift - SPA_BLKPTRSHIFT;

                        parent = dbuf_hold_level(dn, db->db_level+1,
                            db->db_blkid >> epbs, FTAG);
                        ASSERT(parent != NULL);
                        parent_held = TRUE;
                }
                if (drop_struct_lock)
                        rw_exit(&dn->dn_struct_rwlock);
                ASSERT3U(db->db_level+1, ==, parent->db_level);
                di = dbuf_dirty(parent, tx);
                if (parent_held)
                        dbuf_rele(parent, FTAG);

                mutex_enter(&db->db_mtx);
                /*
                 * Since we've dropped the mutex, it's possible that
                 * dbuf_undirty() might have changed this out from under us.
                 */
                if (db->db_last_dirty == dr ||
                    dn->dn_object == DMU_META_DNODE_OBJECT) {
                        mutex_enter(&di->dt.di.dr_mtx);
                        ASSERT3U(di->dr_txg, ==, tx->tx_txg);
                        ASSERT(!list_link_active(&dr->dr_dirty_node));
                        list_insert_tail(&di->dt.di.dr_children, dr);
                        mutex_exit(&di->dt.di.dr_mtx);
                        dr->dr_parent = di;
                }
                mutex_exit(&db->db_mtx);
        } else {
                ASSERT(db->db_level+1 == dn->dn_nlevels);
                ASSERT(db->db_blkid < dn->dn_nblkptr);
                ASSERT(db->db_parent == NULL || db->db_parent == dn->dn_dbuf);
                mutex_enter(&dn->dn_mtx);
                ASSERT(!list_link_active(&dr->dr_dirty_node));
                list_insert_tail(&dn->dn_dirty_records[txgoff], dr);
                mutex_exit(&dn->dn_mtx);
                if (drop_struct_lock)
                        rw_exit(&dn->dn_struct_rwlock);
        }

        dnode_setdirty(dn, tx);
        DB_DNODE_EXIT(db);
        return (dr);
}

/*
 * Undirty a buffer in the transaction group referenced by the given
 * transaction.  Return whether this evicted the dbuf.
 */
static boolean_t
dbuf_undirty(dmu_buf_impl_t *db, dmu_tx_t *tx)
{
        dnode_t *dn;
        uint64_t txg = tx->tx_txg;
        dbuf_dirty_record_t *dr, **drp;

        ASSERT(txg != 0);

        /*
         * Due to our use of dn_nlevels below, this can only be called
         * in open context, unless we are operating on the MOS.
         * From syncing context, dn_nlevels may be different from the
         * dn_nlevels used when dbuf was dirtied.
         */
        ASSERT(db->db_objset ==
            dmu_objset_pool(db->db_objset)->dp_meta_objset ||
            txg != spa_syncing_txg(dmu_objset_spa(db->db_objset)));
        ASSERT(db->db_blkid != DMU_BONUS_BLKID);
        ASSERT0(db->db_level);
        ASSERT(MUTEX_HELD(&db->db_mtx));

        /*
         * If this buffer is not dirty, we're done.
         */
        for (drp = &db->db_last_dirty; (dr = *drp) != NULL; drp = &dr->dr_next)
                if (dr->dr_txg <= txg)
                        break;
        if (dr == NULL || dr->dr_txg < txg)
                return (B_FALSE);
        ASSERT(dr->dr_txg == txg);
        ASSERT(dr->dr_dbuf == db);

        DB_DNODE_ENTER(db);
        dn = DB_DNODE(db);

        dprintf_dbuf(db, "size=%llx\n", (u_longlong_t)db->db.db_size);

        ASSERT(db->db.db_size != 0);

        dsl_pool_undirty_space(dmu_objset_pool(dn->dn_objset),
            dr->dr_accounted, txg);

        *drp = dr->dr_next;

        /*
         * Note that there are three places in dbuf_dirty()
         * where this dirty record may be put on a list.
         * Make sure to do a list_remove corresponding to
         * every one of those list_insert calls.
         */
        if (dr->dr_parent) {
                mutex_enter(&dr->dr_parent->dt.di.dr_mtx);
                list_remove(&dr->dr_parent->dt.di.dr_children, dr);
                mutex_exit(&dr->dr_parent->dt.di.dr_mtx);
        } else if (db->db_blkid == DMU_SPILL_BLKID ||
            db->db_level + 1 == dn->dn_nlevels) {
                ASSERT(db->db_blkptr == NULL || db->db_parent == dn->dn_dbuf);
                mutex_enter(&dn->dn_mtx);
                list_remove(&dn->dn_dirty_records[txg & TXG_MASK], dr);
                mutex_exit(&dn->dn_mtx);
        }
        DB_DNODE_EXIT(db);

        if (db->db_state != DB_NOFILL) {
                dbuf_unoverride(dr);

                ASSERT(db->db_buf != NULL);
                ASSERT(dr->dt.dl.dr_data != NULL);
                if (dr->dt.dl.dr_data != db->db_buf)
                        arc_buf_destroy(dr->dt.dl.dr_data, db);
        }

        kmem_free(dr, sizeof (dbuf_dirty_record_t));

        ASSERT(db->db_dirtycnt > 0);
        db->db_dirtycnt -= 1;

        if (refcount_remove(&db->db_holds, (void *)(uintptr_t)txg) == 0) {
                ASSERT(db->db_state == DB_NOFILL || arc_released(db->db_buf));
                dbuf_destroy(db);
                return (B_TRUE);
        }

        return (B_FALSE);
}

void
dmu_buf_will_dirty(dmu_buf_t *db_fake, dmu_tx_t *tx)
{
        dmu_buf_impl_t *db = (dmu_buf_impl_t *)db_fake;
        int rf = DB_RF_MUST_SUCCEED | DB_RF_NOPREFETCH;

        ASSERT(tx->tx_txg != 0);
        ASSERT(!refcount_is_zero(&db->db_holds));

        /*
         * Quick check for dirtyness.  For already dirty blocks, this
         * reduces runtime of this function by >90%, and overall performance
         * by 50% for some workloads (e.g. file deletion with indirect blocks
         * cached).
         */
        mutex_enter(&db->db_mtx);
        dbuf_dirty_record_t *dr;
        for (dr = db->db_last_dirty;
            dr != NULL && dr->dr_txg >= tx->tx_txg; dr = dr->dr_next) {
                /*
                 * It's possible that it is already dirty but not cached,
                 * because there are some calls to dbuf_dirty() that don't
                 * go through dmu_buf_will_dirty().
                 */
                if (dr->dr_txg == tx->tx_txg && db->db_state == DB_CACHED) {
                        /* This dbuf is already dirty and cached. */
                        dbuf_redirty(dr);
                        mutex_exit(&db->db_mtx);
                        return;
                }
        }
        mutex_exit(&db->db_mtx);

        DB_DNODE_ENTER(db);
        if (RW_WRITE_HELD(&DB_DNODE(db)->dn_struct_rwlock))
                rf |= DB_RF_HAVESTRUCT;
        DB_DNODE_EXIT(db);
        (void) dbuf_read(db, NULL, rf);
        (void) dbuf_dirty(db, tx);
}

void
dmu_buf_will_not_fill(dmu_buf_t *db_fake, dmu_tx_t *tx)
{
        dmu_buf_impl_t *db = (dmu_buf_impl_t *)db_fake;

        db->db_state = DB_NOFILL;

        dmu_buf_will_fill(db_fake, tx);
}

void
dmu_buf_will_fill(dmu_buf_t *db_fake, dmu_tx_t *tx)
{
        dmu_buf_impl_t *db = (dmu_buf_impl_t *)db_fake;

        ASSERT(db->db_blkid != DMU_BONUS_BLKID);
        ASSERT(tx->tx_txg != 0);
        ASSERT(db->db_level == 0);
        ASSERT(!refcount_is_zero(&db->db_holds));

        ASSERT(db->db.db_object != DMU_META_DNODE_OBJECT ||
            dmu_tx_private_ok(tx));

        dbuf_noread(db);
        (void) dbuf_dirty(db, tx);
}

#pragma weak dmu_buf_fill_done = dbuf_fill_done
/* ARGSUSED */
void
dbuf_fill_done(dmu_buf_impl_t *db, dmu_tx_t *tx)
{
        mutex_enter(&db->db_mtx);
        DBUF_VERIFY(db);

        if (db->db_state == DB_FILL) {
                if (db->db_level == 0 && db->db_freed_in_flight) {
                        ASSERT(db->db_blkid != DMU_BONUS_BLKID);
                        /* we were freed while filling */
                        /* XXX dbuf_undirty? */
                        bzero(db->db.db_data, db->db.db_size);
                        db->db_freed_in_flight = FALSE;
                }
                db->db_state = DB_CACHED;
                cv_broadcast(&db->db_changed);
        }
        mutex_exit(&db->db_mtx);
}

void
dmu_buf_write_embedded(dmu_buf_t *dbuf, void *data,
    bp_embedded_type_t etype, enum zio_compress comp,
    int uncompressed_size, int compressed_size, int byteorder,
    dmu_tx_t *tx)
{
        dmu_buf_impl_t *db = (dmu_buf_impl_t *)dbuf;
        struct dirty_leaf *dl;
        dmu_object_type_t type;

        if (etype == BP_EMBEDDED_TYPE_DATA) {
                ASSERT(spa_feature_is_active(dmu_objset_spa(db->db_objset),
                    SPA_FEATURE_EMBEDDED_DATA));
        }

        DB_DNODE_ENTER(db);
        type = DB_DNODE(db)->dn_type;
        DB_DNODE_EXIT(db);

        ASSERT0(db->db_level);
        ASSERT(db->db_blkid != DMU_BONUS_BLKID);

        dmu_buf_will_not_fill(dbuf, tx);

        ASSERT3U(db->db_last_dirty->dr_txg, ==, tx->tx_txg);
        dl = &db->db_last_dirty->dt.dl;
        encode_embedded_bp_compressed(&dl->dr_overridden_by,
            data, comp, uncompressed_size, compressed_size);
        BPE_SET_ETYPE(&dl->dr_overridden_by, etype);
        BP_SET_TYPE(&dl->dr_overridden_by, type);
        BP_SET_LEVEL(&dl->dr_overridden_by, 0);
        BP_SET_BYTEORDER(&dl->dr_overridden_by, byteorder);

        dl->dr_override_state = DR_OVERRIDDEN;
        dl->dr_overridden_by.blk_birth = db->db_last_dirty->dr_txg;
}

/*
 * Directly assign a provided arc buf to a given dbuf if it's not referenced
 * by anybody except our caller. Otherwise copy arcbuf's contents to dbuf.
 */
void
dbuf_assign_arcbuf(dmu_buf_impl_t *db, arc_buf_t *buf, dmu_tx_t *tx)
{
        ASSERT(!refcount_is_zero(&db->db_holds));
        ASSERT(db->db_blkid != DMU_BONUS_BLKID);
        ASSERT(db->db_level == 0);
        ASSERT3U(dbuf_is_metadata(db), ==, arc_is_metadata(buf));
        ASSERT(buf != NULL);
        ASSERT(arc_buf_lsize(buf) == db->db.db_size);
        ASSERT(tx->tx_txg != 0);

        arc_return_buf(buf, db);
        ASSERT(arc_released(buf));

        mutex_enter(&db->db_mtx);

        while (db->db_state == DB_READ || db->db_state == DB_FILL)
                cv_wait(&db->db_changed, &db->db_mtx);

        ASSERT(db->db_state == DB_CACHED || db->db_state == DB_UNCACHED);

        if (db->db_state == DB_CACHED &&
            refcount_count(&db->db_holds) - 1 > db->db_dirtycnt) {
                mutex_exit(&db->db_mtx);
                (void) dbuf_dirty(db, tx);
                bcopy(buf->b_data, db->db.db_data, db->db.db_size);
                arc_buf_destroy(buf, db);
                xuio_stat_wbuf_copied();
                return;
        }

        xuio_stat_wbuf_nocopy();
        if (db->db_state == DB_CACHED) {
                dbuf_dirty_record_t *dr = db->db_last_dirty;

                ASSERT(db->db_buf != NULL);
                if (dr != NULL && dr->dr_txg == tx->tx_txg) {
                        ASSERT(dr->dt.dl.dr_data == db->db_buf);
                        if (!arc_released(db->db_buf)) {
                                ASSERT(dr->dt.dl.dr_override_state ==
                                    DR_OVERRIDDEN);
                                arc_release(db->db_buf, db);
                        }
                        dr->dt.dl.dr_data = buf;
                        arc_buf_destroy(db->db_buf, db);
                } else if (dr == NULL || dr->dt.dl.dr_data != db->db_buf) {
                        arc_release(db->db_buf, db);
                        arc_buf_destroy(db->db_buf, db);
                }
                db->db_buf = NULL;
        }
        ASSERT(db->db_buf == NULL);
        dbuf_set_data(db, buf);
        db->db_state = DB_FILL;
        mutex_exit(&db->db_mtx);
        (void) dbuf_dirty(db, tx);
        dmu_buf_fill_done(&db->db, tx);
}

void
dbuf_destroy(dmu_buf_impl_t *db)
{
        dnode_t *dn;
        dmu_buf_impl_t *parent = db->db_parent;
        dmu_buf_impl_t *dndb;

        ASSERT(MUTEX_HELD(&db->db_mtx));
        ASSERT(refcount_is_zero(&db->db_holds));

        if (db->db_buf != NULL) {
                arc_buf_destroy(db->db_buf, db);
                db->db_buf = NULL;
        }

        if (db->db_blkid == DMU_BONUS_BLKID) {
                ASSERT(db->db.db_data != NULL);
                zio_buf_free(db->db.db_data, DN_MAX_BONUSLEN);
                arc_space_return(DN_MAX_BONUSLEN, ARC_SPACE_OTHER);
                db->db_state = DB_UNCACHED;
        }

        dbuf_clear_data(db);

        if (multilist_link_active(&db->db_cache_link)) {
                ASSERT(db->db_caching_status == DB_DBUF_CACHE ||
                    db->db_caching_status == DB_DBUF_METADATA_CACHE);

                multilist_remove(dbuf_caches[db->db_caching_status].cache, db);
                (void) refcount_remove_many(
                    &dbuf_caches[db->db_caching_status].size,
                    db->db.db_size, db);

                db->db_caching_status = DB_NO_CACHE;
        }

        ASSERT(db->db_state == DB_UNCACHED || db->db_state == DB_NOFILL);
        ASSERT(db->db_data_pending == NULL);

        db->db_state = DB_EVICTING;
        db->db_blkptr = NULL;

        /*
         * Now that db_state is DB_EVICTING, nobody else can find this via
         * the hash table.  We can now drop db_mtx, which allows us to
         * acquire the dn_dbufs_mtx.
         */
        mutex_exit(&db->db_mtx);

        DB_DNODE_ENTER(db);
        dn = DB_DNODE(db);
        dndb = dn->dn_dbuf;
        if (db->db_blkid != DMU_BONUS_BLKID) {
                boolean_t needlock = !MUTEX_HELD(&dn->dn_dbufs_mtx);
                if (needlock)
                        mutex_enter(&dn->dn_dbufs_mtx);
                avl_remove(&dn->dn_dbufs, db);
                atomic_dec_32(&dn->dn_dbufs_count);
                membar_producer();
                DB_DNODE_EXIT(db);
                if (needlock)
                        mutex_exit(&dn->dn_dbufs_mtx);
                /*
                 * Decrementing the dbuf count means that the hold corresponding
                 * to the removed dbuf is no longer discounted in dnode_move(),
                 * so the dnode cannot be moved until after we release the hold.
                 * The membar_producer() ensures visibility of the decremented
                 * value in dnode_move(), since DB_DNODE_EXIT doesn't actually
                 * release any lock.
                 */
                dnode_rele(dn, db);
                db->db_dnode_handle = NULL;

                dbuf_hash_remove(db);
        } else {
                DB_DNODE_EXIT(db);
        }

        ASSERT(refcount_is_zero(&db->db_holds));

        db->db_parent = NULL;

        ASSERT(db->db_buf == NULL);
        ASSERT(db->db.db_data == NULL);
        ASSERT(db->db_hash_next == NULL);
        ASSERT(db->db_blkptr == NULL);
        ASSERT(db->db_data_pending == NULL);
        ASSERT3U(db->db_caching_status, ==, DB_NO_CACHE);
        ASSERT(!multilist_link_active(&db->db_cache_link));

        kmem_cache_free(dbuf_kmem_cache, db);
        arc_space_return(sizeof (dmu_buf_impl_t), ARC_SPACE_OTHER);

        /*
         * If this dbuf is referenced from an indirect dbuf,
         * decrement the ref count on the indirect dbuf.
         */
        if (parent && parent != dndb)
                dbuf_rele(parent, db);
}

/*
 * Note: While bpp will always be updated if the function returns success,
 * parentp will not be updated if the dnode does not have dn_dbuf filled in;
 * this happens when the dnode is the meta-dnode, or a userused or groupused
 * object.
 */
static int
dbuf_findbp(dnode_t *dn, int level, uint64_t blkid, int fail_sparse,
    dmu_buf_impl_t **parentp, blkptr_t **bpp)
{
        *parentp = NULL;
        *bpp = NULL;

        ASSERT(blkid != DMU_BONUS_BLKID);

        if (blkid == DMU_SPILL_BLKID) {
                mutex_enter(&dn->dn_mtx);
                if (dn->dn_have_spill &&
                    (dn->dn_phys->dn_flags & DNODE_FLAG_SPILL_BLKPTR))
                        *bpp = &dn->dn_phys->dn_spill;
                else
                        *bpp = NULL;
                dbuf_add_ref(dn->dn_dbuf, NULL);
                *parentp = dn->dn_dbuf;
                mutex_exit(&dn->dn_mtx);
                return (0);
        }

        int nlevels =
            (dn->dn_phys->dn_nlevels == 0) ? 1 : dn->dn_phys->dn_nlevels;
        int epbs = dn->dn_indblkshift - SPA_BLKPTRSHIFT;

        ASSERT3U(level * epbs, <, 64);
        ASSERT(RW_LOCK_HELD(&dn->dn_struct_rwlock));
        /*
         * This assertion shouldn't trip as long as the max indirect block size
         * is less than 1M.  The reason for this is that up to that point,
         * the number of levels required to address an entire object with blocks
         * of size SPA_MINBLOCKSIZE satisfies nlevels * epbs + 1 <= 64.  In
         * other words, if N * epbs + 1 > 64, then if (N-1) * epbs + 1 > 55
         * (i.e. we can address the entire object), objects will all use at most
         * N-1 levels and the assertion won't overflow.  However, once epbs is
         * 13, 4 * 13 + 1 = 53, but 5 * 13 + 1 = 66.  Then, 4 levels will not be
         * enough to address an entire object, so objects will have 5 levels,
         * but then this assertion will overflow.
         *
         * All this is to say that if we ever increase DN_MAX_INDBLKSHIFT, we
         * need to redo this logic to handle overflows.
         */
        ASSERT(level >= nlevels ||
            ((nlevels - level - 1) * epbs) +
            highbit64(dn->dn_phys->dn_nblkptr) <= 64);
        if (level >= nlevels ||
            blkid >= ((uint64_t)dn->dn_phys->dn_nblkptr <<
            ((nlevels - level - 1) * epbs)) ||
            (fail_sparse &&
            blkid > (dn->dn_phys->dn_maxblkid >> (level * epbs)))) {
                /* the buffer has no parent yet */
                return (SET_ERROR(ENOENT));
        } else if (level < nlevels-1) {
                /* this block is referenced from an indirect block */
                int err = dbuf_hold_impl(dn, level+1,
                    blkid >> epbs, fail_sparse, FALSE, NULL, parentp);
                if (err)
                        return (err);
                err = dbuf_read(*parentp, NULL,
                    (DB_RF_HAVESTRUCT | DB_RF_NOPREFETCH | DB_RF_CANFAIL));
                if (err) {
                        dbuf_rele(*parentp, NULL);
                        *parentp = NULL;
                        return (err);
                }
                *bpp = ((blkptr_t *)(*parentp)->db.db_data) +
                    (blkid & ((1ULL << epbs) - 1));
                if (blkid > (dn->dn_phys->dn_maxblkid >> (level * epbs)))
                        ASSERT(BP_IS_HOLE(*bpp));
                return (0);
        } else {
                /* the block is referenced from the dnode */
                ASSERT3U(level, ==, nlevels-1);
                ASSERT(dn->dn_phys->dn_nblkptr == 0 ||
                    blkid < dn->dn_phys->dn_nblkptr);
                if (dn->dn_dbuf) {
                        dbuf_add_ref(dn->dn_dbuf, NULL);
                        *parentp = dn->dn_dbuf;
                }
                *bpp = &dn->dn_phys->dn_blkptr[blkid];
                return (0);
        }
}

static dmu_buf_impl_t *
dbuf_create(dnode_t *dn, uint8_t level, uint64_t blkid,
    dmu_buf_impl_t *parent, blkptr_t *blkptr)
{
        objset_t *os = dn->dn_objset;
        dmu_buf_impl_t *db, *odb;

        ASSERT(RW_LOCK_HELD(&dn->dn_struct_rwlock));
        ASSERT(dn->dn_type != DMU_OT_NONE);

        db = kmem_cache_alloc(dbuf_kmem_cache, KM_SLEEP);

        db->db_objset = os;
        db->db.db_object = dn->dn_object;
        db->db_level = level;
        db->db_blkid = blkid;
        db->db_last_dirty = NULL;
        db->db_dirtycnt = 0;
        db->db_dnode_handle = dn->dn_handle;
        db->db_parent = parent;
        db->db_blkptr = blkptr;

        db->db_user = NULL;
        db->db_user_immediate_evict = FALSE;
        db->db_freed_in_flight = FALSE;
        db->db_pending_evict = FALSE;

        if (blkid == DMU_BONUS_BLKID) {
                ASSERT3P(parent, ==, dn->dn_dbuf);
                db->db.db_size = DN_MAX_BONUSLEN -
                    (dn->dn_nblkptr-1) * sizeof (blkptr_t);
                ASSERT3U(db->db.db_size, >=, dn->dn_bonuslen);
                db->db.db_offset = DMU_BONUS_BLKID;
                db->db_state = DB_UNCACHED;
                db->db_caching_status = DB_NO_CACHE;
                /* the bonus dbuf is not placed in the hash table */
                arc_space_consume(sizeof (dmu_buf_impl_t), ARC_SPACE_OTHER);
                return (db);
        } else if (blkid == DMU_SPILL_BLKID) {
                db->db.db_size = (blkptr != NULL) ?
                    BP_GET_LSIZE(blkptr) : SPA_MINBLOCKSIZE;
                db->db.db_offset = 0;
        } else {
                int blocksize =
                    db->db_level ? 1 << dn->dn_indblkshift : dn->dn_datablksz;
                db->db.db_size = blocksize;
                db->db.db_offset = db->db_blkid * blocksize;
        }

        /*
         * Hold the dn_dbufs_mtx while we get the new dbuf
         * in the hash table *and* added to the dbufs list.
         * This prevents a possible deadlock with someone
         * trying to look up this dbuf before its added to the
         * dn_dbufs list.
         */
        mutex_enter(&dn->dn_dbufs_mtx);
        db->db_state = DB_EVICTING;
        if ((odb = dbuf_hash_insert(db)) != NULL) {
                /* someone else inserted it first */
                kmem_cache_free(dbuf_kmem_cache, db);
                mutex_exit(&dn->dn_dbufs_mtx);
                return (odb);
        }
        avl_add(&dn->dn_dbufs, db);

        db->db_state = DB_UNCACHED;
        db->db_caching_status = DB_NO_CACHE;
        mutex_exit(&dn->dn_dbufs_mtx);
        arc_space_consume(sizeof (dmu_buf_impl_t), ARC_SPACE_OTHER);

        if (parent && parent != dn->dn_dbuf)
                dbuf_add_ref(parent, db);

        ASSERT(dn->dn_object == DMU_META_DNODE_OBJECT ||
            refcount_count(&dn->dn_holds) > 0);
        (void) refcount_add(&dn->dn_holds, db);
        atomic_inc_32(&dn->dn_dbufs_count);

        dprintf_dbuf(db, "db=%p\n", db);

        return (db);
}

typedef struct dbuf_prefetch_arg {
        spa_t *dpa_spa; /* The spa to issue the prefetch in. */
        zbookmark_phys_t dpa_zb; /* The target block to prefetch. */
        int dpa_epbs; /* Entries (blkptr_t's) Per Block Shift. */
        int dpa_curlevel; /* The current level that we're reading */
        dnode_t *dpa_dnode; /* The dnode associated with the prefetch */
        zio_priority_t dpa_prio; /* The priority I/Os should be issued at. */
        zio_t *dpa_zio; /* The parent zio_t for all prefetches. */
        arc_flags_t dpa_aflags; /* Flags to pass to the final prefetch. */
} dbuf_prefetch_arg_t;

/*
 * Actually issue the prefetch read for the block given.
 */
static void
dbuf_issue_final_prefetch(dbuf_prefetch_arg_t *dpa, blkptr_t *bp)
{
        if (BP_IS_HOLE(bp) || BP_IS_EMBEDDED(bp))
                return;

        arc_flags_t aflags =
            dpa->dpa_aflags | ARC_FLAG_NOWAIT | ARC_FLAG_PREFETCH;

        ASSERT3U(dpa->dpa_curlevel, ==, BP_GET_LEVEL(bp));
        ASSERT3U(dpa->dpa_curlevel, ==, dpa->dpa_zb.zb_level);
        ASSERT(dpa->dpa_zio != NULL);
        (void) arc_read(dpa->dpa_zio, dpa->dpa_spa, bp, NULL, NULL,
            dpa->dpa_prio, ZIO_FLAG_CANFAIL | ZIO_FLAG_SPECULATIVE,
            &aflags, &dpa->dpa_zb);
}

/*
 * Called when an indirect block above our prefetch target is read in.  This
 * will either read in the next indirect block down the tree or issue the actual
 * prefetch if the next block down is our target.
 */
static void
dbuf_prefetch_indirect_done(zio_t *zio, const zbookmark_phys_t *zb,
    const blkptr_t *iobp, arc_buf_t *abuf, void *private)
{
        dbuf_prefetch_arg_t *dpa = private;

        ASSERT3S(dpa->dpa_zb.zb_level, <, dpa->dpa_curlevel);
        ASSERT3S(dpa->dpa_curlevel, >, 0);

        /*
         * The dpa_dnode is only valid if we are called with a NULL
         * zio. This indicates that the arc_read() returned without
         * first calling zio_read() to issue a physical read. Once
         * a physical read is made the dpa_dnode must be invalidated
         * as the locks guarding it may have been dropped. If the
         * dpa_dnode is still valid, then we want to add it to the dbuf
         * cache. To do so, we must hold the dbuf associated with the block
         * we just prefetched, read its contents so that we associate it
         * with an arc_buf_t, and then release it.
         */
        if (zio != NULL) {
                ASSERT3S(BP_GET_LEVEL(zio->io_bp), ==, dpa->dpa_curlevel);
                if (zio->io_flags & ZIO_FLAG_RAW) {
                        ASSERT3U(BP_GET_PSIZE(zio->io_bp), ==, zio->io_size);
                } else {
                        ASSERT3U(BP_GET_LSIZE(zio->io_bp), ==, zio->io_size);
                }
                ASSERT3P(zio->io_spa, ==, dpa->dpa_spa);

                dpa->dpa_dnode = NULL;
        } else if (dpa->dpa_dnode != NULL) {
                uint64_t curblkid = dpa->dpa_zb.zb_blkid >>
                    (dpa->dpa_epbs * (dpa->dpa_curlevel -
                    dpa->dpa_zb.zb_level));
                dmu_buf_impl_t *db = dbuf_hold_level(dpa->dpa_dnode,
                    dpa->dpa_curlevel, curblkid, FTAG);
                (void) dbuf_read(db, NULL,
                    DB_RF_MUST_SUCCEED | DB_RF_NOPREFETCH | DB_RF_HAVESTRUCT);
                dbuf_rele(db, FTAG);
        }

        if (abuf == NULL) {
                kmem_free(dpa, sizeof(*dpa));
                return;
        }
        
        dpa->dpa_curlevel--;

        uint64_t nextblkid = dpa->dpa_zb.zb_blkid >>
            (dpa->dpa_epbs * (dpa->dpa_curlevel - dpa->dpa_zb.zb_level));
        blkptr_t *bp = ((blkptr_t *)abuf->b_data) +
            P2PHASE(nextblkid, 1ULL << dpa->dpa_epbs);
        if (BP_IS_HOLE(bp)) {
                kmem_free(dpa, sizeof (*dpa));
        } else if (dpa->dpa_curlevel == dpa->dpa_zb.zb_level) {
                ASSERT3U(nextblkid, ==, dpa->dpa_zb.zb_blkid);
                dbuf_issue_final_prefetch(dpa, bp);
                kmem_free(dpa, sizeof (*dpa));
        } else {
                arc_flags_t iter_aflags = ARC_FLAG_NOWAIT;
                zbookmark_phys_t zb;

                /* flag if L2ARC eligible, l2arc_noprefetch then decides */
                if (dpa->dpa_aflags & ARC_FLAG_L2CACHE)
                        iter_aflags |= ARC_FLAG_L2CACHE;

                ASSERT3U(dpa->dpa_curlevel, ==, BP_GET_LEVEL(bp));

                SET_BOOKMARK(&zb, dpa->dpa_zb.zb_objset,
                    dpa->dpa_zb.zb_object, dpa->dpa_curlevel, nextblkid);

                (void) arc_read(dpa->dpa_zio, dpa->dpa_spa,
                    bp, dbuf_prefetch_indirect_done, dpa, dpa->dpa_prio,
                    ZIO_FLAG_CANFAIL | ZIO_FLAG_SPECULATIVE,
                    &iter_aflags, &zb);
        }

        arc_buf_destroy(abuf, private);
}

/*
 * Issue prefetch reads for the given block on the given level.  If the indirect
 * blocks above that block are not in memory, we will read them in
 * asynchronously.  As a result, this call never blocks waiting for a read to
 * complete.
 */
void
dbuf_prefetch(dnode_t *dn, int64_t level, uint64_t blkid, zio_priority_t prio,
    arc_flags_t aflags)
{
        blkptr_t bp;
        int epbs, nlevels, curlevel;
        uint64_t curblkid;

        ASSERT(blkid != DMU_BONUS_BLKID);
        ASSERT(RW_LOCK_HELD(&dn->dn_struct_rwlock));

        if (blkid > dn->dn_maxblkid)
                return;

        if (dnode_block_freed(dn, blkid))
                return;

        /*
         * This dnode hasn't been written to disk yet, so there's nothing to
         * prefetch.
         */
        nlevels = dn->dn_phys->dn_nlevels;
        if (level >= nlevels || dn->dn_phys->dn_nblkptr == 0)
                return;

        epbs = dn->dn_phys->dn_indblkshift - SPA_BLKPTRSHIFT;
        if (dn->dn_phys->dn_maxblkid < blkid << (epbs * level))
                return;

        dmu_buf_impl_t *db = dbuf_find(dn->dn_objset, dn->dn_object,
            level, blkid);
        if (db != NULL) {
                mutex_exit(&db->db_mtx);
                /*
                 * This dbuf already exists.  It is either CACHED, or
                 * (we assume) about to be read or filled.
                 */
                return;
        }

        /*
         * Find the closest ancestor (indirect block) of the target block
         * that is present in the cache.  In this indirect block, we will
         * find the bp that is at curlevel, curblkid.
         */
        curlevel = level;
        curblkid = blkid;
        while (curlevel < nlevels - 1) {
                int parent_level = curlevel + 1;
                uint64_t parent_blkid = curblkid >> epbs;
                dmu_buf_impl_t *db;

                if (dbuf_hold_impl(dn, parent_level, parent_blkid,
                    FALSE, TRUE, FTAG, &db) == 0) {
                        blkptr_t *bpp = db->db_buf->b_data;
                        bp = bpp[P2PHASE(curblkid, 1 << epbs)];
                        dbuf_rele(db, FTAG);
                        break;
                }

                curlevel = parent_level;
                curblkid = parent_blkid;
        }

        if (curlevel == nlevels - 1) {
                /* No cached indirect blocks found. */
                ASSERT3U(curblkid, <, dn->dn_phys->dn_nblkptr);
                bp = dn->dn_phys->dn_blkptr[curblkid];
        }
        if (BP_IS_HOLE(&bp))
                return;

        ASSERT3U(curlevel, ==, BP_GET_LEVEL(&bp));

        zio_t *pio = zio_root(dmu_objset_spa(dn->dn_objset), NULL, NULL,
            ZIO_FLAG_CANFAIL);

        dbuf_prefetch_arg_t *dpa = kmem_zalloc(sizeof (*dpa), KM_SLEEP);
        dsl_dataset_t *ds = dn->dn_objset->os_dsl_dataset;
        SET_BOOKMARK(&dpa->dpa_zb, ds != NULL ? ds->ds_object : DMU_META_OBJSET,
            dn->dn_object, level, blkid);
        dpa->dpa_curlevel = curlevel;
        dpa->dpa_prio = prio;
        dpa->dpa_aflags = aflags;
        dpa->dpa_spa = dn->dn_objset->os_spa;
        dpa->dpa_dnode = dn;
        dpa->dpa_epbs = epbs;
        dpa->dpa_zio = pio;

        /* flag if L2ARC eligible, l2arc_noprefetch then decides */
        if (DNODE_LEVEL_IS_L2CACHEABLE(dn, level))
                dpa->dpa_aflags |= ARC_FLAG_L2CACHE;

        /*
         * If we have the indirect just above us, no need to do the asynchronous
         * prefetch chain; we'll just run the last step ourselves.  If we're at
         * a higher level, though, we want to issue the prefetches for all the
         * indirect blocks asynchronously, so we can go on with whatever we were
         * doing.
         */
        if (curlevel == level) {
                ASSERT3U(curblkid, ==, blkid);
                dbuf_issue_final_prefetch(dpa, &bp);
                kmem_free(dpa, sizeof (*dpa));
        } else {
                arc_flags_t iter_aflags = ARC_FLAG_NOWAIT;
                zbookmark_phys_t zb;

                /* flag if L2ARC eligible, l2arc_noprefetch then decides */
                if (DNODE_LEVEL_IS_L2CACHEABLE(dn, level))
                        iter_aflags |= ARC_FLAG_L2CACHE;

                SET_BOOKMARK(&zb, ds != NULL ? ds->ds_object : DMU_META_OBJSET,
                    dn->dn_object, curlevel, curblkid);
                (void) arc_read(dpa->dpa_zio, dpa->dpa_spa,
                    &bp, dbuf_prefetch_indirect_done, dpa, prio,
                    ZIO_FLAG_CANFAIL | ZIO_FLAG_SPECULATIVE,
                    &iter_aflags, &zb);
        }
        /*
         * We use pio here instead of dpa_zio since it's possible that
         * dpa may have already been freed.
         */
        zio_nowait(pio);
}

/*
 * Returns with db_holds incremented, and db_mtx not held.
 * Note: dn_struct_rwlock must be held.
 */
int
dbuf_hold_impl(dnode_t *dn, uint8_t level, uint64_t blkid,
    boolean_t fail_sparse, boolean_t fail_uncached,
    void *tag, dmu_buf_impl_t **dbp)
{
        dmu_buf_impl_t *db, *parent = NULL;

        ASSERT(blkid != DMU_BONUS_BLKID);
        ASSERT(RW_LOCK_HELD(&dn->dn_struct_rwlock));
        ASSERT3U(dn->dn_nlevels, >, level);

        *dbp = NULL;
top:
        /* dbuf_find() returns with db_mtx held */
        db = dbuf_find(dn->dn_objset, dn->dn_object, level, blkid);

        if (db == NULL) {
                blkptr_t *bp = NULL;
                int err;

                if (fail_uncached)
                        return (SET_ERROR(ENOENT));

                ASSERT3P(parent, ==, NULL);
                err = dbuf_findbp(dn, level, blkid, fail_sparse, &parent, &bp);
                if (fail_sparse) {
                        if (err == 0 && bp && BP_IS_HOLE(bp))
                                err = SET_ERROR(ENOENT);
                        if (err) {
                                if (parent)
                                        dbuf_rele(parent, NULL);
                                return (err);
                        }
                }
                if (err && err != ENOENT)
                        return (err);
                db = dbuf_create(dn, level, blkid, parent, bp);
        }

        if (fail_uncached && db->db_state != DB_CACHED) {
                mutex_exit(&db->db_mtx);
                return (SET_ERROR(ENOENT));
        }

        if (db->db_buf != NULL) {
                arc_buf_access(db->db_buf);
                ASSERT3P(db->db.db_data, ==, db->db_buf->b_data);
        }

        ASSERT(db->db_buf == NULL || arc_referenced(db->db_buf));

        /*
         * If this buffer is currently syncing out, and we are are
         * still referencing it from db_data, we need to make a copy
         * of it in case we decide we want to dirty it again in this txg.
         */
        if (db->db_level == 0 && db->db_blkid != DMU_BONUS_BLKID &&
            dn->dn_object != DMU_META_DNODE_OBJECT &&
            db->db_state == DB_CACHED && db->db_data_pending) {
                dbuf_dirty_record_t *dr = db->db_data_pending;

                if (dr->dt.dl.dr_data == db->db_buf) {
                        arc_buf_contents_t type = DBUF_GET_BUFC_TYPE(db);

                        dbuf_set_data(db,
                            arc_alloc_buf(dn->dn_objset->os_spa, db, type,
                            db->db.db_size));
                        bcopy(dr->dt.dl.dr_data->b_data, db->db.db_data,
                            db->db.db_size);
                }
        }

        if (multilist_link_active(&db->db_cache_link)) {
                ASSERT(refcount_is_zero(&db->db_holds));
                ASSERT(db->db_caching_status == DB_DBUF_CACHE ||
                    db->db_caching_status == DB_DBUF_METADATA_CACHE);

                multilist_remove(dbuf_caches[db->db_caching_status].cache, db);
                (void) refcount_remove_many(
                    &dbuf_caches[db->db_caching_status].size,
                    db->db.db_size, db);

                db->db_caching_status = DB_NO_CACHE;
        }
        (void) refcount_add(&db->db_holds, tag);
        DBUF_VERIFY(db);
        mutex_exit(&db->db_mtx);

        /* NOTE: we can't rele the parent until after we drop the db_mtx */
        if (parent)
                dbuf_rele(parent, NULL);

        ASSERT3P(DB_DNODE(db), ==, dn);
        ASSERT3U(db->db_blkid, ==, blkid);
        ASSERT3U(db->db_level, ==, level);
        *dbp = db;

        return (0);
}

dmu_buf_impl_t *
dbuf_hold(dnode_t *dn, uint64_t blkid, void *tag)
{
        return (dbuf_hold_level(dn, 0, blkid, tag));
}

dmu_buf_impl_t *
dbuf_hold_level(dnode_t *dn, int level, uint64_t blkid, void *tag)
{
        dmu_buf_impl_t *db;
        int err = dbuf_hold_impl(dn, level, blkid, FALSE, FALSE, tag, &db);
        return (err ? NULL : db);
}

void
dbuf_create_bonus(dnode_t *dn)
{
        ASSERT(RW_WRITE_HELD(&dn->dn_struct_rwlock));

        ASSERT(dn->dn_bonus == NULL);
        dn->dn_bonus = dbuf_create(dn, 0, DMU_BONUS_BLKID, dn->dn_dbuf, NULL);
}

int
dbuf_spill_set_blksz(dmu_buf_t *db_fake, uint64_t blksz, dmu_tx_t *tx)
{
        dmu_buf_impl_t *db = (dmu_buf_impl_t *)db_fake;
        dnode_t *dn;

        if (db->db_blkid != DMU_SPILL_BLKID)
                return (SET_ERROR(ENOTSUP));
        if (blksz == 0)
                blksz = SPA_MINBLOCKSIZE;
        ASSERT3U(blksz, <=, spa_maxblocksize(dmu_objset_spa(db->db_objset)));
        blksz = P2ROUNDUP(blksz, SPA_MINBLOCKSIZE);

        DB_DNODE_ENTER(db);
        dn = DB_DNODE(db);
        rw_enter(&dn->dn_struct_rwlock, RW_WRITER);
        dbuf_new_size(db, blksz, tx);
        rw_exit(&dn->dn_struct_rwlock);
        DB_DNODE_EXIT(db);

        return (0);
}

void
dbuf_rm_spill(dnode_t *dn, dmu_tx_t *tx)
{
        dbuf_free_range(dn, DMU_SPILL_BLKID, DMU_SPILL_BLKID, tx);
}

#pragma weak dmu_buf_add_ref = dbuf_add_ref
void
dbuf_add_ref(dmu_buf_impl_t *db, void *tag)
{
        int64_t holds = refcount_add(&db->db_holds, tag);
        ASSERT3S(holds, >, 1);
}

#pragma weak dmu_buf_try_add_ref = dbuf_try_add_ref
boolean_t
dbuf_try_add_ref(dmu_buf_t *db_fake, objset_t *os, uint64_t obj, uint64_t blkid,
    void *tag)
{
        dmu_buf_impl_t *db = (dmu_buf_impl_t *)db_fake;
        dmu_buf_impl_t *found_db;
        boolean_t result = B_FALSE;

        if (db->db_blkid == DMU_BONUS_BLKID)
                found_db = dbuf_find_bonus(os, obj);
        else
                found_db = dbuf_find(os, obj, 0, blkid);

        if (found_db != NULL) {
                if (db == found_db && dbuf_refcount(db) > db->db_dirtycnt) {
                        (void) refcount_add(&db->db_holds, tag);
                        result = B_TRUE;
                }
                mutex_exit(&db->db_mtx);
        }
        return (result);
}

/*
 * If you call dbuf_rele() you had better not be referencing the dnode handle
 * unless you have some other direct or indirect hold on the dnode. (An indirect
 * hold is a hold on one of the dnode's dbufs, including the bonus buffer.)
 * Without that, the dbuf_rele() could lead to a dnode_rele() followed by the
 * dnode's parent dbuf evicting its dnode handles.
 */
void
dbuf_rele(dmu_buf_impl_t *db, void *tag)
{
        mutex_enter(&db->db_mtx);
        dbuf_rele_and_unlock(db, tag);
}

void
dmu_buf_rele(dmu_buf_t *db, void *tag)
{
        dbuf_rele((dmu_buf_impl_t *)db, tag);
}

/*
 * dbuf_rele() for an already-locked dbuf.  This is necessary to allow
 * db_dirtycnt and db_holds to be updated atomically.
 */
void
dbuf_rele_and_unlock(dmu_buf_impl_t *db, void *tag)
{
        int64_t holds;

        ASSERT(MUTEX_HELD(&db->db_mtx));
        DBUF_VERIFY(db);

        /*
         * Remove the reference to the dbuf before removing its hold on the
         * dnode so we can guarantee in dnode_move() that a referenced bonus
         * buffer has a corresponding dnode hold.
         */
        holds = refcount_remove(&db->db_holds, tag);
        ASSERT(holds >= 0);

        /*
         * We can't freeze indirects if there is a possibility that they
         * may be modified in the current syncing context.
         */
        if (db->db_buf != NULL &&
            holds == (db->db_level == 0 ? db->db_dirtycnt : 0)) {
                arc_buf_freeze(db->db_buf);
        }

        if (holds == db->db_dirtycnt &&
            db->db_level == 0 && db->db_user_immediate_evict)
                dbuf_evict_user(db);

        if (holds == 0) {
                if (db->db_blkid == DMU_BONUS_BLKID) {
                        dnode_t *dn;
                        boolean_t evict_dbuf = db->db_pending_evict;

                        /*
                         * If the dnode moves here, we cannot cross this
                         * barrier until the move completes.
                         */
                        DB_DNODE_ENTER(db);

                        dn = DB_DNODE(db);
                        atomic_dec_32(&dn->dn_dbufs_count);

                        /*
                         * Decrementing the dbuf count means that the bonus
                         * buffer's dnode hold is no longer discounted in
                         * dnode_move(). The dnode cannot move until after
                         * the dnode_rele() below.
                         */
                        DB_DNODE_EXIT(db);

                        /*
                         * Do not reference db after its lock is dropped.
                         * Another thread may evict it.
                         */
                        mutex_exit(&db->db_mtx);

                        if (evict_dbuf)
                                dnode_evict_bonus(dn);

                        dnode_rele(dn, db);
                } else if (db->db_buf == NULL) {
                        /*
                         * This is a special case: we never associated this
                         * dbuf with any data allocated from the ARC.
                         */
                        ASSERT(db->db_state == DB_UNCACHED ||
                            db->db_state == DB_NOFILL);
                        dbuf_destroy(db);
                } else if (arc_released(db->db_buf)) {
                        /*
                         * This dbuf has anonymous data associated with it.
                         */
                        dbuf_destroy(db);
                } else {
                        boolean_t do_arc_evict = B_FALSE;
                        blkptr_t bp;
                        spa_t *spa = dmu_objset_spa(db->db_objset);

                        if (!DBUF_IS_CACHEABLE(db) &&
                            db->db_blkptr != NULL &&
                            !BP_IS_HOLE(db->db_blkptr) &&
                            !BP_IS_EMBEDDED(db->db_blkptr)) {
                                do_arc_evict = B_TRUE;
                                bp = *db->db_blkptr;
                        }

                        if (!DBUF_IS_CACHEABLE(db) ||
                            db->db_pending_evict) {
                                dbuf_destroy(db);
                        } else if (!multilist_link_active(&db->db_cache_link)) {
                                ASSERT3U(db->db_caching_status, ==,
                                    DB_NO_CACHE);

                                dbuf_cached_state_t dcs =
                                    dbuf_include_in_metadata_cache(db) ?
                                    DB_DBUF_METADATA_CACHE : DB_DBUF_CACHE;
                                db->db_caching_status = dcs;

                                multilist_insert(dbuf_caches[dcs].cache, db);
                                (void) refcount_add_many(&dbuf_caches[dcs].size,
                                    db->db.db_size, db);
                                mutex_exit(&db->db_mtx);

                                if (db->db_caching_status == DB_DBUF_CACHE) {
                                        dbuf_evict_notify();
                                }
                        }

                        if (do_arc_evict)
                                arc_freed(spa, &bp);
                }
        } else {
                mutex_exit(&db->db_mtx);
        }

}

#pragma weak dmu_buf_refcount = dbuf_refcount
uint64_t
dbuf_refcount(dmu_buf_impl_t *db)
{
        return (refcount_count(&db->db_holds));
}

void *
dmu_buf_replace_user(dmu_buf_t *db_fake, dmu_buf_user_t *old_user,
    dmu_buf_user_t *new_user)
{
        dmu_buf_impl_t *db = (dmu_buf_impl_t *)db_fake;

        mutex_enter(&db->db_mtx);
        dbuf_verify_user(db, DBVU_NOT_EVICTING);
        if (db->db_user == old_user)
                db->db_user = new_user;
        else
                old_user = db->db_user;
        dbuf_verify_user(db, DBVU_NOT_EVICTING);
        mutex_exit(&db->db_mtx);

        return (old_user);
}

void *
dmu_buf_set_user(dmu_buf_t *db_fake, dmu_buf_user_t *user)
{
        return (dmu_buf_replace_user(db_fake, NULL, user));
}

void *
dmu_buf_set_user_ie(dmu_buf_t *db_fake, dmu_buf_user_t *user)
{
        dmu_buf_impl_t *db = (dmu_buf_impl_t *)db_fake;

        db->db_user_immediate_evict = TRUE;
        return (dmu_buf_set_user(db_fake, user));
}

void *
dmu_buf_remove_user(dmu_buf_t *db_fake, dmu_buf_user_t *user)
{
        return (dmu_buf_replace_user(db_fake, user, NULL));
}

void *
dmu_buf_get_user(dmu_buf_t *db_fake)
{
        dmu_buf_impl_t *db = (dmu_buf_impl_t *)db_fake;

        dbuf_verify_user(db, DBVU_NOT_EVICTING);
        return (db->db_user);
}

void
dmu_buf_user_evict_wait()
{
        taskq_wait(dbu_evict_taskq);
}

blkptr_t *
dmu_buf_get_blkptr(dmu_buf_t *db)
{
        dmu_buf_impl_t *dbi = (dmu_buf_impl_t *)db;
        return (dbi->db_blkptr);
}

objset_t *
dmu_buf_get_objset(dmu_buf_t *db)
{
        dmu_buf_impl_t *dbi = (dmu_buf_impl_t *)db;
        return (dbi->db_objset);
}

dnode_t *
dmu_buf_dnode_enter(dmu_buf_t *db)
{
        dmu_buf_impl_t *dbi = (dmu_buf_impl_t *)db;
        DB_DNODE_ENTER(dbi);
        return (DB_DNODE(dbi));
}

void
dmu_buf_dnode_exit(dmu_buf_t *db)
{
        dmu_buf_impl_t *dbi = (dmu_buf_impl_t *)db;
        DB_DNODE_EXIT(dbi);
}

static void
dbuf_check_blkptr(dnode_t *dn, dmu_buf_impl_t *db)
{
        /* ASSERT(dmu_tx_is_syncing(tx) */
        ASSERT(MUTEX_HELD(&db->db_mtx));

        if (db->db_blkptr != NULL)
                return;

        if (db->db_blkid == DMU_SPILL_BLKID) {
                db->db_blkptr = &dn->dn_phys->dn_spill;
                BP_ZERO(db->db_blkptr);
                return;
        }
        if (db->db_level == dn->dn_phys->dn_nlevels-1) {
                /*
                 * This buffer was allocated at a time when there was
                 * no available blkptrs from the dnode, or it was
                 * inappropriate to hook it in (i.e., nlevels mis-match).
                 */
                ASSERT(db->db_blkid < dn->dn_phys->dn_nblkptr);
                ASSERT(db->db_parent == NULL);
                db->db_parent = dn->dn_dbuf;
                db->db_blkptr = &dn->dn_phys->dn_blkptr[db->db_blkid];
                DBUF_VERIFY(db);
        } else {
                dmu_buf_impl_t *parent = db->db_parent;
                int epbs = dn->dn_phys->dn_indblkshift - SPA_BLKPTRSHIFT;

                ASSERT(dn->dn_phys->dn_nlevels > 1);
                if (parent == NULL) {
                        mutex_exit(&db->db_mtx);
                        rw_enter(&dn->dn_struct_rwlock, RW_READER);
                        parent = dbuf_hold_level(dn, db->db_level + 1,
                            db->db_blkid >> epbs, db);
                        rw_exit(&dn->dn_struct_rwlock);
                        mutex_enter(&db->db_mtx);
                        db->db_parent = parent;
                }
                db->db_blkptr = (blkptr_t *)parent->db.db_data +
                    (db->db_blkid & ((1ULL << epbs) - 1));
                DBUF_VERIFY(db);
        }
}

static void
dbuf_sync_indirect(dbuf_dirty_record_t *dr, dmu_tx_t *tx)
{
        dmu_buf_impl_t *db = dr->dr_dbuf;
        dnode_t *dn;
        zio_t *zio;

        ASSERT(dmu_tx_is_syncing(tx));

        dprintf_dbuf_bp(db, db->db_blkptr, "blkptr=%p", db->db_blkptr);

        mutex_enter(&db->db_mtx);

        ASSERT(db->db_level > 0);
        DBUF_VERIFY(db);

        /* Read the block if it hasn't been read yet. */
        if (db->db_buf == NULL) {
                mutex_exit(&db->db_mtx);
                (void) dbuf_read(db, NULL, DB_RF_MUST_SUCCEED);
                mutex_enter(&db->db_mtx);
        }
        ASSERT3U(db->db_state, ==, DB_CACHED);
        ASSERT(db->db_buf != NULL);

        DB_DNODE_ENTER(db);
        dn = DB_DNODE(db);
        /* Indirect block size must match what the dnode thinks it is. */
        ASSERT3U(db->db.db_size, ==, 1<<dn->dn_phys->dn_indblkshift);
        dbuf_check_blkptr(dn, db);
        DB_DNODE_EXIT(db);

        /* Provide the pending dirty record to child dbufs */
        db->db_data_pending = dr;

        mutex_exit(&db->db_mtx);

        dbuf_write(dr, db->db_buf, tx);

        zio = dr->dr_zio;
        mutex_enter(&dr->dt.di.dr_mtx);
        dbuf_sync_list(&dr->dt.di.dr_children, db->db_level - 1, tx);
        ASSERT(list_head(&dr->dt.di.dr_children) == NULL);
        mutex_exit(&dr->dt.di.dr_mtx);
        zio_nowait(zio);
}

static void
dbuf_sync_leaf(dbuf_dirty_record_t *dr, dmu_tx_t *tx)
{
        arc_buf_t **datap = &dr->dt.dl.dr_data;
        dmu_buf_impl_t *db = dr->dr_dbuf;
        dnode_t *dn;
        objset_t *os;
        uint64_t txg = tx->tx_txg;

        ASSERT(dmu_tx_is_syncing(tx));

        dprintf_dbuf_bp(db, db->db_blkptr, "blkptr=%p", db->db_blkptr);

        mutex_enter(&db->db_mtx);
        /*
         * To be synced, we must be dirtied.  But we
         * might have been freed after the dirty.
         */
        if (db->db_state == DB_UNCACHED) {
                /* This buffer has been freed since it was dirtied */
                ASSERT(db->db.db_data == NULL);
        } else if (db->db_state == DB_FILL) {
                /* This buffer was freed and is now being re-filled */
                ASSERT(db->db.db_data != dr->dt.dl.dr_data);
        } else {
                ASSERT(db->db_state == DB_CACHED || db->db_state == DB_NOFILL);
        }
        DBUF_VERIFY(db);

        DB_DNODE_ENTER(db);
        dn = DB_DNODE(db);

        if (db->db_blkid == DMU_SPILL_BLKID) {
                mutex_enter(&dn->dn_mtx);
                dn->dn_phys->dn_flags |= DNODE_FLAG_SPILL_BLKPTR;
                mutex_exit(&dn->dn_mtx);
        }

        /*
         * If this is a bonus buffer, simply copy the bonus data into the
         * dnode.  It will be written out when the dnode is synced (and it
         * will be synced, since it must have been dirty for dbuf_sync to
         * be called).
         */
        if (db->db_blkid == DMU_BONUS_BLKID) {
                dbuf_dirty_record_t **drp;

                ASSERT(*datap != NULL);
                ASSERT0(db->db_level);
                ASSERT3U(dn->dn_phys->dn_bonuslen, <=, DN_MAX_BONUSLEN);
                bcopy(*datap, DN_BONUS(dn->dn_phys), dn->dn_phys->dn_bonuslen);
                DB_DNODE_EXIT(db);

                if (*datap != db->db.db_data) {
                        zio_buf_free(*datap, DN_MAX_BONUSLEN);
                        arc_space_return(DN_MAX_BONUSLEN, ARC_SPACE_OTHER);
                }
                db->db_data_pending = NULL;
                drp = &db->db_last_dirty;
                while (*drp != dr)
                        drp = &(*drp)->dr_next;
                ASSERT(dr->dr_next == NULL);
                ASSERT(dr->dr_dbuf == db);
                *drp = dr->dr_next;
                if (dr->dr_dbuf->db_level != 0) {
                        list_destroy(&dr->dt.di.dr_children);
                        mutex_destroy(&dr->dt.di.dr_mtx);
                }
                kmem_free(dr, sizeof (dbuf_dirty_record_t));
                ASSERT(db->db_dirtycnt > 0);
                db->db_dirtycnt -= 1;
                dbuf_rele_and_unlock(db, (void *)(uintptr_t)txg);
                return;
        }

        os = dn->dn_objset;

        /*
         * This function may have dropped the db_mtx lock allowing a dmu_sync
         * operation to sneak in. As a result, we need to ensure that we
         * don't check the dr_override_state until we have returned from
         * dbuf_check_blkptr.
         */
        dbuf_check_blkptr(dn, db);

        /*
         * If this buffer is in the middle of an immediate write,
         * wait for the synchronous IO to complete.
         */
        while (dr->dt.dl.dr_override_state == DR_IN_DMU_SYNC) {
                ASSERT(dn->dn_object != DMU_META_DNODE_OBJECT);
                cv_wait(&db->db_changed, &db->db_mtx);
                ASSERT(dr->dt.dl.dr_override_state != DR_NOT_OVERRIDDEN);
        }

        if (db->db_state != DB_NOFILL &&
            dn->dn_object != DMU_META_DNODE_OBJECT &&
            refcount_count(&db->db_holds) > 1 &&
            dr->dt.dl.dr_override_state != DR_OVERRIDDEN &&
            *datap == db->db_buf) {
                /*
                 * If this buffer is currently "in use" (i.e., there
                 * are active holds and db_data still references it),
                 * then make a copy before we start the write so that
                 * any modifications from the open txg will not leak
                 * into this write.
                 *
                 * NOTE: this copy does not need to be made for
                 * objects only modified in the syncing context (e.g.
                 * DNONE_DNODE blocks).
                 */
                int psize = arc_buf_size(*datap);
                arc_buf_contents_t type = DBUF_GET_BUFC_TYPE(db);
                enum zio_compress compress_type = arc_get_compression(*datap);

                if (compress_type == ZIO_COMPRESS_OFF) {
                        *datap = arc_alloc_buf(os->os_spa, db, type, psize);
                } else {
                        ASSERT3U(type, ==, ARC_BUFC_DATA);
                        int lsize = arc_buf_lsize(*datap);
                        *datap = arc_alloc_compressed_buf(os->os_spa, db,
                            psize, lsize, compress_type);
                }
                bcopy(db->db.db_data, (*datap)->b_data, psize);
        }
        db->db_data_pending = dr;

        mutex_exit(&db->db_mtx);

        dbuf_write(dr, *datap, tx);

        ASSERT(!list_link_active(&dr->dr_dirty_node));
        if (dn->dn_object == DMU_META_DNODE_OBJECT) {
                list_insert_tail(&dn->dn_dirty_records[txg&TXG_MASK], dr);
                DB_DNODE_EXIT(db);
        } else {
                /*
                 * Although zio_nowait() does not "wait for an IO", it does
                 * initiate the IO. If this is an empty write it seems plausible
                 * that the IO could actually be completed before the nowait
                 * returns. We need to DB_DNODE_EXIT() first in case
                 * zio_nowait() invalidates the dbuf.
                 */
                DB_DNODE_EXIT(db);
                zio_nowait(dr->dr_zio);
        }
}

void
dbuf_sync_list(list_t *list, int level, dmu_tx_t *tx)
{
        dbuf_dirty_record_t *dr;

        while (dr = list_head(list)) {
                if (dr->dr_zio != NULL) {
                        /*
                         * If we find an already initialized zio then we
                         * are processing the meta-dnode, and we have finished.
                         * The dbufs for all dnodes are put back on the list
                         * during processing, so that we can zio_wait()
                         * these IOs after initiating all child IOs.
                         */
                        ASSERT3U(dr->dr_dbuf->db.db_object, ==,
                            DMU_META_DNODE_OBJECT);
                        break;
                }
                if (dr->dr_dbuf->db_blkid != DMU_BONUS_BLKID &&
                    dr->dr_dbuf->db_blkid != DMU_SPILL_BLKID) {
                        VERIFY3U(dr->dr_dbuf->db_level, ==, level);
                }
                list_remove(list, dr);
                if (dr->dr_dbuf->db_level > 0)
                        dbuf_sync_indirect(dr, tx);
                else
                        dbuf_sync_leaf(dr, tx);
        }
}

/* ARGSUSED */
static void
dbuf_write_ready(zio_t *zio, arc_buf_t *buf, void *vdb)
{
        dmu_buf_impl_t *db = vdb;
        dnode_t *dn;
        blkptr_t *bp = zio->io_bp;
        blkptr_t *bp_orig = &zio->io_bp_orig;
        spa_t *spa = zio->io_spa;
        int64_t delta;
        uint64_t fill = 0;
        int i;

        ASSERT3P(db->db_blkptr, !=, NULL);
        ASSERT3P(&db->db_data_pending->dr_bp_copy, ==, bp);

        DB_DNODE_ENTER(db);
        dn = DB_DNODE(db);
        delta = bp_get_dsize_sync(spa, bp) - bp_get_dsize_sync(spa, bp_orig);
        dnode_diduse_space(dn, delta - zio->io_prev_space_delta);
        zio->io_prev_space_delta = delta;

        if (bp->blk_birth != 0) {
                ASSERT((db->db_blkid != DMU_SPILL_BLKID &&
                    BP_GET_TYPE(bp) == dn->dn_type) ||
                    (db->db_blkid == DMU_SPILL_BLKID &&
                    BP_GET_TYPE(bp) == dn->dn_bonustype) ||
                    BP_IS_EMBEDDED(bp));
                ASSERT(BP_GET_LEVEL(bp) == db->db_level);
        }

        mutex_enter(&db->db_mtx);

#ifdef ZFS_DEBUG
        if (db->db_blkid == DMU_SPILL_BLKID) {
                ASSERT(dn->dn_phys->dn_flags & DNODE_FLAG_SPILL_BLKPTR);
                ASSERT(!(BP_IS_HOLE(bp)) &&
                    db->db_blkptr == &dn->dn_phys->dn_spill);
        }
#endif

        if (db->db_level == 0) {
                mutex_enter(&dn->dn_mtx);
                if (db->db_blkid > dn->dn_phys->dn_maxblkid &&
                    db->db_blkid != DMU_SPILL_BLKID)
                        dn->dn_phys->dn_maxblkid = db->db_blkid;
                mutex_exit(&dn->dn_mtx);

                if (dn->dn_type == DMU_OT_DNODE) {
                        dnode_phys_t *dnp = db->db.db_data;
                        for (i = db->db.db_size >> DNODE_SHIFT; i > 0;
                            i--, dnp++) {
                                if (dnp->dn_type != DMU_OT_NONE)
                                        fill++;
                        }
                } else {
                        if (BP_IS_HOLE(bp)) {
                                fill = 0;
                        } else {
                                fill = 1;
                        }
                }
        } else {
                blkptr_t *ibp = db->db.db_data;
                ASSERT3U(db->db.db_size, ==, 1<<dn->dn_phys->dn_indblkshift);
                for (i = db->db.db_size >> SPA_BLKPTRSHIFT; i > 0; i--, ibp++) {
                        if (BP_IS_HOLE(ibp))
                                continue;
                        fill += BP_GET_FILL(ibp);
                }
        }
        DB_DNODE_EXIT(db);

        if (!BP_IS_EMBEDDED(bp))
                bp->blk_fill = fill;

        mutex_exit(&db->db_mtx);

        rw_enter(&dn->dn_struct_rwlock, RW_WRITER);
        *db->db_blkptr = *bp;
        rw_exit(&dn->dn_struct_rwlock);
}

/* ARGSUSED */
/*
 * This function gets called just prior to running through the compression
 * stage of the zio pipeline. If we're an indirect block comprised of only
 * holes, then we want this indirect to be compressed away to a hole. In
 * order to do that we must zero out any information about the holes that
 * this indirect points to prior to before we try to compress it.
 */
static void
dbuf_write_children_ready(zio_t *zio, arc_buf_t *buf, void *vdb)
{
        dmu_buf_impl_t *db = vdb;
        dnode_t *dn;
        blkptr_t *bp;
        unsigned int epbs, i;

        ASSERT3U(db->db_level, >, 0);
        DB_DNODE_ENTER(db);
        dn = DB_DNODE(db);
        epbs = dn->dn_phys->dn_indblkshift - SPA_BLKPTRSHIFT;
        ASSERT3U(epbs, <, 31);

        /* Determine if all our children are holes */
        for (i = 0, bp = db->db.db_data; i < 1 << epbs; i++, bp++) {
                if (!BP_IS_HOLE(bp))
                        break;
        }

        /*
         * If all the children are holes, then zero them all out so that
         * we may get compressed away.
         */
        if (i == 1 << epbs) {
                /*
                 * We only found holes. Grab the rwlock to prevent
                 * anybody from reading the blocks we're about to
                 * zero out.
                 */
                rw_enter(&dn->dn_struct_rwlock, RW_WRITER);
                bzero(db->db.db_data, db->db.db_size);
                rw_exit(&dn->dn_struct_rwlock);
        }
        DB_DNODE_EXIT(db);
}

/*
 * The SPA will call this callback several times for each zio - once
 * for every physical child i/o (zio->io_phys_children times).  This
 * allows the DMU to monitor the progress of each logical i/o.  For example,
 * there may be 2 copies of an indirect block, or many fragments of a RAID-Z
 * block.  There may be a long delay before all copies/fragments are completed,
 * so this callback allows us to retire dirty space gradually, as the physical
 * i/os complete.
 */
/* ARGSUSED */
static void
dbuf_write_physdone(zio_t *zio, arc_buf_t *buf, void *arg)
{
        dmu_buf_impl_t *db = arg;
        objset_t *os = db->db_objset;
        dsl_pool_t *dp = dmu_objset_pool(os);
        dbuf_dirty_record_t *dr;
        int delta = 0;

        dr = db->db_data_pending;
        ASSERT3U(dr->dr_txg, ==, zio->io_txg);

        /*
         * The callback will be called io_phys_children times.  Retire one
         * portion of our dirty space each time we are called.  Any rounding
         * error will be cleaned up by dsl_pool_sync()'s call to
         * dsl_pool_undirty_space().
         */
        delta = dr->dr_accounted / zio->io_phys_children;
        dsl_pool_undirty_space(dp, delta, zio->io_txg);
}

/* ARGSUSED */
static void
dbuf_write_done(zio_t *zio, arc_buf_t *buf, void *vdb)
{
        dmu_buf_impl_t *db = vdb;
        blkptr_t *bp_orig = &zio->io_bp_orig;
        blkptr_t *bp = db->db_blkptr;
        objset_t *os = db->db_objset;
        dmu_tx_t *tx = os->os_synctx;
        dbuf_dirty_record_t **drp, *dr;

        ASSERT0(zio->io_error);
        ASSERT(db->db_blkptr == bp);

        /*
         * For nopwrites and rewrites we ensure that the bp matches our
         * original and bypass all the accounting.
         */
        if (zio->io_flags & (ZIO_FLAG_IO_REWRITE | ZIO_FLAG_NOPWRITE)) {
                ASSERT(BP_EQUAL(bp, bp_orig));
        } else {
                dsl_dataset_t *ds = os->os_dsl_dataset;
                (void) dsl_dataset_block_kill(ds, bp_orig, tx, B_TRUE);
                dsl_dataset_block_born(ds, bp, tx);
        }

        mutex_enter(&db->db_mtx);

        DBUF_VERIFY(db);

        drp = &db->db_last_dirty;
        while ((dr = *drp) != db->db_data_pending)
                drp = &dr->dr_next;
        ASSERT(!list_link_active(&dr->dr_dirty_node));
        ASSERT(dr->dr_dbuf == db);
        ASSERT(dr->dr_next == NULL);
        *drp = dr->dr_next;

#ifdef ZFS_DEBUG
        if (db->db_blkid == DMU_SPILL_BLKID) {
                dnode_t *dn;

                DB_DNODE_ENTER(db);
                dn = DB_DNODE(db);
                ASSERT(dn->dn_phys->dn_flags & DNODE_FLAG_SPILL_BLKPTR);
                ASSERT(!(BP_IS_HOLE(db->db_blkptr)) &&
                    db->db_blkptr == &dn->dn_phys->dn_spill);
                DB_DNODE_EXIT(db);
        }
#endif

        if (db->db_level == 0) {
                ASSERT(db->db_blkid != DMU_BONUS_BLKID);
                ASSERT(dr->dt.dl.dr_override_state == DR_NOT_OVERRIDDEN);
                if (db->db_state != DB_NOFILL) {
                        if (dr->dt.dl.dr_data != db->db_buf)
                                arc_buf_destroy(dr->dt.dl.dr_data, db);
                }
        } else {
                dnode_t *dn;

                DB_DNODE_ENTER(db);
                dn = DB_DNODE(db);
                ASSERT(list_head(&dr->dt.di.dr_children) == NULL);
                ASSERT3U(db->db.db_size, ==, 1 << dn->dn_phys->dn_indblkshift);
                if (!BP_IS_HOLE(db->db_blkptr)) {
                        int epbs =
                            dn->dn_phys->dn_indblkshift - SPA_BLKPTRSHIFT;
                        ASSERT3U(db->db_blkid, <=,
                            dn->dn_phys->dn_maxblkid >> (db->db_level * epbs));
                        ASSERT3U(BP_GET_LSIZE(db->db_blkptr), ==,
                            db->db.db_size);
                }
                DB_DNODE_EXIT(db);
                mutex_destroy(&dr->dt.di.dr_mtx);
                list_destroy(&dr->dt.di.dr_children);
        }
        kmem_free(dr, sizeof (dbuf_dirty_record_t));

        cv_broadcast(&db->db_changed);
        ASSERT(db->db_dirtycnt > 0);
        db->db_dirtycnt -= 1;
        db->db_data_pending = NULL;
        dbuf_rele_and_unlock(db, (void *)(uintptr_t)tx->tx_txg);
}

static void
dbuf_write_nofill_ready(zio_t *zio)
{
        dbuf_write_ready(zio, NULL, zio->io_private);
}

static void
dbuf_write_nofill_done(zio_t *zio)
{
        dbuf_write_done(zio, NULL, zio->io_private);
}

static void
dbuf_write_override_ready(zio_t *zio)
{
        dbuf_dirty_record_t *dr = zio->io_private;
        dmu_buf_impl_t *db = dr->dr_dbuf;

        dbuf_write_ready(zio, NULL, db);
}

static void
dbuf_write_override_done(zio_t *zio)
{
        dbuf_dirty_record_t *dr = zio->io_private;
        dmu_buf_impl_t *db = dr->dr_dbuf;
        blkptr_t *obp = &dr->dt.dl.dr_overridden_by;

        mutex_enter(&db->db_mtx);
        if (!BP_EQUAL(zio->io_bp, obp)) {
                if (!BP_IS_HOLE(obp))
                        dsl_free(spa_get_dsl(zio->io_spa), zio->io_txg, obp);
                arc_release(dr->dt.dl.dr_data, db);
        }
        mutex_exit(&db->db_mtx);
        dbuf_write_done(zio, NULL, db);

        if (zio->io_abd != NULL)
                abd_put(zio->io_abd);
}

typedef struct dbuf_remap_impl_callback_arg {
        objset_t        *drica_os;
        uint64_t        drica_blk_birth;
        dmu_tx_t        *drica_tx;
} dbuf_remap_impl_callback_arg_t;

static void
dbuf_remap_impl_callback(uint64_t vdev, uint64_t offset, uint64_t size,
    void *arg)
{
        dbuf_remap_impl_callback_arg_t *drica = arg;
        objset_t *os = drica->drica_os;
        spa_t *spa = dmu_objset_spa(os);
        dmu_tx_t *tx = drica->drica_tx;

        ASSERT(dsl_pool_sync_context(spa_get_dsl(spa)));

        if (os == spa_meta_objset(spa)) {
                spa_vdev_indirect_mark_obsolete(spa, vdev, offset, size, tx);
        } else {
                dsl_dataset_block_remapped(dmu_objset_ds(os), vdev, offset,
                    size, drica->drica_blk_birth, tx);
        }
}

static void
dbuf_remap_impl(dnode_t *dn, blkptr_t *bp, dmu_tx_t *tx)
{
        blkptr_t bp_copy = *bp;
        spa_t *spa = dmu_objset_spa(dn->dn_objset);
        dbuf_remap_impl_callback_arg_t drica;

        ASSERT(dsl_pool_sync_context(spa_get_dsl(spa)));

        drica.drica_os = dn->dn_objset;
        drica.drica_blk_birth = bp->blk_birth;
        drica.drica_tx = tx;
        if (spa_remap_blkptr(spa, &bp_copy, dbuf_remap_impl_callback,
            &drica)) {
                /*
                 * The struct_rwlock prevents dbuf_read_impl() from
                 * dereferencing the BP while we are changing it.  To
                 * avoid lock contention, only grab it when we are actually
                 * changing the BP.
                 */
                rw_enter(&dn->dn_struct_rwlock, RW_WRITER);
                *bp = bp_copy;
                rw_exit(&dn->dn_struct_rwlock);
        }
}

/*
 * Returns true if a dbuf_remap would modify the dbuf. We do this by attempting
 * to remap a copy of every bp in the dbuf.
 */
boolean_t
dbuf_can_remap(const dmu_buf_impl_t *db)
{
        spa_t *spa = dmu_objset_spa(db->db_objset);
        blkptr_t *bp = db->db.db_data;
        boolean_t ret = B_FALSE;

        ASSERT3U(db->db_level, >, 0);
        ASSERT3S(db->db_state, ==, DB_CACHED);

        ASSERT(spa_feature_is_active(spa, SPA_FEATURE_DEVICE_REMOVAL));

        spa_config_enter(spa, SCL_VDEV, FTAG, RW_READER);
        for (int i = 0; i < db->db.db_size >> SPA_BLKPTRSHIFT; i++) {
                blkptr_t bp_copy = bp[i];
                if (spa_remap_blkptr(spa, &bp_copy, NULL, NULL)) {
                        ret = B_TRUE;
                        break;
                }
        }
        spa_config_exit(spa, SCL_VDEV, FTAG);

        return (ret);
}

boolean_t
dnode_needs_remap(const dnode_t *dn)
{
        spa_t *spa = dmu_objset_spa(dn->dn_objset);
        boolean_t ret = B_FALSE;

        if (dn->dn_phys->dn_nlevels == 0) {
                return (B_FALSE);
        }

        ASSERT(spa_feature_is_active(spa, SPA_FEATURE_DEVICE_REMOVAL));

        spa_config_enter(spa, SCL_VDEV, FTAG, RW_READER);
        for (int j = 0; j < dn->dn_phys->dn_nblkptr; j++) {
                blkptr_t bp_copy = dn->dn_phys->dn_blkptr[j];
                if (spa_remap_blkptr(spa, &bp_copy, NULL, NULL)) {
                        ret = B_TRUE;
                        break;
                }
        }
        spa_config_exit(spa, SCL_VDEV, FTAG);

        return (ret);
}

/*
 * Remap any existing BP's to concrete vdevs, if possible.
 */
static void
dbuf_remap(dnode_t *dn, dmu_buf_impl_t *db, dmu_tx_t *tx)
{
        spa_t *spa = dmu_objset_spa(db->db_objset);
        ASSERT(dsl_pool_sync_context(spa_get_dsl(spa)));

        if (!spa_feature_is_active(spa, SPA_FEATURE_DEVICE_REMOVAL))
                return;

        if (db->db_level > 0) {
                blkptr_t *bp = db->db.db_data;
                for (int i = 0; i < db->db.db_size >> SPA_BLKPTRSHIFT; i++) {
                        dbuf_remap_impl(dn, &bp[i], tx);
                }
        } else if (db->db.db_object == DMU_META_DNODE_OBJECT) {
                dnode_phys_t *dnp = db->db.db_data;
                ASSERT3U(db->db_dnode_handle->dnh_dnode->dn_type, ==,
                    DMU_OT_DNODE);
                for (int i = 0; i < db->db.db_size >> DNODE_SHIFT; i++) {
                        for (int j = 0; j < dnp[i].dn_nblkptr; j++) {
                                dbuf_remap_impl(dn, &dnp[i].dn_blkptr[j], tx);
                        }
                }
        }
}


/* Issue I/O to commit a dirty buffer to disk. */
static void
dbuf_write(dbuf_dirty_record_t *dr, arc_buf_t *data, dmu_tx_t *tx)
{
        dmu_buf_impl_t *db = dr->dr_dbuf;
        dnode_t *dn;
        objset_t *os;
        dmu_buf_impl_t *parent = db->db_parent;
        uint64_t txg = tx->tx_txg;
        zbookmark_phys_t zb;
        zio_prop_t zp;
        zio_t *zio;
        int wp_flag = 0;

        ASSERT(dmu_tx_is_syncing(tx));

        DB_DNODE_ENTER(db);
        dn = DB_DNODE(db);
        os = dn->dn_objset;

        if (db->db_state != DB_NOFILL) {
                if (db->db_level > 0 || dn->dn_type == DMU_OT_DNODE) {
                        /*
                         * Private object buffers are released here rather
                         * than in dbuf_dirty() since they are only modified
                         * in the syncing context and we don't want the
                         * overhead of making multiple copies of the data.
                         */
                        if (BP_IS_HOLE(db->db_blkptr)) {
                                arc_buf_thaw(data);
                        } else {
                                dbuf_release_bp(db);
                        }
                        dbuf_remap(dn, db, tx);
                }
        }

        if (parent != dn->dn_dbuf) {
                /* Our parent is an indirect block. */
                /* We have a dirty parent that has been scheduled for write. */
                ASSERT(parent && parent->db_data_pending);
                /* Our parent's buffer is one level closer to the dnode. */
                ASSERT(db->db_level == parent->db_level-1);
                /*
                 * We're about to modify our parent's db_data by modifying
                 * our block pointer, so the parent must be released.
                 */
                ASSERT(arc_released(parent->db_buf));
                zio = parent->db_data_pending->dr_zio;
        } else {
                /* Our parent is the dnode itself. */
                ASSERT((db->db_level == dn->dn_phys->dn_nlevels-1 &&
                    db->db_blkid != DMU_SPILL_BLKID) ||
                    (db->db_blkid == DMU_SPILL_BLKID && db->db_level == 0));
                if (db->db_blkid != DMU_SPILL_BLKID)
                        ASSERT3P(db->db_blkptr, ==,
                            &dn->dn_phys->dn_blkptr[db->db_blkid]);
                zio = dn->dn_zio;
        }

        ASSERT(db->db_level == 0 || data == db->db_buf);
        ASSERT3U(db->db_blkptr->blk_birth, <=, txg);
        ASSERT(zio);

        SET_BOOKMARK(&zb, os->os_dsl_dataset ?
            os->os_dsl_dataset->ds_object : DMU_META_OBJSET,
            db->db.db_object, db->db_level, db->db_blkid);

        if (db->db_blkid == DMU_SPILL_BLKID)
                wp_flag = WP_SPILL;
        wp_flag |= (db->db_state == DB_NOFILL) ? WP_NOFILL : 0;

        dmu_write_policy(os, dn, db->db_level, wp_flag, &zp);
        DB_DNODE_EXIT(db);

        /*
         * We copy the blkptr now (rather than when we instantiate the dirty
         * record), because its value can change between open context and
         * syncing context. We do not need to hold dn_struct_rwlock to read
         * db_blkptr because we are in syncing context.
         */
        dr->dr_bp_copy = *db->db_blkptr;

        if (db->db_level == 0 &&
            dr->dt.dl.dr_override_state == DR_OVERRIDDEN) {
                /*
                 * The BP for this block has been provided by open context
                 * (by dmu_sync() or dmu_buf_write_embedded()).
                 */
                abd_t *contents = (data != NULL) ?
                    abd_get_from_buf(data->b_data, arc_buf_size(data)) : NULL;

                dr->dr_zio = zio_write(zio, os->os_spa, txg, &dr->dr_bp_copy,
                    contents, db->db.db_size, db->db.db_size, &zp,
                    dbuf_write_override_ready, NULL, NULL,
                    dbuf_write_override_done,
                    dr, ZIO_PRIORITY_ASYNC_WRITE, ZIO_FLAG_MUSTSUCCEED, &zb);
                mutex_enter(&db->db_mtx);
                dr->dt.dl.dr_override_state = DR_NOT_OVERRIDDEN;
                zio_write_override(dr->dr_zio, &dr->dt.dl.dr_overridden_by,
                    dr->dt.dl.dr_copies, dr->dt.dl.dr_nopwrite);
                mutex_exit(&db->db_mtx);
        } else if (db->db_state == DB_NOFILL) {
                ASSERT(zp.zp_checksum == ZIO_CHECKSUM_OFF ||
                    zp.zp_checksum == ZIO_CHECKSUM_NOPARITY);
                dr->dr_zio = zio_write(zio, os->os_spa, txg,
                    &dr->dr_bp_copy, NULL, db->db.db_size, db->db.db_size, &zp,
                    dbuf_write_nofill_ready, NULL, NULL,
                    dbuf_write_nofill_done, db,
                    ZIO_PRIORITY_ASYNC_WRITE,
                    ZIO_FLAG_MUSTSUCCEED | ZIO_FLAG_NODATA, &zb);
        } else {
                ASSERT(arc_released(data));

                /*
                 * For indirect blocks, we want to setup the children
                 * ready callback so that we can properly handle an indirect
                 * block that only contains holes.
                 */
                arc_write_done_func_t *children_ready_cb = NULL;
                if (db->db_level != 0)
                        children_ready_cb = dbuf_write_children_ready;

                dr->dr_zio = arc_write(zio, os->os_spa, txg,
                    &dr->dr_bp_copy, data, DBUF_IS_L2CACHEABLE(db),
                    &zp, dbuf_write_ready, children_ready_cb,
                    dbuf_write_physdone, dbuf_write_done, db,
                    ZIO_PRIORITY_ASYNC_WRITE, ZIO_FLAG_MUSTSUCCEED, &zb);
        }
}