zfs: merge openzfs/zfs@3522f57b6 (master)

[FreeBSD/FreeBSD.git] / sys / kern / vfs_bio.c

/*-
 * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
 *
 * Copyright (c) 2004 Poul-Henning Kamp
 * Copyright (c) 1994,1997 John S. Dyson
 * Copyright (c) 2013 The FreeBSD Foundation
 * All rights reserved.
 *
 * Portions of this software were developed by Konstantin Belousov
 * under sponsorship from the FreeBSD Foundation.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */

/*
 * this file contains a new buffer I/O scheme implementing a coherent
 * VM object and buffer cache scheme.  Pains have been taken to make
 * sure that the performance degradation associated with schemes such
 * as this is not realized.
 *
 * Author:  John S. Dyson
 * Significant help during the development and debugging phases
 * had been provided by David Greenman, also of the FreeBSD core team.
 *
 * see man buf(9) for more info.
 */

#include <sys/cdefs.h>
__FBSDID("$FreeBSD$");

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/asan.h>
#include <sys/bio.h>
#include <sys/bitset.h>
#include <sys/conf.h>
#include <sys/counter.h>
#include <sys/buf.h>
#include <sys/devicestat.h>
#include <sys/eventhandler.h>
#include <sys/fail.h>
#include <sys/ktr.h>
#include <sys/limits.h>
#include <sys/lock.h>
#include <sys/malloc.h>
#include <sys/mount.h>
#include <sys/mutex.h>
#include <sys/kernel.h>
#include <sys/kthread.h>
#include <sys/proc.h>
#include <sys/racct.h>
#include <sys/refcount.h>
#include <sys/resourcevar.h>
#include <sys/rwlock.h>
#include <sys/smp.h>
#include <sys/sysctl.h>
#include <sys/syscallsubr.h>
#include <sys/vmem.h>
#include <sys/vmmeter.h>
#include <sys/vnode.h>
#include <sys/watchdog.h>
#include <geom/geom.h>
#include <vm/vm.h>
#include <vm/vm_param.h>
#include <vm/vm_kern.h>
#include <vm/vm_object.h>
#include <vm/vm_page.h>
#include <vm/vm_pageout.h>
#include <vm/vm_pager.h>
#include <vm/vm_extern.h>
#include <vm/vm_map.h>
#include <vm/swap_pager.h>

static MALLOC_DEFINE(M_BIOBUF, "biobuf", "BIO buffer");

struct  bio_ops bioops;         /* I/O operation notification */

struct  buf_ops buf_ops_bio = {
        .bop_name       =       "buf_ops_bio",
        .bop_write      =       bufwrite,
        .bop_strategy   =       bufstrategy,
        .bop_sync       =       bufsync,
        .bop_bdflush    =       bufbdflush,
};

struct bufqueue {
        struct mtx_padalign     bq_lock;
        TAILQ_HEAD(, buf)       bq_queue;
        uint8_t                 bq_index;
        uint16_t                bq_subqueue;
        int                     bq_len;
} __aligned(CACHE_LINE_SIZE);

#define BQ_LOCKPTR(bq)          (&(bq)->bq_lock)
#define BQ_LOCK(bq)             mtx_lock(BQ_LOCKPTR((bq)))
#define BQ_UNLOCK(bq)           mtx_unlock(BQ_LOCKPTR((bq)))
#define BQ_ASSERT_LOCKED(bq)    mtx_assert(BQ_LOCKPTR((bq)), MA_OWNED)

struct bufdomain {
        struct bufqueue bd_subq[MAXCPU + 1]; /* Per-cpu sub queues + global */
        struct bufqueue bd_dirtyq;
        struct bufqueue *bd_cleanq;
        struct mtx_padalign bd_run_lock;
        /* Constants */
        long            bd_maxbufspace;
        long            bd_hibufspace;
        long            bd_lobufspace;
        long            bd_bufspacethresh;
        int             bd_hifreebuffers;
        int             bd_lofreebuffers;
        int             bd_hidirtybuffers;
        int             bd_lodirtybuffers;
        int             bd_dirtybufthresh;
        int             bd_lim;
        /* atomics */
        int             bd_wanted;
        int __aligned(CACHE_LINE_SIZE)  bd_numdirtybuffers;
        int __aligned(CACHE_LINE_SIZE)  bd_running;
        long __aligned(CACHE_LINE_SIZE) bd_bufspace;
        int __aligned(CACHE_LINE_SIZE)  bd_freebuffers;
} __aligned(CACHE_LINE_SIZE);

#define BD_LOCKPTR(bd)          (&(bd)->bd_cleanq->bq_lock)
#define BD_LOCK(bd)             mtx_lock(BD_LOCKPTR((bd)))
#define BD_UNLOCK(bd)           mtx_unlock(BD_LOCKPTR((bd)))
#define BD_ASSERT_LOCKED(bd)    mtx_assert(BD_LOCKPTR((bd)), MA_OWNED)
#define BD_RUN_LOCKPTR(bd)      (&(bd)->bd_run_lock)
#define BD_RUN_LOCK(bd)         mtx_lock(BD_RUN_LOCKPTR((bd)))
#define BD_RUN_UNLOCK(bd)       mtx_unlock(BD_RUN_LOCKPTR((bd)))
#define BD_DOMAIN(bd)           (bd - bdomain)

static char *buf;               /* buffer header pool */
static struct buf *
nbufp(unsigned i)
{
        return ((struct buf *)(buf + (sizeof(struct buf) +
            sizeof(vm_page_t) * atop(maxbcachebuf)) * i));
}

caddr_t __read_mostly unmapped_buf;

/* Used below and for softdep flushing threads in ufs/ffs/ffs_softdep.c */
struct proc *bufdaemonproc;

static void vm_hold_free_pages(struct buf *bp, int newbsize);
static void vm_hold_load_pages(struct buf *bp, vm_offset_t from,
                vm_offset_t to);
static void vfs_page_set_valid(struct buf *bp, vm_ooffset_t off, vm_page_t m);
static void vfs_page_set_validclean(struct buf *bp, vm_ooffset_t off,
                vm_page_t m);
static void vfs_clean_pages_dirty_buf(struct buf *bp);
static void vfs_setdirty_range(struct buf *bp);
static void vfs_vmio_invalidate(struct buf *bp);
static void vfs_vmio_truncate(struct buf *bp, int npages);
static void vfs_vmio_extend(struct buf *bp, int npages, int size);
static int vfs_bio_clcheck(struct vnode *vp, int size,
                daddr_t lblkno, daddr_t blkno);
static void breada(struct vnode *, daddr_t *, int *, int, struct ucred *, int,
                void (*)(struct buf *));
static int buf_flush(struct vnode *vp, struct bufdomain *, int);
static int flushbufqueues(struct vnode *, struct bufdomain *, int, int);
static void buf_daemon(void);
static __inline void bd_wakeup(void);
static int sysctl_runningspace(SYSCTL_HANDLER_ARGS);
static void bufkva_reclaim(vmem_t *, int);
static void bufkva_free(struct buf *);
static int buf_import(void *, void **, int, int, int);
static void buf_release(void *, void **, int);
static void maxbcachebuf_adjust(void);
static inline struct bufdomain *bufdomain(struct buf *);
static void bq_remove(struct bufqueue *bq, struct buf *bp);
static void bq_insert(struct bufqueue *bq, struct buf *bp, bool unlock);
static int buf_recycle(struct bufdomain *, bool kva);
static void bq_init(struct bufqueue *bq, int qindex, int cpu,
            const char *lockname);
static void bd_init(struct bufdomain *bd);
static int bd_flushall(struct bufdomain *bd);
static int sysctl_bufdomain_long(SYSCTL_HANDLER_ARGS);
static int sysctl_bufdomain_int(SYSCTL_HANDLER_ARGS);

static int sysctl_bufspace(SYSCTL_HANDLER_ARGS);
int vmiodirenable = TRUE;
SYSCTL_INT(_vfs, OID_AUTO, vmiodirenable, CTLFLAG_RW, &vmiodirenable, 0,
    "Use the VM system for directory writes");
long runningbufspace;
SYSCTL_LONG(_vfs, OID_AUTO, runningbufspace, CTLFLAG_RD, &runningbufspace, 0,
    "Amount of presently outstanding async buffer io");
SYSCTL_PROC(_vfs, OID_AUTO, bufspace, CTLTYPE_LONG|CTLFLAG_MPSAFE|CTLFLAG_RD,
    NULL, 0, sysctl_bufspace, "L", "Physical memory used for buffers");
static counter_u64_t bufkvaspace;
SYSCTL_COUNTER_U64(_vfs, OID_AUTO, bufkvaspace, CTLFLAG_RD, &bufkvaspace,
    "Kernel virtual memory used for buffers");
static long maxbufspace;
SYSCTL_PROC(_vfs, OID_AUTO, maxbufspace,
    CTLTYPE_LONG|CTLFLAG_MPSAFE|CTLFLAG_RW, &maxbufspace,
    __offsetof(struct bufdomain, bd_maxbufspace), sysctl_bufdomain_long, "L",
    "Maximum allowed value of bufspace (including metadata)");
static long bufmallocspace;
SYSCTL_LONG(_vfs, OID_AUTO, bufmallocspace, CTLFLAG_RD, &bufmallocspace, 0,
    "Amount of malloced memory for buffers");
static long maxbufmallocspace;
SYSCTL_LONG(_vfs, OID_AUTO, maxmallocbufspace, CTLFLAG_RW, &maxbufmallocspace,
    0, "Maximum amount of malloced memory for buffers");
static long lobufspace;
SYSCTL_PROC(_vfs, OID_AUTO, lobufspace,
    CTLTYPE_LONG|CTLFLAG_MPSAFE|CTLFLAG_RW, &lobufspace,
    __offsetof(struct bufdomain, bd_lobufspace), sysctl_bufdomain_long, "L",
    "Minimum amount of buffers we want to have");
long hibufspace;
SYSCTL_PROC(_vfs, OID_AUTO, hibufspace,
    CTLTYPE_LONG|CTLFLAG_MPSAFE|CTLFLAG_RW, &hibufspace,
    __offsetof(struct bufdomain, bd_hibufspace), sysctl_bufdomain_long, "L",
    "Maximum allowed value of bufspace (excluding metadata)");
long bufspacethresh;
SYSCTL_PROC(_vfs, OID_AUTO, bufspacethresh,
    CTLTYPE_LONG|CTLFLAG_MPSAFE|CTLFLAG_RW, &bufspacethresh,
    __offsetof(struct bufdomain, bd_bufspacethresh), sysctl_bufdomain_long, "L",
    "Bufspace consumed before waking the daemon to free some");
static counter_u64_t buffreekvacnt;
SYSCTL_COUNTER_U64(_vfs, OID_AUTO, buffreekvacnt, CTLFLAG_RW, &buffreekvacnt,
    "Number of times we have freed the KVA space from some buffer");
static counter_u64_t bufdefragcnt;
SYSCTL_COUNTER_U64(_vfs, OID_AUTO, bufdefragcnt, CTLFLAG_RW, &bufdefragcnt,
    "Number of times we have had to repeat buffer allocation to defragment");
static long lorunningspace;
SYSCTL_PROC(_vfs, OID_AUTO, lorunningspace, CTLTYPE_LONG | CTLFLAG_MPSAFE |
    CTLFLAG_RW, &lorunningspace, 0, sysctl_runningspace, "L",
    "Minimum preferred space used for in-progress I/O");
static long hirunningspace;
SYSCTL_PROC(_vfs, OID_AUTO, hirunningspace, CTLTYPE_LONG | CTLFLAG_MPSAFE |
    CTLFLAG_RW, &hirunningspace, 0, sysctl_runningspace, "L",
    "Maximum amount of space to use for in-progress I/O");
int dirtybufferflushes;
SYSCTL_INT(_vfs, OID_AUTO, dirtybufferflushes, CTLFLAG_RW, &dirtybufferflushes,
    0, "Number of bdwrite to bawrite conversions to limit dirty buffers");
int bdwriteskip;
SYSCTL_INT(_vfs, OID_AUTO, bdwriteskip, CTLFLAG_RW, &bdwriteskip,
    0, "Number of buffers supplied to bdwrite with snapshot deadlock risk");
int altbufferflushes;
SYSCTL_INT(_vfs, OID_AUTO, altbufferflushes, CTLFLAG_RW | CTLFLAG_STATS,
    &altbufferflushes, 0, "Number of fsync flushes to limit dirty buffers");
static int recursiveflushes;
SYSCTL_INT(_vfs, OID_AUTO, recursiveflushes, CTLFLAG_RW | CTLFLAG_STATS,
    &recursiveflushes, 0, "Number of flushes skipped due to being recursive");
static int sysctl_numdirtybuffers(SYSCTL_HANDLER_ARGS);
SYSCTL_PROC(_vfs, OID_AUTO, numdirtybuffers,
    CTLTYPE_INT|CTLFLAG_MPSAFE|CTLFLAG_RD, NULL, 0, sysctl_numdirtybuffers, "I",
    "Number of buffers that are dirty (has unwritten changes) at the moment");
static int lodirtybuffers;
SYSCTL_PROC(_vfs, OID_AUTO, lodirtybuffers,
    CTLTYPE_INT|CTLFLAG_MPSAFE|CTLFLAG_RW, &lodirtybuffers,
    __offsetof(struct bufdomain, bd_lodirtybuffers), sysctl_bufdomain_int, "I",
    "How many buffers we want to have free before bufdaemon can sleep");
static int hidirtybuffers;
SYSCTL_PROC(_vfs, OID_AUTO, hidirtybuffers,
    CTLTYPE_INT|CTLFLAG_MPSAFE|CTLFLAG_RW, &hidirtybuffers,
    __offsetof(struct bufdomain, bd_hidirtybuffers), sysctl_bufdomain_int, "I",
    "When the number of dirty buffers is considered severe");
int dirtybufthresh;
SYSCTL_PROC(_vfs, OID_AUTO, dirtybufthresh,
    CTLTYPE_INT|CTLFLAG_MPSAFE|CTLFLAG_RW, &dirtybufthresh,
    __offsetof(struct bufdomain, bd_dirtybufthresh), sysctl_bufdomain_int, "I",
    "Number of bdwrite to bawrite conversions to clear dirty buffers");
static int numfreebuffers;
SYSCTL_INT(_vfs, OID_AUTO, numfreebuffers, CTLFLAG_RD, &numfreebuffers, 0,
    "Number of free buffers");
static int lofreebuffers;
SYSCTL_PROC(_vfs, OID_AUTO, lofreebuffers,
    CTLTYPE_INT|CTLFLAG_MPSAFE|CTLFLAG_RW, &lofreebuffers,
    __offsetof(struct bufdomain, bd_lofreebuffers), sysctl_bufdomain_int, "I",
   "Target number of free buffers");
static int hifreebuffers;
SYSCTL_PROC(_vfs, OID_AUTO, hifreebuffers,
    CTLTYPE_INT|CTLFLAG_MPSAFE|CTLFLAG_RW, &hifreebuffers,
    __offsetof(struct bufdomain, bd_hifreebuffers), sysctl_bufdomain_int, "I",
   "Threshold for clean buffer recycling");
static counter_u64_t getnewbufcalls;
SYSCTL_COUNTER_U64(_vfs, OID_AUTO, getnewbufcalls, CTLFLAG_RD,
   &getnewbufcalls, "Number of calls to getnewbuf");
static counter_u64_t getnewbufrestarts;
SYSCTL_COUNTER_U64(_vfs, OID_AUTO, getnewbufrestarts, CTLFLAG_RD,
    &getnewbufrestarts,
    "Number of times getnewbuf has had to restart a buffer acquisition");
static counter_u64_t mappingrestarts;
SYSCTL_COUNTER_U64(_vfs, OID_AUTO, mappingrestarts, CTLFLAG_RD,
    &mappingrestarts,
    "Number of times getblk has had to restart a buffer mapping for "
    "unmapped buffer");
static counter_u64_t numbufallocfails;
SYSCTL_COUNTER_U64(_vfs, OID_AUTO, numbufallocfails, CTLFLAG_RW,
    &numbufallocfails, "Number of times buffer allocations failed");
static int flushbufqtarget = 100;
SYSCTL_INT(_vfs, OID_AUTO, flushbufqtarget, CTLFLAG_RW, &flushbufqtarget, 0,
    "Amount of work to do in flushbufqueues when helping bufdaemon");
static counter_u64_t notbufdflushes;
SYSCTL_COUNTER_U64(_vfs, OID_AUTO, notbufdflushes, CTLFLAG_RD, &notbufdflushes,
    "Number of dirty buffer flushes done by the bufdaemon helpers");
static long barrierwrites;
SYSCTL_LONG(_vfs, OID_AUTO, barrierwrites, CTLFLAG_RW | CTLFLAG_STATS,
    &barrierwrites, 0, "Number of barrier writes");
SYSCTL_INT(_vfs, OID_AUTO, unmapped_buf_allowed, CTLFLAG_RD,
    &unmapped_buf_allowed, 0,
    "Permit the use of the unmapped i/o");
int maxbcachebuf = MAXBCACHEBUF;
SYSCTL_INT(_vfs, OID_AUTO, maxbcachebuf, CTLFLAG_RDTUN, &maxbcachebuf, 0,
    "Maximum size of a buffer cache block");

/*
 * This lock synchronizes access to bd_request.
 */
static struct mtx_padalign __exclusive_cache_line bdlock;

/*
 * This lock protects the runningbufreq and synchronizes runningbufwakeup and
 * waitrunningbufspace().
 */
static struct mtx_padalign __exclusive_cache_line rbreqlock;

/*
 * Lock that protects bdirtywait.
 */
static struct mtx_padalign __exclusive_cache_line bdirtylock;

/*
 * Wakeup point for bufdaemon, as well as indicator of whether it is already
 * active.  Set to 1 when the bufdaemon is already "on" the queue, 0 when it
 * is idling.
 */
static int bd_request;

/*
 * Request for the buf daemon to write more buffers than is indicated by
 * lodirtybuf.  This may be necessary to push out excess dependencies or
 * defragment the address space where a simple count of the number of dirty
 * buffers is insufficient to characterize the demand for flushing them.
 */
static int bd_speedupreq;

/*
 * Synchronization (sleep/wakeup) variable for active buffer space requests.
 * Set when wait starts, cleared prior to wakeup().
 * Used in runningbufwakeup() and waitrunningbufspace().
 */
static int runningbufreq;

/*
 * Synchronization for bwillwrite() waiters.
 */
static int bdirtywait;

/*
 * Definitions for the buffer free lists.
 */
#define QUEUE_NONE      0       /* on no queue */
#define QUEUE_EMPTY     1       /* empty buffer headers */
#define QUEUE_DIRTY     2       /* B_DELWRI buffers */
#define QUEUE_CLEAN     3       /* non-B_DELWRI buffers */
#define QUEUE_SENTINEL  4       /* not an queue index, but mark for sentinel */

/* Maximum number of buffer domains. */
#define BUF_DOMAINS     8

struct bufdomainset bdlodirty;          /* Domains > lodirty */
struct bufdomainset bdhidirty;          /* Domains > hidirty */

/* Configured number of clean queues. */
static int __read_mostly buf_domains;

BITSET_DEFINE(bufdomainset, BUF_DOMAINS);
struct bufdomain __exclusive_cache_line bdomain[BUF_DOMAINS];
struct bufqueue __exclusive_cache_line bqempty;

/*
 * per-cpu empty buffer cache.
 */
uma_zone_t buf_zone;

/*
 * Single global constant for BUF_WMESG, to avoid getting multiple references.
 * buf_wmesg is referred from macros.
 */
const char *buf_wmesg = BUF_WMESG;

static int
sysctl_runningspace(SYSCTL_HANDLER_ARGS)
{
        long value;
        int error;

        value = *(long *)arg1;
        error = sysctl_handle_long(oidp, &value, 0, req);
        if (error != 0 || req->newptr == NULL)
                return (error);
        mtx_lock(&rbreqlock);
        if (arg1 == &hirunningspace) {
                if (value < lorunningspace)
                        error = EINVAL;
                else
                        hirunningspace = value;
        } else {
                KASSERT(arg1 == &lorunningspace,
                    ("%s: unknown arg1", __func__));
                if (value > hirunningspace)
                        error = EINVAL;
                else
                        lorunningspace = value;
        }
        mtx_unlock(&rbreqlock);
        return (error);
}

static int
sysctl_bufdomain_int(SYSCTL_HANDLER_ARGS)
{
        int error;
        int value;
        int i;

        value = *(int *)arg1;
        error = sysctl_handle_int(oidp, &value, 0, req);
        if (error != 0 || req->newptr == NULL)
                return (error);
        *(int *)arg1 = value;
        for (i = 0; i < buf_domains; i++)
                *(int *)(uintptr_t)(((uintptr_t)&bdomain[i]) + arg2) =
                    value / buf_domains;

        return (error);
}

static int
sysctl_bufdomain_long(SYSCTL_HANDLER_ARGS)
{
        long value;
        int error;
        int i;

        value = *(long *)arg1;
        error = sysctl_handle_long(oidp, &value, 0, req);
        if (error != 0 || req->newptr == NULL)
                return (error);
        *(long *)arg1 = value;
        for (i = 0; i < buf_domains; i++)
                *(long *)(uintptr_t)(((uintptr_t)&bdomain[i]) + arg2) =
                    value / buf_domains;

        return (error);
}

#if defined(COMPAT_FREEBSD4) || defined(COMPAT_FREEBSD5) || \
    defined(COMPAT_FREEBSD6) || defined(COMPAT_FREEBSD7)
static int
sysctl_bufspace(SYSCTL_HANDLER_ARGS)
{
        long lvalue;
        int ivalue;
        int i;

        lvalue = 0;
        for (i = 0; i < buf_domains; i++)
                lvalue += bdomain[i].bd_bufspace;
        if (sizeof(int) == sizeof(long) || req->oldlen >= sizeof(long))
                return (sysctl_handle_long(oidp, &lvalue, 0, req));
        if (lvalue > INT_MAX)
                /* On overflow, still write out a long to trigger ENOMEM. */
                return (sysctl_handle_long(oidp, &lvalue, 0, req));
        ivalue = lvalue;
        return (sysctl_handle_int(oidp, &ivalue, 0, req));
}
#else
static int
sysctl_bufspace(SYSCTL_HANDLER_ARGS)
{
        long lvalue;
        int i;

        lvalue = 0;
        for (i = 0; i < buf_domains; i++)
                lvalue += bdomain[i].bd_bufspace;
        return (sysctl_handle_long(oidp, &lvalue, 0, req));
}
#endif

static int
sysctl_numdirtybuffers(SYSCTL_HANDLER_ARGS)
{
        int value;
        int i;

        value = 0;
        for (i = 0; i < buf_domains; i++)
                value += bdomain[i].bd_numdirtybuffers;
        return (sysctl_handle_int(oidp, &value, 0, req));
}

/*
 *      bdirtywakeup:
 *
 *      Wakeup any bwillwrite() waiters.
 */
static void
bdirtywakeup(void)
{
        mtx_lock(&bdirtylock);
        if (bdirtywait) {
                bdirtywait = 0;
                wakeup(&bdirtywait);
        }
        mtx_unlock(&bdirtylock);
}

/*
 *      bd_clear:
 *
 *      Clear a domain from the appropriate bitsets when dirtybuffers
 *      is decremented.
 */
static void
bd_clear(struct bufdomain *bd)
{

        mtx_lock(&bdirtylock);
        if (bd->bd_numdirtybuffers <= bd->bd_lodirtybuffers)
                BIT_CLR(BUF_DOMAINS, BD_DOMAIN(bd), &bdlodirty);
        if (bd->bd_numdirtybuffers <= bd->bd_hidirtybuffers)
                BIT_CLR(BUF_DOMAINS, BD_DOMAIN(bd), &bdhidirty);
        mtx_unlock(&bdirtylock);
}

/*
 *      bd_set:
 *
 *      Set a domain in the appropriate bitsets when dirtybuffers
 *      is incremented.
 */
static void
bd_set(struct bufdomain *bd)
{

        mtx_lock(&bdirtylock);
        if (bd->bd_numdirtybuffers > bd->bd_lodirtybuffers)
                BIT_SET(BUF_DOMAINS, BD_DOMAIN(bd), &bdlodirty);
        if (bd->bd_numdirtybuffers > bd->bd_hidirtybuffers)
                BIT_SET(BUF_DOMAINS, BD_DOMAIN(bd), &bdhidirty);
        mtx_unlock(&bdirtylock);
}

/*
 *      bdirtysub:
 *
 *      Decrement the numdirtybuffers count by one and wakeup any
 *      threads blocked in bwillwrite().
 */
static void
bdirtysub(struct buf *bp)
{
        struct bufdomain *bd;
        int num;

        bd = bufdomain(bp);
        num = atomic_fetchadd_int(&bd->bd_numdirtybuffers, -1);
        if (num == (bd->bd_lodirtybuffers + bd->bd_hidirtybuffers) / 2)
                bdirtywakeup();
        if (num == bd->bd_lodirtybuffers || num == bd->bd_hidirtybuffers)
                bd_clear(bd);
}

/*
 *      bdirtyadd:
 *
 *      Increment the numdirtybuffers count by one and wakeup the buf 
 *      daemon if needed.
 */
static void
bdirtyadd(struct buf *bp)
{
        struct bufdomain *bd;
        int num;

        /*
         * Only do the wakeup once as we cross the boundary.  The
         * buf daemon will keep running until the condition clears.
         */
        bd = bufdomain(bp);
        num = atomic_fetchadd_int(&bd->bd_numdirtybuffers, 1);
        if (num == (bd->bd_lodirtybuffers + bd->bd_hidirtybuffers) / 2)
                bd_wakeup();
        if (num == bd->bd_lodirtybuffers || num == bd->bd_hidirtybuffers)
                bd_set(bd);
}

/*
 *      bufspace_daemon_wakeup:
 *
 *      Wakeup the daemons responsible for freeing clean bufs.
 */
static void
bufspace_daemon_wakeup(struct bufdomain *bd)
{

        /*
         * avoid the lock if the daemon is running.
         */
        if (atomic_fetchadd_int(&bd->bd_running, 1) == 0) {
                BD_RUN_LOCK(bd);
                atomic_store_int(&bd->bd_running, 1);
                wakeup(&bd->bd_running);
                BD_RUN_UNLOCK(bd);
        }
}

/*
 *      bufspace_daemon_wait:
 *
 *      Sleep until the domain falls below a limit or one second passes.
 */
static void
bufspace_daemon_wait(struct bufdomain *bd)
{
        /*
         * Re-check our limits and sleep.  bd_running must be
         * cleared prior to checking the limits to avoid missed
         * wakeups.  The waker will adjust one of bufspace or
         * freebuffers prior to checking bd_running.
         */
        BD_RUN_LOCK(bd);
        atomic_store_int(&bd->bd_running, 0);
        if (bd->bd_bufspace < bd->bd_bufspacethresh &&
            bd->bd_freebuffers > bd->bd_lofreebuffers) {
                msleep(&bd->bd_running, BD_RUN_LOCKPTR(bd), PRIBIO|PDROP,
                    "-", hz);
        } else {
                /* Avoid spurious wakeups while running. */
                atomic_store_int(&bd->bd_running, 1);
                BD_RUN_UNLOCK(bd);
        }
}

/*
 *      bufspace_adjust:
 *
 *      Adjust the reported bufspace for a KVA managed buffer, possibly
 *      waking any waiters.
 */
static void
bufspace_adjust(struct buf *bp, int bufsize)
{
        struct bufdomain *bd;
        long space;
        int diff;

        KASSERT((bp->b_flags & B_MALLOC) == 0,
            ("bufspace_adjust: malloc buf %p", bp));
        bd = bufdomain(bp);
        diff = bufsize - bp->b_bufsize;
        if (diff < 0) {
                atomic_subtract_long(&bd->bd_bufspace, -diff);
        } else if (diff > 0) {
                space = atomic_fetchadd_long(&bd->bd_bufspace, diff);
                /* Wake up the daemon on the transition. */
                if (space < bd->bd_bufspacethresh &&
                    space + diff >= bd->bd_bufspacethresh)
                        bufspace_daemon_wakeup(bd);
        }
        bp->b_bufsize = bufsize;
}

/*
 *      bufspace_reserve:
 *
 *      Reserve bufspace before calling allocbuf().  metadata has a
 *      different space limit than data.
 */
static int
bufspace_reserve(struct bufdomain *bd, int size, bool metadata)
{
        long limit, new;
        long space;

        if (metadata)
                limit = bd->bd_maxbufspace;
        else
                limit = bd->bd_hibufspace;
        space = atomic_fetchadd_long(&bd->bd_bufspace, size);
        new = space + size;
        if (new > limit) {
                atomic_subtract_long(&bd->bd_bufspace, size);
                return (ENOSPC);
        }

        /* Wake up the daemon on the transition. */
        if (space < bd->bd_bufspacethresh && new >= bd->bd_bufspacethresh)
                bufspace_daemon_wakeup(bd);

        return (0);
}

/*
 *      bufspace_release:
 *
 *      Release reserved bufspace after bufspace_adjust() has consumed it.
 */
static void
bufspace_release(struct bufdomain *bd, int size)
{

        atomic_subtract_long(&bd->bd_bufspace, size);
}

/*
 *      bufspace_wait:
 *
 *      Wait for bufspace, acting as the buf daemon if a locked vnode is
 *      supplied.  bd_wanted must be set prior to polling for space.  The
 *      operation must be re-tried on return.
 */
static void
bufspace_wait(struct bufdomain *bd, struct vnode *vp, int gbflags,
    int slpflag, int slptimeo)
{
        struct thread *td;
        int error, fl, norunbuf;

        if ((gbflags & GB_NOWAIT_BD) != 0)
                return;

        td = curthread;
        BD_LOCK(bd);
        while (bd->bd_wanted) {
                if (vp != NULL && vp->v_type != VCHR &&
                    (td->td_pflags & TDP_BUFNEED) == 0) {
                        BD_UNLOCK(bd);
                        /*
                         * getblk() is called with a vnode locked, and
                         * some majority of the dirty buffers may as
                         * well belong to the vnode.  Flushing the
                         * buffers there would make a progress that
                         * cannot be achieved by the buf_daemon, that
                         * cannot lock the vnode.
                         */
                        norunbuf = ~(TDP_BUFNEED | TDP_NORUNNINGBUF) |
                            (td->td_pflags & TDP_NORUNNINGBUF);

                        /*
                         * Play bufdaemon.  The getnewbuf() function
                         * may be called while the thread owns lock
                         * for another dirty buffer for the same
                         * vnode, which makes it impossible to use
                         * VOP_FSYNC() there, due to the buffer lock
                         * recursion.
                         */
                        td->td_pflags |= TDP_BUFNEED | TDP_NORUNNINGBUF;
                        fl = buf_flush(vp, bd, flushbufqtarget);
                        td->td_pflags &= norunbuf;
                        BD_LOCK(bd);
                        if (fl != 0)
                                continue;
                        if (bd->bd_wanted == 0)
                                break;
                }
                error = msleep(&bd->bd_wanted, BD_LOCKPTR(bd),
                    (PRIBIO + 4) | slpflag, "newbuf", slptimeo);
                if (error != 0)
                        break;
        }
        BD_UNLOCK(bd);
}

/*
 *      bufspace_daemon:
 *
 *      buffer space management daemon.  Tries to maintain some marginal
 *      amount of free buffer space so that requesting processes neither
 *      block nor work to reclaim buffers.
 */
static void
bufspace_daemon(void *arg)
{
        struct bufdomain *bd;

        EVENTHANDLER_REGISTER(shutdown_pre_sync, kthread_shutdown, curthread,
            SHUTDOWN_PRI_LAST + 100);

        bd = arg;
        for (;;) {
                kthread_suspend_check();

                /*
                 * Free buffers from the clean queue until we meet our
                 * targets.
                 *
                 * Theory of operation:  The buffer cache is most efficient
                 * when some free buffer headers and space are always
                 * available to getnewbuf().  This daemon attempts to prevent
                 * the excessive blocking and synchronization associated
                 * with shortfall.  It goes through three phases according
                 * demand:
                 *
                 * 1)   The daemon wakes up voluntarily once per-second
                 *      during idle periods when the counters are below
                 *      the wakeup thresholds (bufspacethresh, lofreebuffers).
                 *
                 * 2)   The daemon wakes up as we cross the thresholds
                 *      ahead of any potential blocking.  This may bounce
                 *      slightly according to the rate of consumption and
                 *      release.
                 *
                 * 3)   The daemon and consumers are starved for working
                 *      clean buffers.  This is the 'bufspace' sleep below
                 *      which will inefficiently trade bufs with bqrelse
                 *      until we return to condition 2.
                 */
                while (bd->bd_bufspace > bd->bd_lobufspace ||
                    bd->bd_freebuffers < bd->bd_hifreebuffers) {
                        if (buf_recycle(bd, false) != 0) {
                                if (bd_flushall(bd))
                                        continue;
                                /*
                                 * Speedup dirty if we've run out of clean
                                 * buffers.  This is possible in particular
                                 * because softdep may held many bufs locked
                                 * pending writes to other bufs which are
                                 * marked for delayed write, exhausting
                                 * clean space until they are written.
                                 */
                                bd_speedup();
                                BD_LOCK(bd);
                                if (bd->bd_wanted) {
                                        msleep(&bd->bd_wanted, BD_LOCKPTR(bd),
                                            PRIBIO|PDROP, "bufspace", hz/10);
                                } else
                                        BD_UNLOCK(bd);
                        }
                        maybe_yield();
                }
                bufspace_daemon_wait(bd);
        }
}

/*
 *      bufmallocadjust:
 *
 *      Adjust the reported bufspace for a malloc managed buffer, possibly
 *      waking any waiters.
 */
static void
bufmallocadjust(struct buf *bp, int bufsize)
{
        int diff;

        KASSERT((bp->b_flags & B_MALLOC) != 0,
            ("bufmallocadjust: non-malloc buf %p", bp));
        diff = bufsize - bp->b_bufsize;
        if (diff < 0)
                atomic_subtract_long(&bufmallocspace, -diff);
        else
                atomic_add_long(&bufmallocspace, diff);
        bp->b_bufsize = bufsize;
}

/*
 *      runningwakeup:
 *
 *      Wake up processes that are waiting on asynchronous writes to fall
 *      below lorunningspace.
 */
static void
runningwakeup(void)
{

        mtx_lock(&rbreqlock);
        if (runningbufreq) {
                runningbufreq = 0;
                wakeup(&runningbufreq);
        }
        mtx_unlock(&rbreqlock);
}

/*
 *      runningbufwakeup:
 *
 *      Decrement the outstanding write count according.
 */
void
runningbufwakeup(struct buf *bp)
{
        long space, bspace;

        bspace = bp->b_runningbufspace;
        if (bspace == 0)
                return;
        space = atomic_fetchadd_long(&runningbufspace, -bspace);
        KASSERT(space >= bspace, ("runningbufspace underflow %ld %ld",
            space, bspace));
        bp->b_runningbufspace = 0;
        /*
         * Only acquire the lock and wakeup on the transition from exceeding
         * the threshold to falling below it.
         */
        if (space < lorunningspace)
                return;
        if (space - bspace > lorunningspace)
                return;
        runningwakeup();
}

/*
 *      waitrunningbufspace()
 *
 *      runningbufspace is a measure of the amount of I/O currently
 *      running.  This routine is used in async-write situations to
 *      prevent creating huge backups of pending writes to a device.
 *      Only asynchronous writes are governed by this function.
 *
 *      This does NOT turn an async write into a sync write.  It waits  
 *      for earlier writes to complete and generally returns before the
 *      caller's write has reached the device.
 */
void
waitrunningbufspace(void)
{

        mtx_lock(&rbreqlock);
        while (runningbufspace > hirunningspace) {
                runningbufreq = 1;
                msleep(&runningbufreq, &rbreqlock, PVM, "wdrain", 0);
        }
        mtx_unlock(&rbreqlock);
}

/*
 *      vfs_buf_test_cache:
 *
 *      Called when a buffer is extended.  This function clears the B_CACHE
 *      bit if the newly extended portion of the buffer does not contain
 *      valid data.
 */
static __inline void
vfs_buf_test_cache(struct buf *bp, vm_ooffset_t foff, vm_offset_t off,
    vm_offset_t size, vm_page_t m)
{

        /*
         * This function and its results are protected by higher level
         * synchronization requiring vnode and buf locks to page in and
         * validate pages.
         */
        if (bp->b_flags & B_CACHE) {
                int base = (foff + off) & PAGE_MASK;
                if (vm_page_is_valid(m, base, size) == 0)
                        bp->b_flags &= ~B_CACHE;
        }
}

/* Wake up the buffer daemon if necessary */
static void
bd_wakeup(void)
{

        mtx_lock(&bdlock);
        if (bd_request == 0) {
                bd_request = 1;
                wakeup(&bd_request);
        }
        mtx_unlock(&bdlock);
}

/*
 * Adjust the maxbcachbuf tunable.
 */
static void
maxbcachebuf_adjust(void)
{
        int i;

        /*
         * maxbcachebuf must be a power of 2 >= MAXBSIZE.
         */
        i = 2;
        while (i * 2 <= maxbcachebuf)
                i *= 2;
        maxbcachebuf = i;
        if (maxbcachebuf < MAXBSIZE)
                maxbcachebuf = MAXBSIZE;
        if (maxbcachebuf > maxphys)
                maxbcachebuf = maxphys;
        if (bootverbose != 0 && maxbcachebuf != MAXBCACHEBUF)
                printf("maxbcachebuf=%d\n", maxbcachebuf);
}

/*
 * bd_speedup - speedup the buffer cache flushing code
 */
void
bd_speedup(void)
{
        int needwake;

        mtx_lock(&bdlock);
        needwake = 0;
        if (bd_speedupreq == 0 || bd_request == 0)
                needwake = 1;
        bd_speedupreq = 1;
        bd_request = 1;
        if (needwake)
                wakeup(&bd_request);
        mtx_unlock(&bdlock);
}

#ifdef __i386__
#define TRANSIENT_DENOM 5
#else
#define TRANSIENT_DENOM 10
#endif

/*
 * Calculating buffer cache scaling values and reserve space for buffer
 * headers.  This is called during low level kernel initialization and
 * may be called more then once.  We CANNOT write to the memory area
 * being reserved at this time.
 */
caddr_t
kern_vfs_bio_buffer_alloc(caddr_t v, long physmem_est)
{
        int tuned_nbuf;
        long maxbuf, maxbuf_sz, buf_sz, biotmap_sz;

#ifdef KASAN
        /*
         * With KASAN enabled, the kernel map is shadowed.  Account for this
         * when sizing maps based on the amount of physical memory available.
         */
        physmem_est = (physmem_est * KASAN_SHADOW_SCALE) /
            (KASAN_SHADOW_SCALE + 1);
#endif

        /*
         * physmem_est is in pages.  Convert it to kilobytes (assumes
         * PAGE_SIZE is >= 1K)
         */
        physmem_est = physmem_est * (PAGE_SIZE / 1024);

        maxbcachebuf_adjust();
        /*
         * The nominal buffer size (and minimum KVA allocation) is BKVASIZE.
         * For the first 64MB of ram nominally allocate sufficient buffers to
         * cover 1/4 of our ram.  Beyond the first 64MB allocate additional
         * buffers to cover 1/10 of our ram over 64MB.  When auto-sizing
         * the buffer cache we limit the eventual kva reservation to
         * maxbcache bytes.
         *
         * factor represents the 1/4 x ram conversion.
         */
        if (nbuf == 0) {
                int factor = 4 * BKVASIZE / 1024;

                nbuf = 50;
                if (physmem_est > 4096)
                        nbuf += min((physmem_est - 4096) / factor,
                            65536 / factor);
                if (physmem_est > 65536)
                        nbuf += min((physmem_est - 65536) * 2 / (factor * 5),
                            32 * 1024 * 1024 / (factor * 5));

                if (maxbcache && nbuf > maxbcache / BKVASIZE)
                        nbuf = maxbcache / BKVASIZE;
                tuned_nbuf = 1;
        } else
                tuned_nbuf = 0;

        /* XXX Avoid unsigned long overflows later on with maxbufspace. */
        maxbuf = (LONG_MAX / 3) / BKVASIZE;
        if (nbuf > maxbuf) {
                if (!tuned_nbuf)
                        printf("Warning: nbufs lowered from %d to %ld\n", nbuf,
                            maxbuf);
                nbuf = maxbuf;
        }

        /*
         * Ideal allocation size for the transient bio submap is 10%
         * of the maximal space buffer map.  This roughly corresponds
         * to the amount of the buffer mapped for typical UFS load.
         *
         * Clip the buffer map to reserve space for the transient
         * BIOs, if its extent is bigger than 90% (80% on i386) of the
         * maximum buffer map extent on the platform.
         *
         * The fall-back to the maxbuf in case of maxbcache unset,
         * allows to not trim the buffer KVA for the architectures
         * with ample KVA space.
         */
        if (bio_transient_maxcnt == 0 && unmapped_buf_allowed) {
                maxbuf_sz = maxbcache != 0 ? maxbcache : maxbuf * BKVASIZE;
                buf_sz = (long)nbuf * BKVASIZE;
                if (buf_sz < maxbuf_sz / TRANSIENT_DENOM *
                    (TRANSIENT_DENOM - 1)) {
                        /*
                         * There is more KVA than memory.  Do not
                         * adjust buffer map size, and assign the rest
                         * of maxbuf to transient map.
                         */
                        biotmap_sz = maxbuf_sz - buf_sz;
                } else {
                        /*
                         * Buffer map spans all KVA we could afford on
                         * this platform.  Give 10% (20% on i386) of
                         * the buffer map to the transient bio map.
                         */
                        biotmap_sz = buf_sz / TRANSIENT_DENOM;
                        buf_sz -= biotmap_sz;
                }
                if (biotmap_sz / INT_MAX > maxphys)
                        bio_transient_maxcnt = INT_MAX;
                else
                        bio_transient_maxcnt = biotmap_sz / maxphys;
                /*
                 * Artificially limit to 1024 simultaneous in-flight I/Os
                 * using the transient mapping.
                 */
                if (bio_transient_maxcnt > 1024)
                        bio_transient_maxcnt = 1024;
                if (tuned_nbuf)
                        nbuf = buf_sz / BKVASIZE;
        }

        if (nswbuf == 0) {
                nswbuf = min(nbuf / 4, 256);
                if (nswbuf < NSWBUF_MIN)
                        nswbuf = NSWBUF_MIN;
        }

        /*
         * Reserve space for the buffer cache buffers
         */
        buf = (char *)v;
        v = (caddr_t)buf + (sizeof(struct buf) + sizeof(vm_page_t) *
            atop(maxbcachebuf)) * nbuf;

        return (v);
}

/* Initialize the buffer subsystem.  Called before use of any buffers. */
void
bufinit(void)
{
        struct buf *bp;
        int i;

        KASSERT(maxbcachebuf >= MAXBSIZE,
            ("maxbcachebuf (%d) must be >= MAXBSIZE (%d)\n", maxbcachebuf,
            MAXBSIZE));
        bq_init(&bqempty, QUEUE_EMPTY, -1, "bufq empty lock");
        mtx_init(&rbreqlock, "runningbufspace lock", NULL, MTX_DEF);
        mtx_init(&bdlock, "buffer daemon lock", NULL, MTX_DEF);
        mtx_init(&bdirtylock, "dirty buf lock", NULL, MTX_DEF);

        unmapped_buf = (caddr_t)kva_alloc(maxphys);

        /* finally, initialize each buffer header and stick on empty q */
        for (i = 0; i < nbuf; i++) {
                bp = nbufp(i);
                bzero(bp, sizeof(*bp) + sizeof(vm_page_t) * atop(maxbcachebuf));
                bp->b_flags = B_INVAL;
                bp->b_rcred = NOCRED;
                bp->b_wcred = NOCRED;
                bp->b_qindex = QUEUE_NONE;
                bp->b_domain = -1;
                bp->b_subqueue = mp_maxid + 1;
                bp->b_xflags = 0;
                bp->b_data = bp->b_kvabase = unmapped_buf;
                LIST_INIT(&bp->b_dep);
                BUF_LOCKINIT(bp);
                bq_insert(&bqempty, bp, false);
        }

        /*
         * maxbufspace is the absolute maximum amount of buffer space we are 
         * allowed to reserve in KVM and in real terms.  The absolute maximum
         * is nominally used by metadata.  hibufspace is the nominal maximum
         * used by most other requests.  The differential is required to 
         * ensure that metadata deadlocks don't occur.
         *
         * maxbufspace is based on BKVASIZE.  Allocating buffers larger then
         * this may result in KVM fragmentation which is not handled optimally
         * by the system. XXX This is less true with vmem.  We could use
         * PAGE_SIZE.
         */
        maxbufspace = (long)nbuf * BKVASIZE;
        hibufspace = lmax(3 * maxbufspace / 4, maxbufspace - maxbcachebuf * 10);
        lobufspace = (hibufspace / 20) * 19; /* 95% */
        bufspacethresh = lobufspace + (hibufspace - lobufspace) / 2;

        /*
         * Note: The 16 MiB upper limit for hirunningspace was chosen
         * arbitrarily and may need further tuning. It corresponds to
         * 128 outstanding write IO requests (if IO size is 128 KiB),
         * which fits with many RAID controllers' tagged queuing limits.
         * The lower 1 MiB limit is the historical upper limit for
         * hirunningspace.
         */
        hirunningspace = lmax(lmin(roundup(hibufspace / 64, maxbcachebuf),
            16 * 1024 * 1024), 1024 * 1024);
        lorunningspace = roundup((hirunningspace * 2) / 3, maxbcachebuf);

        /*
         * Limit the amount of malloc memory since it is wired permanently into
         * the kernel space.  Even though this is accounted for in the buffer
         * allocation, we don't want the malloced region to grow uncontrolled.
         * The malloc scheme improves memory utilization significantly on
         * average (small) directories.
         */
        maxbufmallocspace = hibufspace / 20;

        /*
         * Reduce the chance of a deadlock occurring by limiting the number
         * of delayed-write dirty buffers we allow to stack up.
         */
        hidirtybuffers = nbuf / 4 + 20;
        dirtybufthresh = hidirtybuffers * 9 / 10;
        /*
         * To support extreme low-memory systems, make sure hidirtybuffers
         * cannot eat up all available buffer space.  This occurs when our
         * minimum cannot be met.  We try to size hidirtybuffers to 3/4 our
         * buffer space assuming BKVASIZE'd buffers.
         */
        while ((long)hidirtybuffers * BKVASIZE > 3 * hibufspace / 4) {
                hidirtybuffers >>= 1;
        }
        lodirtybuffers = hidirtybuffers / 2;

        /*
         * lofreebuffers should be sufficient to avoid stalling waiting on
         * buf headers under heavy utilization.  The bufs in per-cpu caches
         * are counted as free but will be unavailable to threads executing
         * on other cpus.
         *
         * hifreebuffers is the free target for the bufspace daemon.  This
         * should be set appropriately to limit work per-iteration.
         */
        lofreebuffers = MIN((nbuf / 25) + (20 * mp_ncpus), 128 * mp_ncpus);
        hifreebuffers = (3 * lofreebuffers) / 2;
        numfreebuffers = nbuf;

        /* Setup the kva and free list allocators. */
        vmem_set_reclaim(buffer_arena, bufkva_reclaim);
        buf_zone = uma_zcache_create("buf free cache",
            sizeof(struct buf) + sizeof(vm_page_t) * atop(maxbcachebuf),
            NULL, NULL, NULL, NULL, buf_import, buf_release, NULL, 0);

        /*
         * Size the clean queue according to the amount of buffer space.
         * One queue per-256mb up to the max.  More queues gives better
         * concurrency but less accurate LRU.
         */
        buf_domains = MIN(howmany(maxbufspace, 256*1024*1024), BUF_DOMAINS);
        for (i = 0 ; i < buf_domains; i++) {
                struct bufdomain *bd;

                bd = &bdomain[i];
                bd_init(bd);
                bd->bd_freebuffers = nbuf / buf_domains;
                bd->bd_hifreebuffers = hifreebuffers / buf_domains;
                bd->bd_lofreebuffers = lofreebuffers / buf_domains;
                bd->bd_bufspace = 0;
                bd->bd_maxbufspace = maxbufspace / buf_domains;
                bd->bd_hibufspace = hibufspace / buf_domains;
                bd->bd_lobufspace = lobufspace / buf_domains;
                bd->bd_bufspacethresh = bufspacethresh / buf_domains;
                bd->bd_numdirtybuffers = 0;
                bd->bd_hidirtybuffers = hidirtybuffers / buf_domains;
                bd->bd_lodirtybuffers = lodirtybuffers / buf_domains;
                bd->bd_dirtybufthresh = dirtybufthresh / buf_domains;
                /* Don't allow more than 2% of bufs in the per-cpu caches. */
                bd->bd_lim = nbuf / buf_domains / 50 / mp_ncpus;
        }
        getnewbufcalls = counter_u64_alloc(M_WAITOK);
        getnewbufrestarts = counter_u64_alloc(M_WAITOK);
        mappingrestarts = counter_u64_alloc(M_WAITOK);
        numbufallocfails = counter_u64_alloc(M_WAITOK);
        notbufdflushes = counter_u64_alloc(M_WAITOK);
        buffreekvacnt = counter_u64_alloc(M_WAITOK);
        bufdefragcnt = counter_u64_alloc(M_WAITOK);
        bufkvaspace = counter_u64_alloc(M_WAITOK);
}

#ifdef INVARIANTS
static inline void
vfs_buf_check_mapped(struct buf *bp)
{

        KASSERT(bp->b_kvabase != unmapped_buf,
            ("mapped buf: b_kvabase was not updated %p", bp));
        KASSERT(bp->b_data != unmapped_buf,
            ("mapped buf: b_data was not updated %p", bp));
        KASSERT(bp->b_data < unmapped_buf || bp->b_data >= unmapped_buf +
            maxphys, ("b_data + b_offset unmapped %p", bp));
}

static inline void
vfs_buf_check_unmapped(struct buf *bp)
{

        KASSERT(bp->b_data == unmapped_buf,
            ("unmapped buf: corrupted b_data %p", bp));
}

#define BUF_CHECK_MAPPED(bp) vfs_buf_check_mapped(bp)
#define BUF_CHECK_UNMAPPED(bp) vfs_buf_check_unmapped(bp)
#else
#define BUF_CHECK_MAPPED(bp) do {} while (0)
#define BUF_CHECK_UNMAPPED(bp) do {} while (0)
#endif

static int
isbufbusy(struct buf *bp)
{
        if (((bp->b_flags & B_INVAL) == 0 && BUF_ISLOCKED(bp)) ||
            ((bp->b_flags & (B_DELWRI | B_INVAL)) == B_DELWRI))
                return (1);
        return (0);
}

/*
 * Shutdown the system cleanly to prepare for reboot, halt, or power off.
 */
void
bufshutdown(int show_busybufs)
{
        static int first_buf_printf = 1;
        struct buf *bp;
        int i, iter, nbusy, pbusy;
#ifndef PREEMPTION
        int subiter;
#endif

        /*
         * Sync filesystems for shutdown
         */
        wdog_kern_pat(WD_LASTVAL);
        kern_sync(curthread);

        /*
         * With soft updates, some buffers that are
         * written will be remarked as dirty until other
         * buffers are written.
         */
        for (iter = pbusy = 0; iter < 20; iter++) {
                nbusy = 0;
                for (i = nbuf - 1; i >= 0; i--) {
                        bp = nbufp(i);
                        if (isbufbusy(bp))
                                nbusy++;
                }
                if (nbusy == 0) {
                        if (first_buf_printf)
                                printf("All buffers synced.");
                        break;
                }
                if (first_buf_printf) {
                        printf("Syncing disks, buffers remaining... ");
                        first_buf_printf = 0;
                }
                printf("%d ", nbusy);
                if (nbusy < pbusy)
                        iter = 0;
                pbusy = nbusy;

                wdog_kern_pat(WD_LASTVAL);
                kern_sync(curthread);

#ifdef PREEMPTION
                /*
                 * Spin for a while to allow interrupt threads to run.
                 */
                DELAY(50000 * iter);
#else
                /*
                 * Context switch several times to allow interrupt
                 * threads to run.
                 */
                for (subiter = 0; subiter < 50 * iter; subiter++) {
                        thread_lock(curthread);
                        mi_switch(SW_VOL);
                        DELAY(1000);
                }
#endif
        }
        printf("\n");
        /*
         * Count only busy local buffers to prevent forcing 
         * a fsck if we're just a client of a wedged NFS server
         */
        nbusy = 0;
        for (i = nbuf - 1; i >= 0; i--) {
                bp = nbufp(i);
                if (isbufbusy(bp)) {
#if 0
/* XXX: This is bogus.  We should probably have a BO_REMOTE flag instead */
                        if (bp->b_dev == NULL) {
                                TAILQ_REMOVE(&mountlist,
                                    bp->b_vp->v_mount, mnt_list);
                                continue;
                        }
#endif
                        nbusy++;
                        if (show_busybufs > 0) {
                                printf(
            "%d: buf:%p, vnode:%p, flags:%0x, blkno:%jd, lblkno:%jd, buflock:",
                                    nbusy, bp, bp->b_vp, bp->b_flags,
                                    (intmax_t)bp->b_blkno,
                                    (intmax_t)bp->b_lblkno);
                                BUF_LOCKPRINTINFO(bp);
                                if (show_busybufs > 1)
                                        vn_printf(bp->b_vp,
                                            "vnode content: ");
                        }
                }
        }
        if (nbusy) {
                /*
                 * Failed to sync all blocks. Indicate this and don't
                 * unmount filesystems (thus forcing an fsck on reboot).
                 */
                printf("Giving up on %d buffers\n", nbusy);
                DELAY(5000000); /* 5 seconds */
        } else {
                if (!first_buf_printf)
                        printf("Final sync complete\n");
                /*
                 * Unmount filesystems
                 */
                if (!KERNEL_PANICKED())
                        vfs_unmountall();
        }
        swapoff_all();
        DELAY(100000);          /* wait for console output to finish */
}

static void
bpmap_qenter(struct buf *bp)
{

        BUF_CHECK_MAPPED(bp);

        /*
         * bp->b_data is relative to bp->b_offset, but
         * bp->b_offset may be offset into the first page.
         */
        bp->b_data = (caddr_t)trunc_page((vm_offset_t)bp->b_data);
        pmap_qenter((vm_offset_t)bp->b_data, bp->b_pages, bp->b_npages);
        bp->b_data = (caddr_t)((vm_offset_t)bp->b_data |
            (vm_offset_t)(bp->b_offset & PAGE_MASK));
}

static inline struct bufdomain *
bufdomain(struct buf *bp)
{

        return (&bdomain[bp->b_domain]);
}

static struct bufqueue *
bufqueue(struct buf *bp)
{

        switch (bp->b_qindex) {
        case QUEUE_NONE:
                /* FALLTHROUGH */
        case QUEUE_SENTINEL:
                return (NULL);
        case QUEUE_EMPTY:
                return (&bqempty);
        case QUEUE_DIRTY:
                return (&bufdomain(bp)->bd_dirtyq);
        case QUEUE_CLEAN:
                return (&bufdomain(bp)->bd_subq[bp->b_subqueue]);
        default:
                break;
        }
        panic("bufqueue(%p): Unhandled type %d\n", bp, bp->b_qindex);
}

/*
 * Return the locked bufqueue that bp is a member of.
 */
static struct bufqueue *
bufqueue_acquire(struct buf *bp)
{
        struct bufqueue *bq, *nbq;

        /*
         * bp can be pushed from a per-cpu queue to the
         * cleanq while we're waiting on the lock.  Retry
         * if the queues don't match.
         */
        bq = bufqueue(bp);
        BQ_LOCK(bq);
        for (;;) {
                nbq = bufqueue(bp);
                if (bq == nbq)
                        break;
                BQ_UNLOCK(bq);
                BQ_LOCK(nbq);
                bq = nbq;
        }
        return (bq);
}

/*
 *      binsfree:
 *
 *      Insert the buffer into the appropriate free list.  Requires a
 *      locked buffer on entry and buffer is unlocked before return.
 */
static void
binsfree(struct buf *bp, int qindex)
{
        struct bufdomain *bd;
        struct bufqueue *bq;

        KASSERT(qindex == QUEUE_CLEAN || qindex == QUEUE_DIRTY,
            ("binsfree: Invalid qindex %d", qindex));
        BUF_ASSERT_XLOCKED(bp);

        /*
         * Handle delayed bremfree() processing.
         */
        if (bp->b_flags & B_REMFREE) {
                if (bp->b_qindex == qindex) {
                        bp->b_flags |= B_REUSE;
                        bp->b_flags &= ~B_REMFREE;
                        BUF_UNLOCK(bp);
                        return;
                }
                bq = bufqueue_acquire(bp);
                bq_remove(bq, bp);
                BQ_UNLOCK(bq);
        }
        bd = bufdomain(bp);
        if (qindex == QUEUE_CLEAN) {
                if (bd->bd_lim != 0)
                        bq = &bd->bd_subq[PCPU_GET(cpuid)];
                else
                        bq = bd->bd_cleanq;
        } else
                bq = &bd->bd_dirtyq;
        bq_insert(bq, bp, true);
}

/*
 * buf_free:
 *
 *      Free a buffer to the buf zone once it no longer has valid contents.
 */
static void
buf_free(struct buf *bp)
{

        if (bp->b_flags & B_REMFREE)
                bremfreef(bp);
        if (bp->b_vflags & BV_BKGRDINPROG)
                panic("losing buffer 1");
        if (bp->b_rcred != NOCRED) {
                crfree(bp->b_rcred);
                bp->b_rcred = NOCRED;
        }
        if (bp->b_wcred != NOCRED) {
                crfree(bp->b_wcred);
                bp->b_wcred = NOCRED;
        }
        if (!LIST_EMPTY(&bp->b_dep))
                buf_deallocate(bp);
        bufkva_free(bp);
        atomic_add_int(&bufdomain(bp)->bd_freebuffers, 1);
        MPASS((bp->b_flags & B_MAXPHYS) == 0);
        BUF_UNLOCK(bp);
        uma_zfree(buf_zone, bp);
}

/*
 * buf_import:
 *
 *      Import bufs into the uma cache from the buf list.  The system still
 *      expects a static array of bufs and much of the synchronization
 *      around bufs assumes type stable storage.  As a result, UMA is used
 *      only as a per-cpu cache of bufs still maintained on a global list.
 */
static int
buf_import(void *arg, void **store, int cnt, int domain, int flags)
{
        struct buf *bp;
        int i;

        BQ_LOCK(&bqempty);
        for (i = 0; i < cnt; i++) {
                bp = TAILQ_FIRST(&bqempty.bq_queue);
                if (bp == NULL)
                        break;
                bq_remove(&bqempty, bp);
                store[i] = bp;
        }
        BQ_UNLOCK(&bqempty);

        return (i);
}

/*
 * buf_release:
 *
 *      Release bufs from the uma cache back to the buffer queues.
 */
static void
buf_release(void *arg, void **store, int cnt)
{
        struct bufqueue *bq;
        struct buf *bp;
        int i;

        bq = &bqempty;
        BQ_LOCK(bq);
        for (i = 0; i < cnt; i++) {
                bp = store[i];
                /* Inline bq_insert() to batch locking. */
                TAILQ_INSERT_TAIL(&bq->bq_queue, bp, b_freelist);
                bp->b_flags &= ~(B_AGE | B_REUSE);
                bq->bq_len++;
                bp->b_qindex = bq->bq_index;
        }
        BQ_UNLOCK(bq);
}

/*
 * buf_alloc:
 *
 *      Allocate an empty buffer header.
 */
static struct buf *
buf_alloc(struct bufdomain *bd)
{
        struct buf *bp;
        int freebufs, error;

        /*
         * We can only run out of bufs in the buf zone if the average buf
         * is less than BKVASIZE.  In this case the actual wait/block will
         * come from buf_reycle() failing to flush one of these small bufs.
         */
        bp = NULL;
        freebufs = atomic_fetchadd_int(&bd->bd_freebuffers, -1);
        if (freebufs > 0)
                bp = uma_zalloc(buf_zone, M_NOWAIT);
        if (bp == NULL) {
                atomic_add_int(&bd->bd_freebuffers, 1);
                bufspace_daemon_wakeup(bd);
                counter_u64_add(numbufallocfails, 1);
                return (NULL);
        }
        /*
         * Wake-up the bufspace daemon on transition below threshold.
         */
        if (freebufs == bd->bd_lofreebuffers)
                bufspace_daemon_wakeup(bd);

        error = BUF_LOCK(bp, LK_EXCLUSIVE, NULL);
        KASSERT(error == 0, ("%s: BUF_LOCK on free buf %p: %d.", __func__, bp,
            error));
        (void)error;

        KASSERT(bp->b_vp == NULL,
            ("bp: %p still has vnode %p.", bp, bp->b_vp));
        KASSERT((bp->b_flags & (B_DELWRI | B_NOREUSE)) == 0,
            ("invalid buffer %p flags %#x", bp, bp->b_flags));
        KASSERT((bp->b_xflags & (BX_VNCLEAN|BX_VNDIRTY)) == 0,
            ("bp: %p still on a buffer list. xflags %X", bp, bp->b_xflags));
        KASSERT(bp->b_npages == 0,
            ("bp: %p still has %d vm pages\n", bp, bp->b_npages));
        KASSERT(bp->b_kvasize == 0, ("bp: %p still has kva\n", bp));
        KASSERT(bp->b_bufsize == 0, ("bp: %p still has bufspace\n", bp));
        MPASS((bp->b_flags & B_MAXPHYS) == 0);

        bp->b_domain = BD_DOMAIN(bd);
        bp->b_flags = 0;
        bp->b_ioflags = 0;
        bp->b_xflags = 0;
        bp->b_vflags = 0;
        bp->b_vp = NULL;
        bp->b_blkno = bp->b_lblkno = 0;
        bp->b_offset = NOOFFSET;
        bp->b_iodone = 0;
        bp->b_error = 0;
        bp->b_resid = 0;
        bp->b_bcount = 0;
        bp->b_npages = 0;
        bp->b_dirtyoff = bp->b_dirtyend = 0;
        bp->b_bufobj = NULL;
        bp->b_data = bp->b_kvabase = unmapped_buf;
        bp->b_fsprivate1 = NULL;
        bp->b_fsprivate2 = NULL;
        bp->b_fsprivate3 = NULL;
        LIST_INIT(&bp->b_dep);

        return (bp);
}

/*
 *      buf_recycle:
 *
 *      Free a buffer from the given bufqueue.  kva controls whether the
 *      freed buf must own some kva resources.  This is used for
 *      defragmenting.
 */
static int
buf_recycle(struct bufdomain *bd, bool kva)
{
        struct bufqueue *bq;
        struct buf *bp, *nbp;

        if (kva)
                counter_u64_add(bufdefragcnt, 1);
        nbp = NULL;
        bq = bd->bd_cleanq;
        BQ_LOCK(bq);
        KASSERT(BQ_LOCKPTR(bq) == BD_LOCKPTR(bd),
            ("buf_recycle: Locks don't match"));
        nbp = TAILQ_FIRST(&bq->bq_queue);

        /*
         * Run scan, possibly freeing data and/or kva mappings on the fly
         * depending.
         */
        while ((bp = nbp) != NULL) {
                /*
                 * Calculate next bp (we can only use it if we do not
                 * release the bqlock).
                 */
                nbp = TAILQ_NEXT(bp, b_freelist);

                /*
                 * If we are defragging then we need a buffer with 
                 * some kva to reclaim.
                 */
                if (kva && bp->b_kvasize == 0)
                        continue;

                if (BUF_LOCK(bp, LK_EXCLUSIVE | LK_NOWAIT, NULL) != 0)
                        continue;

                /*
                 * Implement a second chance algorithm for frequently
                 * accessed buffers.
                 */
                if ((bp->b_flags & B_REUSE) != 0) {
                        TAILQ_REMOVE(&bq->bq_queue, bp, b_freelist);
                        TAILQ_INSERT_TAIL(&bq->bq_queue, bp, b_freelist);
                        bp->b_flags &= ~B_REUSE;
                        BUF_UNLOCK(bp);
                        continue;
                }

                /*
                 * Skip buffers with background writes in progress.
                 */
                if ((bp->b_vflags & BV_BKGRDINPROG) != 0) {
                        BUF_UNLOCK(bp);
                        continue;
                }

                KASSERT(bp->b_qindex == QUEUE_CLEAN,
                    ("buf_recycle: inconsistent queue %d bp %p",
                    bp->b_qindex, bp));
                KASSERT(bp->b_domain == BD_DOMAIN(bd),
                    ("getnewbuf: queue domain %d doesn't match request %d",
                    bp->b_domain, (int)BD_DOMAIN(bd)));
                /*
                 * NOTE:  nbp is now entirely invalid.  We can only restart
                 * the scan from this point on.
                 */
                bq_remove(bq, bp);
                BQ_UNLOCK(bq);

                /*
                 * Requeue the background write buffer with error and
                 * restart the scan.
                 */
                if ((bp->b_vflags & BV_BKGRDERR) != 0) {
                        bqrelse(bp);
                        BQ_LOCK(bq);
                        nbp = TAILQ_FIRST(&bq->bq_queue);
                        continue;
                }
                bp->b_flags |= B_INVAL;
                brelse(bp);
                return (0);
        }
        bd->bd_wanted = 1;
        BQ_UNLOCK(bq);

        return (ENOBUFS);
}

/*
 *      bremfree:
 *
 *      Mark the buffer for removal from the appropriate free list.
 *
 */
void
bremfree(struct buf *bp)
{

        CTR3(KTR_BUF, "bremfree(%p) vp %p flags %X", bp, bp->b_vp, bp->b_flags);
        KASSERT((bp->b_flags & B_REMFREE) == 0,
            ("bremfree: buffer %p already marked for delayed removal.", bp));
        KASSERT(bp->b_qindex != QUEUE_NONE,
            ("bremfree: buffer %p not on a queue.", bp));
        BUF_ASSERT_XLOCKED(bp);

        bp->b_flags |= B_REMFREE;
}

/*
 *      bremfreef:
 *
 *      Force an immediate removal from a free list.  Used only in nfs when
 *      it abuses the b_freelist pointer.
 */
void
bremfreef(struct buf *bp)
{
        struct bufqueue *bq;

        bq = bufqueue_acquire(bp);
        bq_remove(bq, bp);
        BQ_UNLOCK(bq);
}

static void
bq_init(struct bufqueue *bq, int qindex, int subqueue, const char *lockname)
{

        mtx_init(&bq->bq_lock, lockname, NULL, MTX_DEF);
        TAILQ_INIT(&bq->bq_queue);
        bq->bq_len = 0;
        bq->bq_index = qindex;
        bq->bq_subqueue = subqueue;
}

static void
bd_init(struct bufdomain *bd)
{
        int i;

        bd->bd_cleanq = &bd->bd_subq[mp_maxid + 1];
        bq_init(bd->bd_cleanq, QUEUE_CLEAN, mp_maxid + 1, "bufq clean lock");
        bq_init(&bd->bd_dirtyq, QUEUE_DIRTY, -1, "bufq dirty lock");
        for (i = 0; i <= mp_maxid; i++)
                bq_init(&bd->bd_subq[i], QUEUE_CLEAN, i,
                    "bufq clean subqueue lock");
        mtx_init(&bd->bd_run_lock, "bufspace daemon run lock", NULL, MTX_DEF);
}

/*
 *      bq_remove:
 *
 *      Removes a buffer from the free list, must be called with the
 *      correct qlock held.
 */
static void
bq_remove(struct bufqueue *bq, struct buf *bp)
{

        CTR3(KTR_BUF, "bq_remove(%p) vp %p flags %X",
            bp, bp->b_vp, bp->b_flags);
        KASSERT(bp->b_qindex != QUEUE_NONE,
            ("bq_remove: buffer %p not on a queue.", bp));
        KASSERT(bufqueue(bp) == bq,
            ("bq_remove: Remove buffer %p from wrong queue.", bp));

        BQ_ASSERT_LOCKED(bq);
        if (bp->b_qindex != QUEUE_EMPTY) {
                BUF_ASSERT_XLOCKED(bp);
        }
        KASSERT(bq->bq_len >= 1,
            ("queue %d underflow", bp->b_qindex));
        TAILQ_REMOVE(&bq->bq_queue, bp, b_freelist);
        bq->bq_len--;
        bp->b_qindex = QUEUE_NONE;
        bp->b_flags &= ~(B_REMFREE | B_REUSE);
}

static void
bd_flush(struct bufdomain *bd, struct bufqueue *bq)
{
        struct buf *bp;

        BQ_ASSERT_LOCKED(bq);
        if (bq != bd->bd_cleanq) {
                BD_LOCK(bd);
                while ((bp = TAILQ_FIRST(&bq->bq_queue)) != NULL) {
                        TAILQ_REMOVE(&bq->bq_queue, bp, b_freelist);
                        TAILQ_INSERT_TAIL(&bd->bd_cleanq->bq_queue, bp,
                            b_freelist);
                        bp->b_subqueue = bd->bd_cleanq->bq_subqueue;
                }
                bd->bd_cleanq->bq_len += bq->bq_len;
                bq->bq_len = 0;
        }
        if (bd->bd_wanted) {
                bd->bd_wanted = 0;
                wakeup(&bd->bd_wanted);
        }
        if (bq != bd->bd_cleanq)
                BD_UNLOCK(bd);
}

static int
bd_flushall(struct bufdomain *bd)
{
        struct bufqueue *bq;
        int flushed;
        int i;

        if (bd->bd_lim == 0)
                return (0);
        flushed = 0;
        for (i = 0; i <= mp_maxid; i++) {
                bq = &bd->bd_subq[i];
                if (bq->bq_len == 0)
                        continue;
                BQ_LOCK(bq);
                bd_flush(bd, bq);
                BQ_UNLOCK(bq);
                flushed++;
        }

        return (flushed);
}

static void
bq_insert(struct bufqueue *bq, struct buf *bp, bool unlock)
{
        struct bufdomain *bd;

        if (bp->b_qindex != QUEUE_NONE)
                panic("bq_insert: free buffer %p onto another queue?", bp);

        bd = bufdomain(bp);
        if (bp->b_flags & B_AGE) {
                /* Place this buf directly on the real queue. */
                if (bq->bq_index == QUEUE_CLEAN)
                        bq = bd->bd_cleanq;
                BQ_LOCK(bq);
                TAILQ_INSERT_HEAD(&bq->bq_queue, bp, b_freelist);
        } else {
                BQ_LOCK(bq);
                TAILQ_INSERT_TAIL(&bq->bq_queue, bp, b_freelist);
        }
        bp->b_flags &= ~(B_AGE | B_REUSE);
        bq->bq_len++;
        bp->b_qindex = bq->bq_index;
        bp->b_subqueue = bq->bq_subqueue;

        /*
         * Unlock before we notify so that we don't wakeup a waiter that
         * fails a trylock on the buf and sleeps again.
         */
        if (unlock)
                BUF_UNLOCK(bp);

        if (bp->b_qindex == QUEUE_CLEAN) {
                /*
                 * Flush the per-cpu queue and notify any waiters.
                 */
                if (bd->bd_wanted || (bq != bd->bd_cleanq &&
                    bq->bq_len >= bd->bd_lim))
                        bd_flush(bd, bq);
        }
        BQ_UNLOCK(bq);
}

/*
 *      bufkva_free:
 *
 *      Free the kva allocation for a buffer.
 *
 */
static void
bufkva_free(struct buf *bp)
{

#ifdef INVARIANTS
        if (bp->b_kvasize == 0) {
                KASSERT(bp->b_kvabase == unmapped_buf &&
                    bp->b_data == unmapped_buf,
                    ("Leaked KVA space on %p", bp));
        } else if (buf_mapped(bp))
                BUF_CHECK_MAPPED(bp);
        else
                BUF_CHECK_UNMAPPED(bp);
#endif
        if (bp->b_kvasize == 0)
                return;

        vmem_free(buffer_arena, (vm_offset_t)bp->b_kvabase, bp->b_kvasize);
        counter_u64_add(bufkvaspace, -bp->b_kvasize);
        counter_u64_add(buffreekvacnt, 1);
        bp->b_data = bp->b_kvabase = unmapped_buf;
        bp->b_kvasize = 0;
}

/*
 *      bufkva_alloc:
 *
 *      Allocate the buffer KVA and set b_kvasize and b_kvabase.
 */
static int
bufkva_alloc(struct buf *bp, int maxsize, int gbflags)
{
        vm_offset_t addr;
        int error;

        KASSERT((gbflags & GB_UNMAPPED) == 0 || (gbflags & GB_KVAALLOC) != 0,
            ("Invalid gbflags 0x%x in %s", gbflags, __func__));
        MPASS((bp->b_flags & B_MAXPHYS) == 0);
        KASSERT(maxsize <= maxbcachebuf,
            ("bufkva_alloc kva too large %d %u", maxsize, maxbcachebuf));

        bufkva_free(bp);

        addr = 0;
        error = vmem_alloc(buffer_arena, maxsize, M_BESTFIT | M_NOWAIT, &addr);
        if (error != 0) {
                /*
                 * Buffer map is too fragmented.  Request the caller
                 * to defragment the map.
                 */
                return (error);
        }
        bp->b_kvabase = (caddr_t)addr;
        bp->b_kvasize = maxsize;
        counter_u64_add(bufkvaspace, bp->b_kvasize);
        if ((gbflags & GB_UNMAPPED) != 0) {
                bp->b_data = unmapped_buf;
                BUF_CHECK_UNMAPPED(bp);
        } else {
                bp->b_data = bp->b_kvabase;
                BUF_CHECK_MAPPED(bp);
        }
        return (0);
}

/*
 *      bufkva_reclaim:
 *
 *      Reclaim buffer kva by freeing buffers holding kva.  This is a vmem
 *      callback that fires to avoid returning failure.
 */
static void
bufkva_reclaim(vmem_t *vmem, int flags)
{
        bool done;
        int q;
        int i;

        done = false;
        for (i = 0; i < 5; i++) {
                for (q = 0; q < buf_domains; q++)
                        if (buf_recycle(&bdomain[q], true) != 0)
                                done = true;
                if (done)
                        break;
        }
        return;
}

/*
 * Attempt to initiate asynchronous I/O on read-ahead blocks.  We must
 * clear BIO_ERROR and B_INVAL prior to initiating I/O . If B_CACHE is set,
 * the buffer is valid and we do not have to do anything.
 */
static void
breada(struct vnode * vp, daddr_t * rablkno, int * rabsize, int cnt,
    struct ucred * cred, int flags, void (*ckhashfunc)(struct buf *))
{
        struct buf *rabp;
        struct thread *td;
        int i;

        td = curthread;

        for (i = 0; i < cnt; i++, rablkno++, rabsize++) {
                if (inmem(vp, *rablkno))
                        continue;
                rabp = getblk(vp, *rablkno, *rabsize, 0, 0, 0);
                if ((rabp->b_flags & B_CACHE) != 0) {
                        brelse(rabp);
                        continue;
                }
#ifdef RACCT
                if (racct_enable) {
                        PROC_LOCK(curproc);
                        racct_add_buf(curproc, rabp, 0);
                        PROC_UNLOCK(curproc);
                }
#endif /* RACCT */
                td->td_ru.ru_inblock++;
                rabp->b_flags |= B_ASYNC;
                rabp->b_flags &= ~B_INVAL;
                if ((flags & GB_CKHASH) != 0) {
                        rabp->b_flags |= B_CKHASH;
                        rabp->b_ckhashcalc = ckhashfunc;
                }
                rabp->b_ioflags &= ~BIO_ERROR;
                rabp->b_iocmd = BIO_READ;
                if (rabp->b_rcred == NOCRED && cred != NOCRED)
                        rabp->b_rcred = crhold(cred);
                vfs_busy_pages(rabp, 0);
                BUF_KERNPROC(rabp);
                rabp->b_iooffset = dbtob(rabp->b_blkno);
                bstrategy(rabp);
        }
}

/*
 * Entry point for bread() and breadn() via #defines in sys/buf.h.
 *
 * Get a buffer with the specified data.  Look in the cache first.  We
 * must clear BIO_ERROR and B_INVAL prior to initiating I/O.  If B_CACHE
 * is set, the buffer is valid and we do not have to do anything, see
 * getblk(). Also starts asynchronous I/O on read-ahead blocks.
 *
 * Always return a NULL buffer pointer (in bpp) when returning an error.
 *
 * The blkno parameter is the logical block being requested. Normally
 * the mapping of logical block number to disk block address is done
 * by calling VOP_BMAP(). However, if the mapping is already known, the
 * disk block address can be passed using the dblkno parameter. If the
 * disk block address is not known, then the same value should be passed
 * for blkno and dblkno.
 */
int
breadn_flags(struct vnode *vp, daddr_t blkno, daddr_t dblkno, int size,
    daddr_t *rablkno, int *rabsize, int cnt, struct ucred *cred, int flags,
    void (*ckhashfunc)(struct buf *), struct buf **bpp)
{
        struct buf *bp;
        struct thread *td;
        int error, readwait, rv;

        CTR3(KTR_BUF, "breadn(%p, %jd, %d)", vp, blkno, size);
        td = curthread;
        /*
         * Can only return NULL if GB_LOCK_NOWAIT or GB_SPARSE flags
         * are specified.
         */
        error = getblkx(vp, blkno, dblkno, size, 0, 0, flags, &bp);
        if (error != 0) {
                *bpp = NULL;
                return (error);
        }
        KASSERT(blkno == bp->b_lblkno,
            ("getblkx returned buffer for blkno %jd instead of blkno %jd",
            (intmax_t)bp->b_lblkno, (intmax_t)blkno));
        flags &= ~GB_NOSPARSE;
        *bpp = bp;

        /*
         * If not found in cache, do some I/O
         */
        readwait = 0;
        if ((bp->b_flags & B_CACHE) == 0) {
#ifdef RACCT
                if (racct_enable) {
                        PROC_LOCK(td->td_proc);
                        racct_add_buf(td->td_proc, bp, 0);
                        PROC_UNLOCK(td->td_proc);
                }
#endif /* RACCT */
                td->td_ru.ru_inblock++;
                bp->b_iocmd = BIO_READ;
                bp->b_flags &= ~B_INVAL;
                if ((flags & GB_CKHASH) != 0) {
                        bp->b_flags |= B_CKHASH;
                        bp->b_ckhashcalc = ckhashfunc;
                }
                if ((flags & GB_CVTENXIO) != 0)
                        bp->b_xflags |= BX_CVTENXIO;
                bp->b_ioflags &= ~BIO_ERROR;
                if (bp->b_rcred == NOCRED && cred != NOCRED)
                        bp->b_rcred = crhold(cred);
                vfs_busy_pages(bp, 0);
                bp->b_iooffset = dbtob(bp->b_blkno);
                bstrategy(bp);
                ++readwait;
        }

        /*
         * Attempt to initiate asynchronous I/O on read-ahead blocks.
         */
        breada(vp, rablkno, rabsize, cnt, cred, flags, ckhashfunc);

        rv = 0;
        if (readwait) {
                rv = bufwait(bp);
                if (rv != 0) {
                        brelse(bp);
                        *bpp = NULL;
                }
        }
        return (rv);
}

/*
 * Write, release buffer on completion.  (Done by iodone
 * if async).  Do not bother writing anything if the buffer
 * is invalid.
 *
 * Note that we set B_CACHE here, indicating that buffer is
 * fully valid and thus cacheable.  This is true even of NFS
 * now so we set it generally.  This could be set either here 
 * or in biodone() since the I/O is synchronous.  We put it
 * here.
 */
int
bufwrite(struct buf *bp)
{
        int oldflags;
        struct vnode *vp;
        long space;
        int vp_md;

        CTR3(KTR_BUF, "bufwrite(%p) vp %p flags %X", bp, bp->b_vp, bp->b_flags);
        if ((bp->b_bufobj->bo_flag & BO_DEAD) != 0) {
                bp->b_flags |= B_INVAL | B_RELBUF;
                bp->b_flags &= ~B_CACHE;
                brelse(bp);
                return (ENXIO);
        }
        if (bp->b_flags & B_INVAL) {
                brelse(bp);
                return (0);
        }

        if (bp->b_flags & B_BARRIER)
                atomic_add_long(&barrierwrites, 1);

        oldflags = bp->b_flags;

        KASSERT(!(bp->b_vflags & BV_BKGRDINPROG),
            ("FFS background buffer should not get here %p", bp));

        vp = bp->b_vp;
        if (vp)
                vp_md = vp->v_vflag & VV_MD;
        else
                vp_md = 0;

        /*
         * Mark the buffer clean.  Increment the bufobj write count
         * before bundirty() call, to prevent other thread from seeing
         * empty dirty list and zero counter for writes in progress,
         * falsely indicating that the bufobj is clean.
         */
        bufobj_wref(bp->b_bufobj);
        bundirty(bp);

        bp->b_flags &= ~B_DONE;
        bp->b_ioflags &= ~BIO_ERROR;
        bp->b_flags |= B_CACHE;
        bp->b_iocmd = BIO_WRITE;

        vfs_busy_pages(bp, 1);

        /*
         * Normal bwrites pipeline writes
         */
        bp->b_runningbufspace = bp->b_bufsize;
        space = atomic_fetchadd_long(&runningbufspace, bp->b_runningbufspace);

#ifdef RACCT
        if (racct_enable) {
                PROC_LOCK(curproc);
                racct_add_buf(curproc, bp, 1);
                PROC_UNLOCK(curproc);
        }
#endif /* RACCT */
        curthread->td_ru.ru_oublock++;
        if (oldflags & B_ASYNC)
                BUF_KERNPROC(bp);
        bp->b_iooffset = dbtob(bp->b_blkno);
        buf_track(bp, __func__);
        bstrategy(bp);

        if ((oldflags & B_ASYNC) == 0) {
                int rtval = bufwait(bp);
                brelse(bp);
                return (rtval);
        } else if (space > hirunningspace) {
                /*
                 * don't allow the async write to saturate the I/O
                 * system.  We will not deadlock here because
                 * we are blocking waiting for I/O that is already in-progress
                 * to complete. We do not block here if it is the update
                 * or syncer daemon trying to clean up as that can lead
                 * to deadlock.
                 */
                if ((curthread->td_pflags & TDP_NORUNNINGBUF) == 0 && !vp_md)
                        waitrunningbufspace();
        }

        return (0);
}

void
bufbdflush(struct bufobj *bo, struct buf *bp)
{
        struct buf *nbp;
        struct bufdomain *bd;

        bd = &bdomain[bo->bo_domain];
        if (bo->bo_dirty.bv_cnt > bd->bd_dirtybufthresh + 10) {
                (void) VOP_FSYNC(bp->b_vp, MNT_NOWAIT, curthread);
                altbufferflushes++;
        } else if (bo->bo_dirty.bv_cnt > bd->bd_dirtybufthresh) {
                BO_LOCK(bo);
                /*
                 * Try to find a buffer to flush.
                 */
                TAILQ_FOREACH(nbp, &bo->bo_dirty.bv_hd, b_bobufs) {
                        if ((nbp->b_vflags & BV_BKGRDINPROG) ||
                            BUF_LOCK(nbp,
                                     LK_EXCLUSIVE | LK_NOWAIT, NULL))
                                continue;
                        if (bp == nbp)
                                panic("bdwrite: found ourselves");
                        BO_UNLOCK(bo);
                        /* Don't countdeps with the bo lock held. */
                        if (buf_countdeps(nbp, 0)) {
                                BO_LOCK(bo);
                                BUF_UNLOCK(nbp);
                                continue;
                        }
                        if (nbp->b_flags & B_CLUSTEROK) {
                                vfs_bio_awrite(nbp);
                        } else {
                                bremfree(nbp);
                                bawrite(nbp);
                        }
                        dirtybufferflushes++;
                        break;
                }
                if (nbp == NULL)
                        BO_UNLOCK(bo);
        }
}

/*
 * Delayed write. (Buffer is marked dirty).  Do not bother writing
 * anything if the buffer is marked invalid.
 *
 * Note that since the buffer must be completely valid, we can safely
 * set B_CACHE.  In fact, we have to set B_CACHE here rather then in
 * biodone() in order to prevent getblk from writing the buffer
 * out synchronously.
 */
void
bdwrite(struct buf *bp)
{
        struct thread *td = curthread;
        struct vnode *vp;
        struct bufobj *bo;

        CTR3(KTR_BUF, "bdwrite(%p) vp %p flags %X", bp, bp->b_vp, bp->b_flags);
        KASSERT(bp->b_bufobj != NULL, ("No b_bufobj %p", bp));
        KASSERT((bp->b_flags & B_BARRIER) == 0,
            ("Barrier request in delayed write %p", bp));

        if (bp->b_flags & B_INVAL) {
                brelse(bp);
                return;
        }

        /*
         * If we have too many dirty buffers, don't create any more.
         * If we are wildly over our limit, then force a complete
         * cleanup. Otherwise, just keep the situation from getting
         * out of control. Note that we have to avoid a recursive
         * disaster and not try to clean up after our own cleanup!
         */
        vp = bp->b_vp;
        bo = bp->b_bufobj;
        if ((td->td_pflags & (TDP_COWINPROGRESS|TDP_INBDFLUSH)) == 0) {
                td->td_pflags |= TDP_INBDFLUSH;
                BO_BDFLUSH(bo, bp);
                td->td_pflags &= ~TDP_INBDFLUSH;
        } else
                recursiveflushes++;

        bdirty(bp);
        /*
         * Set B_CACHE, indicating that the buffer is fully valid.  This is
         * true even of NFS now.
         */
        bp->b_flags |= B_CACHE;

        /*
         * This bmap keeps the system from needing to do the bmap later,
         * perhaps when the system is attempting to do a sync.  Since it
         * is likely that the indirect block -- or whatever other datastructure
         * that the filesystem needs is still in memory now, it is a good
         * thing to do this.  Note also, that if the pageout daemon is
         * requesting a sync -- there might not be enough memory to do
         * the bmap then...  So, this is important to do.
         */
        if (vp->v_type != VCHR && bp->b_lblkno == bp->b_blkno) {
                VOP_BMAP(vp, bp->b_lblkno, NULL, &bp->b_blkno, NULL, NULL);
        }

        buf_track(bp, __func__);

        /*
         * Set the *dirty* buffer range based upon the VM system dirty
         * pages.
         *
         * Mark the buffer pages as clean.  We need to do this here to
         * satisfy the vnode_pager and the pageout daemon, so that it
         * thinks that the pages have been "cleaned".  Note that since
         * the pages are in a delayed write buffer -- the VFS layer
         * "will" see that the pages get written out on the next sync,
         * or perhaps the cluster will be completed.
         */
        vfs_clean_pages_dirty_buf(bp);
        bqrelse(bp);

        /*
         * note: we cannot initiate I/O from a bdwrite even if we wanted to,
         * due to the softdep code.
         */
}

/*
 *      bdirty:
 *
 *      Turn buffer into delayed write request.  We must clear BIO_READ and
 *      B_RELBUF, and we must set B_DELWRI.  We reassign the buffer to 
 *      itself to properly update it in the dirty/clean lists.  We mark it
 *      B_DONE to ensure that any asynchronization of the buffer properly
 *      clears B_DONE ( else a panic will occur later ).  
 *
 *      bdirty() is kinda like bdwrite() - we have to clear B_INVAL which
 *      might have been set pre-getblk().  Unlike bwrite/bdwrite, bdirty()
 *      should only be called if the buffer is known-good.
 *
 *      Since the buffer is not on a queue, we do not update the numfreebuffers
 *      count.
 *
 *      The buffer must be on QUEUE_NONE.
 */
void
bdirty(struct buf *bp)
{

        CTR3(KTR_BUF, "bdirty(%p) vp %p flags %X",
            bp, bp->b_vp, bp->b_flags);
        KASSERT(bp->b_bufobj != NULL, ("No b_bufobj %p", bp));
        KASSERT(bp->b_flags & B_REMFREE || bp->b_qindex == QUEUE_NONE,
            ("bdirty: buffer %p still on queue %d", bp, bp->b_qindex));
        bp->b_flags &= ~(B_RELBUF);
        bp->b_iocmd = BIO_WRITE;

        if ((bp->b_flags & B_DELWRI) == 0) {
                bp->b_flags |= /* XXX B_DONE | */ B_DELWRI;
                reassignbuf(bp);
                bdirtyadd(bp);
        }
}

/*
 *      bundirty:
 *
 *      Clear B_DELWRI for buffer.
 *
 *      Since the buffer is not on a queue, we do not update the numfreebuffers
 *      count.
 *
 *      The buffer must be on QUEUE_NONE.
 */

void
bundirty(struct buf *bp)
{

        CTR3(KTR_BUF, "bundirty(%p) vp %p flags %X", bp, bp->b_vp, bp->b_flags);
        KASSERT(bp->b_bufobj != NULL, ("No b_bufobj %p", bp));
        KASSERT(bp->b_flags & B_REMFREE || bp->b_qindex == QUEUE_NONE,
            ("bundirty: buffer %p still on queue %d", bp, bp->b_qindex));

        if (bp->b_flags & B_DELWRI) {
                bp->b_flags &= ~B_DELWRI;
                reassignbuf(bp);
                bdirtysub(bp);
        }
        /*
         * Since it is now being written, we can clear its deferred write flag.
         */
        bp->b_flags &= ~B_DEFERRED;
}

/*
 *      bawrite:
 *
 *      Asynchronous write.  Start output on a buffer, but do not wait for
 *      it to complete.  The buffer is released when the output completes.
 *
 *      bwrite() ( or the VOP routine anyway ) is responsible for handling 
 *      B_INVAL buffers.  Not us.
 */
void
bawrite(struct buf *bp)
{

        bp->b_flags |= B_ASYNC;
        (void) bwrite(bp);
}

/*
 *      babarrierwrite:
 *
 *      Asynchronous barrier write.  Start output on a buffer, but do not
 *      wait for it to complete.  Place a write barrier after this write so
 *      that this buffer and all buffers written before it are committed to
 *      the disk before any buffers written after this write are committed
 *      to the disk.  The buffer is released when the output completes.
 */
void
babarrierwrite(struct buf *bp)
{

        bp->b_flags |= B_ASYNC | B_BARRIER;
        (void) bwrite(bp);
}

/*
 *      bbarrierwrite:
 *
 *      Synchronous barrier write.  Start output on a buffer and wait for
 *      it to complete.  Place a write barrier after this write so that
 *      this buffer and all buffers written before it are committed to 
 *      the disk before any buffers written after this write are committed
 *      to the disk.  The buffer is released when the output completes.
 */
int
bbarrierwrite(struct buf *bp)
{

        bp->b_flags |= B_BARRIER;
        return (bwrite(bp));
}

/*
 *      bwillwrite:
 *
 *      Called prior to the locking of any vnodes when we are expecting to
 *      write.  We do not want to starve the buffer cache with too many
 *      dirty buffers so we block here.  By blocking prior to the locking
 *      of any vnodes we attempt to avoid the situation where a locked vnode
 *      prevents the various system daemons from flushing related buffers.
 */
void
bwillwrite(void)
{

        if (buf_dirty_count_severe()) {
                mtx_lock(&bdirtylock);
                while (buf_dirty_count_severe()) {
                        bdirtywait = 1;
                        msleep(&bdirtywait, &bdirtylock, (PRIBIO + 4),
                            "flswai", 0);
                }
                mtx_unlock(&bdirtylock);
        }
}

/*
 * Return true if we have too many dirty buffers.
 */
int
buf_dirty_count_severe(void)
{

        return (!BIT_EMPTY(BUF_DOMAINS, &bdhidirty));
}

/*
 *      brelse:
 *
 *      Release a busy buffer and, if requested, free its resources.  The
 *      buffer will be stashed in the appropriate bufqueue[] allowing it
 *      to be accessed later as a cache entity or reused for other purposes.
 */
void
brelse(struct buf *bp)
{
        struct mount *v_mnt;
        int qindex;

        /*
         * Many functions erroneously call brelse with a NULL bp under rare
         * error conditions. Simply return when called with a NULL bp.
         */
        if (bp == NULL)
                return;
        CTR3(KTR_BUF, "brelse(%p) vp %p flags %X",
            bp, bp->b_vp, bp->b_flags);
        KASSERT(!(bp->b_flags & (B_CLUSTER|B_PAGING)),
            ("brelse: inappropriate B_PAGING or B_CLUSTER bp %p", bp));
        KASSERT((bp->b_flags & B_VMIO) != 0 || (bp->b_flags & B_NOREUSE) == 0,
            ("brelse: non-VMIO buffer marked NOREUSE"));

        if (BUF_LOCKRECURSED(bp)) {
                /*
                 * Do not process, in particular, do not handle the
                 * B_INVAL/B_RELBUF and do not release to free list.
                 */
                BUF_UNLOCK(bp);
                return;
        }

        if (bp->b_flags & B_MANAGED) {
                bqrelse(bp);
                return;
        }

        if (LIST_EMPTY(&bp->b_dep)) {
                bp->b_flags &= ~B_IOSTARTED;
        } else {
                KASSERT((bp->b_flags & B_IOSTARTED) == 0,
                    ("brelse: SU io not finished bp %p", bp));
        }

        if ((bp->b_vflags & (BV_BKGRDINPROG | BV_BKGRDERR)) == BV_BKGRDERR) {
                BO_LOCK(bp->b_bufobj);
                bp->b_vflags &= ~BV_BKGRDERR;
                BO_UNLOCK(bp->b_bufobj);
                bdirty(bp);
        }

        if (bp->b_iocmd == BIO_WRITE && (bp->b_ioflags & BIO_ERROR) &&
            (bp->b_flags & B_INVALONERR)) {
                /*
                 * Forced invalidation of dirty buffer contents, to be used
                 * after a failed write in the rare case that the loss of the
                 * contents is acceptable.  The buffer is invalidated and
                 * freed.
                 */
                bp->b_flags |= B_INVAL | B_RELBUF | B_NOCACHE;
                bp->b_flags &= ~(B_ASYNC | B_CACHE);
        }

        if (bp->b_iocmd == BIO_WRITE && (bp->b_ioflags & BIO_ERROR) &&
            (bp->b_error != ENXIO || !LIST_EMPTY(&bp->b_dep)) &&
            !(bp->b_flags & B_INVAL)) {
                /*
                 * Failed write, redirty.  All errors except ENXIO (which
                 * means the device is gone) are treated as being
                 * transient.
                 *
                 * XXX Treating EIO as transient is not correct; the
                 * contract with the local storage device drivers is that
                 * they will only return EIO once the I/O is no longer
                 * retriable.  Network I/O also respects this through the
                 * guarantees of TCP and/or the internal retries of NFS.
                 * ENOMEM might be transient, but we also have no way of
                 * knowing when its ok to retry/reschedule.  In general,
                 * this entire case should be made obsolete through better
                 * error handling/recovery and resource scheduling.
                 *
                 * Do this also for buffers that failed with ENXIO, but have
                 * non-empty dependencies - the soft updates code might need
                 * to access the buffer to untangle them.
                 *
                 * Must clear BIO_ERROR to prevent pages from being scrapped.
                 */
                bp->b_ioflags &= ~BIO_ERROR;
                bdirty(bp);
        } else if ((bp->b_flags & (B_NOCACHE | B_INVAL)) ||
            (bp->b_ioflags & BIO_ERROR) || (bp->b_bufsize <= 0)) {
                /*
                 * Either a failed read I/O, or we were asked to free or not
                 * cache the buffer, or we failed to write to a device that's
                 * no longer present.
                 */
                bp->b_flags |= B_INVAL;
                if (!LIST_EMPTY(&bp->b_dep))
                        buf_deallocate(bp);
                if (bp->b_flags & B_DELWRI)
                        bdirtysub(bp);
                bp->b_flags &= ~(B_DELWRI | B_CACHE);
                if ((bp->b_flags & B_VMIO) == 0) {
                        allocbuf(bp, 0);
                        if (bp->b_vp)
                                brelvp(bp);
                }
        }

        /*
         * We must clear B_RELBUF if B_DELWRI is set.  If vfs_vmio_truncate() 
         * is called with B_DELWRI set, the underlying pages may wind up
         * getting freed causing a previous write (bdwrite()) to get 'lost'
         * because pages associated with a B_DELWRI bp are marked clean.
         * 
         * We still allow the B_INVAL case to call vfs_vmio_truncate(), even
         * if B_DELWRI is set.
         */
        if (bp->b_flags & B_DELWRI)
                bp->b_flags &= ~B_RELBUF;

        /*
         * VMIO buffer rundown.  It is not very necessary to keep a VMIO buffer
         * constituted, not even NFS buffers now.  Two flags effect this.  If
         * B_INVAL, the struct buf is invalidated but the VM object is kept
         * around ( i.e. so it is trivial to reconstitute the buffer later ).
         *
         * If BIO_ERROR or B_NOCACHE is set, pages in the VM object will be
         * invalidated.  BIO_ERROR cannot be set for a failed write unless the
         * buffer is also B_INVAL because it hits the re-dirtying code above.
         *
         * Normally we can do this whether a buffer is B_DELWRI or not.  If
         * the buffer is an NFS buffer, it is tracking piecemeal writes or
         * the commit state and we cannot afford to lose the buffer. If the
         * buffer has a background write in progress, we need to keep it
         * around to prevent it from being reconstituted and starting a second
         * background write.
         */

        v_mnt = bp->b_vp != NULL ? bp->b_vp->v_mount : NULL;

        if ((bp->b_flags & B_VMIO) && (bp->b_flags & B_NOCACHE ||
            (bp->b_ioflags & BIO_ERROR && bp->b_iocmd == BIO_READ)) &&
            (v_mnt == NULL || (v_mnt->mnt_vfc->vfc_flags & VFCF_NETWORK) == 0 ||
            vn_isdisk(bp->b_vp) || (bp->b_flags & B_DELWRI) == 0)) {
                vfs_vmio_invalidate(bp);
                allocbuf(bp, 0);
        }

        if ((bp->b_flags & (B_INVAL | B_RELBUF)) != 0 ||
            (bp->b_flags & (B_DELWRI | B_NOREUSE)) == B_NOREUSE) {
                allocbuf(bp, 0);
                bp->b_flags &= ~B_NOREUSE;
                if (bp->b_vp != NULL)
                        brelvp(bp);
        }

        /*
         * If the buffer has junk contents signal it and eventually
         * clean up B_DELWRI and diassociate the vnode so that gbincore()
         * doesn't find it.
         */
        if (bp->b_bufsize == 0 || (bp->b_ioflags & BIO_ERROR) != 0 ||
            (bp->b_flags & (B_INVAL | B_NOCACHE | B_RELBUF)) != 0)
                bp->b_flags |= B_INVAL;
        if (bp->b_flags & B_INVAL) {
                if (bp->b_flags & B_DELWRI)
                        bundirty(bp);
                if (bp->b_vp)
                        brelvp(bp);
        }

        buf_track(bp, __func__);

        /* buffers with no memory */
        if (bp->b_bufsize == 0) {
                buf_free(bp);
                return;
        }
        /* buffers with junk contents */
        if (bp->b_flags & (B_INVAL | B_NOCACHE | B_RELBUF) ||
            (bp->b_ioflags & BIO_ERROR)) {
                bp->b_xflags &= ~(BX_BKGRDWRITE | BX_ALTDATA);
                if (bp->b_vflags & BV_BKGRDINPROG)
                        panic("losing buffer 2");
                qindex = QUEUE_CLEAN;
                bp->b_flags |= B_AGE;
        /* remaining buffers */
        } else if (bp->b_flags & B_DELWRI)
                qindex = QUEUE_DIRTY;
        else
                qindex = QUEUE_CLEAN;

        if ((bp->b_flags & B_DELWRI) == 0 && (bp->b_xflags & BX_VNDIRTY))
                panic("brelse: not dirty");

        bp->b_flags &= ~(B_ASYNC | B_NOCACHE | B_RELBUF | B_DIRECT);
        bp->b_xflags &= ~(BX_CVTENXIO);
        /* binsfree unlocks bp. */
        binsfree(bp, qindex);
}

/*
 * Release a buffer back to the appropriate queue but do not try to free
 * it.  The buffer is expected to be used again soon.
 *
 * bqrelse() is used by bdwrite() to requeue a delayed write, and used by
 * biodone() to requeue an async I/O on completion.  It is also used when
 * known good buffers need to be requeued but we think we may need the data
 * again soon.
 *
 * XXX we should be able to leave the B_RELBUF hint set on completion.
 */
void
bqrelse(struct buf *bp)
{
        int qindex;

        CTR3(KTR_BUF, "bqrelse(%p) vp %p flags %X", bp, bp->b_vp, bp->b_flags);
        KASSERT(!(bp->b_flags & (B_CLUSTER|B_PAGING)),
            ("bqrelse: inappropriate B_PAGING or B_CLUSTER bp %p", bp));

        qindex = QUEUE_NONE;
        if (BUF_LOCKRECURSED(bp)) {
                /* do not release to free list */
                BUF_UNLOCK(bp);
                return;
        }
        bp->b_flags &= ~(B_ASYNC | B_NOCACHE | B_AGE | B_RELBUF);
        bp->b_xflags &= ~(BX_CVTENXIO);

        if (LIST_EMPTY(&bp->b_dep)) {
                bp->b_flags &= ~B_IOSTARTED;
        } else {
                KASSERT((bp->b_flags & B_IOSTARTED) == 0,
                    ("bqrelse: SU io not finished bp %p", bp));
        }

        if (bp->b_flags & B_MANAGED) {
                if (bp->b_flags & B_REMFREE)
                        bremfreef(bp);
                goto out;
        }

        /* buffers with stale but valid contents */
        if ((bp->b_flags & B_DELWRI) != 0 || (bp->b_vflags & (BV_BKGRDINPROG |
            BV_BKGRDERR)) == BV_BKGRDERR) {
                BO_LOCK(bp->b_bufobj);
                bp->b_vflags &= ~BV_BKGRDERR;
                BO_UNLOCK(bp->b_bufobj);
                qindex = QUEUE_DIRTY;
        } else {
                if ((bp->b_flags & B_DELWRI) == 0 &&
                    (bp->b_xflags & BX_VNDIRTY))
                        panic("bqrelse: not dirty");
                if ((bp->b_flags & B_NOREUSE) != 0) {
                        brelse(bp);
                        return;
                }
                qindex = QUEUE_CLEAN;
        }
        buf_track(bp, __func__);
        /* binsfree unlocks bp. */
        binsfree(bp, qindex);
        return;

out:
        buf_track(bp, __func__);
        /* unlock */
        BUF_UNLOCK(bp);
}

/*
 * Complete I/O to a VMIO backed page.  Validate the pages as appropriate,
 * restore bogus pages.
 */
static void
vfs_vmio_iodone(struct buf *bp)
{
        vm_ooffset_t foff;
        vm_page_t m;
        vm_object_t obj;
        struct vnode *vp __unused;
        int i, iosize, resid;
        bool bogus;

        obj = bp->b_bufobj->bo_object;
        KASSERT(blockcount_read(&obj->paging_in_progress) >= bp->b_npages,
            ("vfs_vmio_iodone: paging in progress(%d) < b_npages(%d)",
            blockcount_read(&obj->paging_in_progress), bp->b_npages));

        vp = bp->b_vp;
        VNPASS(vp->v_holdcnt > 0, vp);
        VNPASS(vp->v_object != NULL, vp);

        foff = bp->b_offset;
        KASSERT(bp->b_offset != NOOFFSET,
            ("vfs_vmio_iodone: bp %p has no buffer offset", bp));

        bogus = false;
        iosize = bp->b_bcount - bp->b_resid;
        for (i = 0; i < bp->b_npages; i++) {
                resid = ((foff + PAGE_SIZE) & ~(off_t)PAGE_MASK) - foff;
                if (resid > iosize)
                        resid = iosize;

                /*
                 * cleanup bogus pages, restoring the originals
                 */
                m = bp->b_pages[i];
                if (m == bogus_page) {
                        bogus = true;
                        m = vm_page_relookup(obj, OFF_TO_IDX(foff));
                        if (m == NULL)
                                panic("biodone: page disappeared!");
                        bp->b_pages[i] = m;
                } else if ((bp->b_iocmd == BIO_READ) && resid > 0) {
                        /*
                         * In the write case, the valid and clean bits are
                         * already changed correctly ( see bdwrite() ), so we 
                         * only need to do this here in the read case.
                         */
                        KASSERT((m->dirty & vm_page_bits(foff & PAGE_MASK,
                            resid)) == 0, ("vfs_vmio_iodone: page %p "
                            "has unexpected dirty bits", m));
                        vfs_page_set_valid(bp, foff, m);
                }
                KASSERT(OFF_TO_IDX(foff) == m->pindex,
                    ("vfs_vmio_iodone: foff(%jd)/pindex(%ju) mismatch",
                    (intmax_t)foff, (uintmax_t)m->pindex));

                vm_page_sunbusy(m);
                foff = (foff + PAGE_SIZE) & ~(off_t)PAGE_MASK;
                iosize -= resid;
        }
        vm_object_pip_wakeupn(obj, bp->b_npages);
        if (bogus && buf_mapped(bp)) {
                BUF_CHECK_MAPPED(bp);
                pmap_qenter(trunc_page((vm_offset_t)bp->b_data),
                    bp->b_pages, bp->b_npages);
        }
}

/*
 * Perform page invalidation when a buffer is released.  The fully invalid
 * pages will be reclaimed later in vfs_vmio_truncate().
 */
static void
vfs_vmio_invalidate(struct buf *bp)
{
        vm_object_t obj;
        vm_page_t m;
        int flags, i, resid, poffset, presid;

        if (buf_mapped(bp)) {
                BUF_CHECK_MAPPED(bp);
                pmap_qremove(trunc_page((vm_offset_t)bp->b_data), bp->b_npages);
        } else
                BUF_CHECK_UNMAPPED(bp);
        /*
         * Get the base offset and length of the buffer.  Note that 
         * in the VMIO case if the buffer block size is not
         * page-aligned then b_data pointer may not be page-aligned.
         * But our b_pages[] array *IS* page aligned.
         *
         * block sizes less then DEV_BSIZE (usually 512) are not 
         * supported due to the page granularity bits (m->valid,
         * m->dirty, etc...). 
         *
         * See man buf(9) for more information
         */
        flags = (bp->b_flags & B_NOREUSE) != 0 ? VPR_NOREUSE : 0;
        obj = bp->b_bufobj->bo_object;
        resid = bp->b_bufsize;
        poffset = bp->b_offset & PAGE_MASK;
        VM_OBJECT_WLOCK(obj);
        for (i = 0; i < bp->b_npages; i++) {
                m = bp->b_pages[i];
                if (m == bogus_page)
                        panic("vfs_vmio_invalidate: Unexpected bogus page.");
                bp->b_pages[i] = NULL;

                presid = resid > (PAGE_SIZE - poffset) ?
                    (PAGE_SIZE - poffset) : resid;
                KASSERT(presid >= 0, ("brelse: extra page"));
                vm_page_busy_acquire(m, VM_ALLOC_SBUSY);
                if (pmap_page_wired_mappings(m) == 0)
                        vm_page_set_invalid(m, poffset, presid);
                vm_page_sunbusy(m);
                vm_page_release_locked(m, flags);
                resid -= presid;
                poffset = 0;
        }
        VM_OBJECT_WUNLOCK(obj);
        bp->b_npages = 0;
}

/*
 * Page-granular truncation of an existing VMIO buffer.
 */
static void
vfs_vmio_truncate(struct buf *bp, int desiredpages)
{
        vm_object_t obj;
        vm_page_t m;
        int flags, i;

        if (bp->b_npages == desiredpages)
                return;

        if (buf_mapped(bp)) {
                BUF_CHECK_MAPPED(bp);
                pmap_qremove((vm_offset_t)trunc_page((vm_offset_t)bp->b_data) +
                    (desiredpages << PAGE_SHIFT), bp->b_npages - desiredpages);
        } else
                BUF_CHECK_UNMAPPED(bp);

        /*
         * The object lock is needed only if we will attempt to free pages.
         */
        flags = (bp->b_flags & B_NOREUSE) != 0 ? VPR_NOREUSE : 0;
        if ((bp->b_flags & B_DIRECT) != 0) {
                flags |= VPR_TRYFREE;
                obj = bp->b_bufobj->bo_object;
                VM_OBJECT_WLOCK(obj);
        } else {
                obj = NULL;
        }
        for (i = desiredpages; i < bp->b_npages; i++) {
                m = bp->b_pages[i];
                KASSERT(m != bogus_page, ("allocbuf: bogus page found"));
                bp->b_pages[i] = NULL;
                if (obj != NULL)
                        vm_page_release_locked(m, flags);
                else
                        vm_page_release(m, flags);
        }
        if (obj != NULL)
                VM_OBJECT_WUNLOCK(obj);
        bp->b_npages = desiredpages;
}

/*
 * Byte granular extension of VMIO buffers.
 */
static void
vfs_vmio_extend(struct buf *bp, int desiredpages, int size)
{
        /*
         * We are growing the buffer, possibly in a 
         * byte-granular fashion.
         */
        vm_object_t obj;
        vm_offset_t toff;
        vm_offset_t tinc;
        vm_page_t m;

        /*
         * Step 1, bring in the VM pages from the object, allocating
         * them if necessary.  We must clear B_CACHE if these pages
         * are not valid for the range covered by the buffer.
         */
        obj = bp->b_bufobj->bo_object;
        if (bp->b_npages < desiredpages) {
                KASSERT(desiredpages <= atop(maxbcachebuf),
                    ("vfs_vmio_extend past maxbcachebuf %p %d %u",
                    bp, desiredpages, maxbcachebuf));

                /*
                 * We must allocate system pages since blocking
                 * here could interfere with paging I/O, no
                 * matter which process we are.
                 *
                 * Only exclusive busy can be tested here.
                 * Blocking on shared busy might lead to
                 * deadlocks once allocbuf() is called after
                 * pages are vfs_busy_pages().
                 */
                (void)vm_page_grab_pages_unlocked(obj,
                    OFF_TO_IDX(bp->b_offset) + bp->b_npages,
                    VM_ALLOC_SYSTEM | VM_ALLOC_IGN_SBUSY |
                    VM_ALLOC_NOBUSY | VM_ALLOC_WIRED,
                    &bp->b_pages[bp->b_npages], desiredpages - bp->b_npages);
                bp->b_npages = desiredpages;
        }

        /*
         * Step 2.  We've loaded the pages into the buffer,
         * we have to figure out if we can still have B_CACHE
         * set.  Note that B_CACHE is set according to the
         * byte-granular range ( bcount and size ), not the
         * aligned range ( newbsize ).
         *
         * The VM test is against m->valid, which is DEV_BSIZE
         * aligned.  Needless to say, the validity of the data
         * needs to also be DEV_BSIZE aligned.  Note that this
         * fails with NFS if the server or some other client
         * extends the file's EOF.  If our buffer is resized, 
         * B_CACHE may remain set! XXX
         */
        toff = bp->b_bcount;
        tinc = PAGE_SIZE - ((bp->b_offset + toff) & PAGE_MASK);
        while ((bp->b_flags & B_CACHE) && toff < size) {
                vm_pindex_t pi;

                if (tinc > (size - toff))
                        tinc = size - toff;
                pi = ((bp->b_offset & PAGE_MASK) + toff) >> PAGE_SHIFT;
                m = bp->b_pages[pi];
                vfs_buf_test_cache(bp, bp->b_offset, toff, tinc, m);
                toff += tinc;
                tinc = PAGE_SIZE;
        }

        /*
         * Step 3, fixup the KVA pmap.
         */
        if (buf_mapped(bp))
                bpmap_qenter(bp);
        else
                BUF_CHECK_UNMAPPED(bp);
}

/*
 * Check to see if a block at a particular lbn is available for a clustered
 * write.
 */
static int
vfs_bio_clcheck(struct vnode *vp, int size, daddr_t lblkno, daddr_t blkno)
{
        struct buf *bpa;
        int match;

        match = 0;

        /* If the buf isn't in core skip it */
        if ((bpa = gbincore(&vp->v_bufobj, lblkno)) == NULL)
                return (0);

        /* If the buf is busy we don't want to wait for it */
        if (BUF_LOCK(bpa, LK_EXCLUSIVE | LK_NOWAIT, NULL) != 0)
                return (0);

        /* Only cluster with valid clusterable delayed write buffers */
        if ((bpa->b_flags & (B_DELWRI | B_CLUSTEROK | B_INVAL)) !=
            (B_DELWRI | B_CLUSTEROK))
                goto done;

        if (bpa->b_bufsize != size)
                goto done;

        /*
         * Check to see if it is in the expected place on disk and that the
         * block has been mapped.
         */
        if ((bpa->b_blkno != bpa->b_lblkno) && (bpa->b_blkno == blkno))
                match = 1;
done:
        BUF_UNLOCK(bpa);
        return (match);
}

/*
 *      vfs_bio_awrite:
 *
 *      Implement clustered async writes for clearing out B_DELWRI buffers.
 *      This is much better then the old way of writing only one buffer at
 *      a time.  Note that we may not be presented with the buffers in the 
 *      correct order, so we search for the cluster in both directions.
 */
int
vfs_bio_awrite(struct buf *bp)
{
        struct bufobj *bo;
        int i;
        int j;
        daddr_t lblkno = bp->b_lblkno;
        struct vnode *vp = bp->b_vp;
        int ncl;
        int nwritten;
        int size;
        int maxcl;
        int gbflags;

        bo = &vp->v_bufobj;
        gbflags = (bp->b_data == unmapped_buf) ? GB_UNMAPPED : 0;
        /*
         * right now we support clustered writing only to regular files.  If
         * we find a clusterable block we could be in the middle of a cluster
         * rather then at the beginning.
         */
        if ((vp->v_type == VREG) && 
            (vp->v_mount != 0) && /* Only on nodes that have the size info */
            (bp->b_flags & (B_CLUSTEROK | B_INVAL)) == B_CLUSTEROK) {
                size = vp->v_mount->mnt_stat.f_iosize;
                maxcl = maxphys / size;

                BO_RLOCK(bo);
                for (i = 1; i < maxcl; i++)
                        if (vfs_bio_clcheck(vp, size, lblkno + i,
                            bp->b_blkno + ((i * size) >> DEV_BSHIFT)) == 0)
                                break;

                for (j = 1; i + j <= maxcl && j <= lblkno; j++) 
                        if (vfs_bio_clcheck(vp, size, lblkno - j,
                            bp->b_blkno - ((j * size) >> DEV_BSHIFT)) == 0)
                                break;
                BO_RUNLOCK(bo);
                --j;
                ncl = i + j;
                /*
                 * this is a possible cluster write
                 */
                if (ncl != 1) {
                        BUF_UNLOCK(bp);
                        nwritten = cluster_wbuild(vp, size, lblkno - j, ncl,
                            gbflags);
                        return (nwritten);
                }
        }
        bremfree(bp);
        bp->b_flags |= B_ASYNC;
        /*
         * default (old) behavior, writing out only one block
         *
         * XXX returns b_bufsize instead of b_bcount for nwritten?
         */
        nwritten = bp->b_bufsize;
        (void) bwrite(bp);

        return (nwritten);
}

/*
 *      getnewbuf_kva:
 *
 *      Allocate KVA for an empty buf header according to gbflags.
 */
static int
getnewbuf_kva(struct buf *bp, int gbflags, int maxsize)
{

        if ((gbflags & (GB_UNMAPPED | GB_KVAALLOC)) != GB_UNMAPPED) {
                /*
                 * In order to keep fragmentation sane we only allocate kva
                 * in BKVASIZE chunks.  XXX with vmem we can do page size.
                 */
                maxsize = (maxsize + BKVAMASK) & ~BKVAMASK;

                if (maxsize != bp->b_kvasize &&
                    bufkva_alloc(bp, maxsize, gbflags))
                        return (ENOSPC);
        }
        return (0);
}

/*
 *      getnewbuf:
 *
 *      Find and initialize a new buffer header, freeing up existing buffers
 *      in the bufqueues as necessary.  The new buffer is returned locked.
 *
 *      We block if:
 *              We have insufficient buffer headers
 *              We have insufficient buffer space
 *              buffer_arena is too fragmented ( space reservation fails )
 *              If we have to flush dirty buffers ( but we try to avoid this )
 *
 *      The caller is responsible for releasing the reserved bufspace after
 *      allocbuf() is called.
 */
static struct buf *
getnewbuf(struct vnode *vp, int slpflag, int slptimeo, int maxsize, int gbflags)
{
        struct bufdomain *bd;
        struct buf *bp;
        bool metadata, reserved;

        bp = NULL;
        KASSERT((gbflags & (GB_UNMAPPED | GB_KVAALLOC)) != GB_KVAALLOC,
            ("GB_KVAALLOC only makes sense with GB_UNMAPPED"));
        if (!unmapped_buf_allowed)
                gbflags &= ~(GB_UNMAPPED | GB_KVAALLOC);

        if (vp == NULL || (vp->v_vflag & (VV_MD | VV_SYSTEM)) != 0 ||
            vp->v_type == VCHR)
                metadata = true;
        else
                metadata = false;
        if (vp == NULL)
                bd = &bdomain[0];
        else
                bd = &bdomain[vp->v_bufobj.bo_domain];

        counter_u64_add(getnewbufcalls, 1);
        reserved = false;
        do {
                if (reserved == false &&
                    bufspace_reserve(bd, maxsize, metadata) != 0) {
                        counter_u64_add(getnewbufrestarts, 1);
                        continue;
                }
                reserved = true;
                if ((bp = buf_alloc(bd)) == NULL) {
                        counter_u64_add(getnewbufrestarts, 1);
                        continue;
                }
                if (getnewbuf_kva(bp, gbflags, maxsize) == 0)
                        return (bp);
                break;
        } while (buf_recycle(bd, false) == 0);

        if (reserved)
                bufspace_release(bd, maxsize);
        if (bp != NULL) {
                bp->b_flags |= B_INVAL;
                brelse(bp);
        }
        bufspace_wait(bd, vp, gbflags, slpflag, slptimeo);

        return (NULL);
}

/*
 *      buf_daemon:
 *
 *      buffer flushing daemon.  Buffers are normally flushed by the
 *      update daemon but if it cannot keep up this process starts to
 *      take the load in an attempt to prevent getnewbuf() from blocking.
 */
static struct kproc_desc buf_kp = {
        "bufdaemon",
        buf_daemon,
        &bufdaemonproc
};
SYSINIT(bufdaemon, SI_SUB_KTHREAD_BUF, SI_ORDER_FIRST, kproc_start, &buf_kp);

static int
buf_flush(struct vnode *vp, struct bufdomain *bd, int target)
{
        int flushed;

        flushed = flushbufqueues(vp, bd, target, 0);
        if (flushed == 0) {
                /*
                 * Could not find any buffers without rollback
                 * dependencies, so just write the first one
                 * in the hopes of eventually making progress.
                 */
                if (vp != NULL && target > 2)
                        target /= 2;
                flushbufqueues(vp, bd, target, 1);
        }
        return (flushed);
}

static void
buf_daemon()
{
        struct bufdomain *bd;
        int speedupreq;
        int lodirty;
        int i;

        /*
         * This process needs to be suspended prior to shutdown sync.
         */
        EVENTHANDLER_REGISTER(shutdown_pre_sync, kthread_shutdown, curthread,
            SHUTDOWN_PRI_LAST + 100);

        /*
         * Start the buf clean daemons as children threads.
         */
        for (i = 0 ; i < buf_domains; i++) {
                int error;

                error = kthread_add((void (*)(void *))bufspace_daemon,
                    &bdomain[i], curproc, NULL, 0, 0, "bufspacedaemon-%d", i);
                if (error)
                        panic("error %d spawning bufspace daemon", error);
        }

        /*
         * This process is allowed to take the buffer cache to the limit
         */
        curthread->td_pflags |= TDP_NORUNNINGBUF | TDP_BUFNEED;
        mtx_lock(&bdlock);
        for (;;) {
                bd_request = 0;
                mtx_unlock(&bdlock);

                kthread_suspend_check();

                /*
                 * Save speedupreq for this pass and reset to capture new
                 * requests.
                 */
                speedupreq = bd_speedupreq;
                bd_speedupreq = 0;

                /*
                 * Flush each domain sequentially according to its level and
                 * the speedup request.
                 */
                for (i = 0; i < buf_domains; i++) {
                        bd = &bdomain[i];
                        if (speedupreq)
                                lodirty = bd->bd_numdirtybuffers / 2;
                        else
                                lodirty = bd->bd_lodirtybuffers;
                        while (bd->bd_numdirtybuffers > lodirty) {
                                if (buf_flush(NULL, bd,
                                    bd->bd_numdirtybuffers - lodirty) == 0)
                                        break;
                                kern_yield(PRI_USER);
                        }
                }

                /*
                 * Only clear bd_request if we have reached our low water
                 * mark.  The buf_daemon normally waits 1 second and
                 * then incrementally flushes any dirty buffers that have
                 * built up, within reason.
                 *
                 * If we were unable to hit our low water mark and couldn't
                 * find any flushable buffers, we sleep for a short period
                 * to avoid endless loops on unlockable buffers.
                 */
                mtx_lock(&bdlock);
                if (!BIT_EMPTY(BUF_DOMAINS, &bdlodirty)) {
                        /*
                         * We reached our low water mark, reset the
                         * request and sleep until we are needed again.
                         * The sleep is just so the suspend code works.
                         */
                        bd_request = 0;
                        /*
                         * Do an extra wakeup in case dirty threshold
                         * changed via sysctl and the explicit transition
                         * out of shortfall was missed.
                         */
                        bdirtywakeup();
                        if (runningbufspace <= lorunningspace)
                                runningwakeup();
                        msleep(&bd_request, &bdlock, PVM, "psleep", hz);
                } else {
                        /*
                         * We couldn't find any flushable dirty buffers but
                         * still have too many dirty buffers, we
                         * have to sleep and try again.  (rare)
                         */
                        msleep(&bd_request, &bdlock, PVM, "qsleep", hz / 10);
                }
        }
}

/*
 *      flushbufqueues:
 *
 *      Try to flush a buffer in the dirty queue.  We must be careful to
 *      free up B_INVAL buffers instead of write them, which NFS is 
 *      particularly sensitive to.
 */
static int flushwithdeps = 0;
SYSCTL_INT(_vfs, OID_AUTO, flushwithdeps, CTLFLAG_RW | CTLFLAG_STATS,
    &flushwithdeps, 0,
    "Number of buffers flushed with dependecies that require rollbacks");

static int
flushbufqueues(struct vnode *lvp, struct bufdomain *bd, int target,
    int flushdeps)
{
        struct bufqueue *bq;
        struct buf *sentinel;
        struct vnode *vp;
        struct mount *mp;
        struct buf *bp;
        int hasdeps;
        int flushed;
        int error;
        bool unlock;

        flushed = 0;
        bq = &bd->bd_dirtyq;
        bp = NULL;
        sentinel = malloc(sizeof(struct buf), M_TEMP, M_WAITOK | M_ZERO);
        sentinel->b_qindex = QUEUE_SENTINEL;
        BQ_LOCK(bq);
        TAILQ_INSERT_HEAD(&bq->bq_queue, sentinel, b_freelist);
        BQ_UNLOCK(bq);
        while (flushed != target) {
                maybe_yield();
                BQ_LOCK(bq);
                bp = TAILQ_NEXT(sentinel, b_freelist);
                if (bp != NULL) {
                        TAILQ_REMOVE(&bq->bq_queue, sentinel, b_freelist);
                        TAILQ_INSERT_AFTER(&bq->bq_queue, bp, sentinel,
                            b_freelist);
                } else {
                        BQ_UNLOCK(bq);
                        break;
                }
                /*
                 * Skip sentinels inserted by other invocations of the
                 * flushbufqueues(), taking care to not reorder them.
                 *
                 * Only flush the buffers that belong to the
                 * vnode locked by the curthread.
                 */
                if (bp->b_qindex == QUEUE_SENTINEL || (lvp != NULL &&
                    bp->b_vp != lvp)) {
                        BQ_UNLOCK(bq);
                        continue;
                }
                error = BUF_LOCK(bp, LK_EXCLUSIVE | LK_NOWAIT, NULL);
                BQ_UNLOCK(bq);
                if (error != 0)
                        continue;

                /*
                 * BKGRDINPROG can only be set with the buf and bufobj
                 * locks both held.  We tolerate a race to clear it here.
                 */
                if ((bp->b_vflags & BV_BKGRDINPROG) != 0 ||
                    (bp->b_flags & B_DELWRI) == 0) {
                        BUF_UNLOCK(bp);
                        continue;
                }
                if (bp->b_flags & B_INVAL) {
                        bremfreef(bp);
                        brelse(bp);
                        flushed++;
                        continue;
                }

                if (!LIST_EMPTY(&bp->b_dep) && buf_countdeps(bp, 0)) {
                        if (flushdeps == 0) {
                                BUF_UNLOCK(bp);
                                continue;
                        }
                        hasdeps = 1;
                } else
                        hasdeps = 0;
                /*
                 * We must hold the lock on a vnode before writing
                 * one of its buffers. Otherwise we may confuse, or
                 * in the case of a snapshot vnode, deadlock the
                 * system.
                 *
                 * The lock order here is the reverse of the normal
                 * of vnode followed by buf lock.  This is ok because
                 * the NOWAIT will prevent deadlock.
                 */
                vp = bp->b_vp;
                if (vn_start_write(vp, &mp, V_NOWAIT) != 0) {
                        BUF_UNLOCK(bp);
                        continue;
                }
                if (lvp == NULL) {
                        unlock = true;
                        error = vn_lock(vp, LK_EXCLUSIVE | LK_NOWAIT);
                } else {
                        ASSERT_VOP_LOCKED(vp, "getbuf");
                        unlock = false;
                        error = VOP_ISLOCKED(vp) == LK_EXCLUSIVE ? 0 :
                            vn_lock(vp, LK_TRYUPGRADE);
                }
                if (error == 0) {
                        CTR3(KTR_BUF, "flushbufqueue(%p) vp %p flags %X",
                            bp, bp->b_vp, bp->b_flags);
                        if (curproc == bufdaemonproc) {
                                vfs_bio_awrite(bp);
                        } else {
                                bremfree(bp);
                                bwrite(bp);
                                counter_u64_add(notbufdflushes, 1);
                        }
                        vn_finished_write(mp);
                        if (unlock)
                                VOP_UNLOCK(vp);
                        flushwithdeps += hasdeps;
                        flushed++;

                        /*
                         * Sleeping on runningbufspace while holding
                         * vnode lock leads to deadlock.
                         */
                        if (curproc == bufdaemonproc &&
                            runningbufspace > hirunningspace)
                                waitrunningbufspace();
                        continue;
                }
                vn_finished_write(mp);
                BUF_UNLOCK(bp);
        }
        BQ_LOCK(bq);
        TAILQ_REMOVE(&bq->bq_queue, sentinel, b_freelist);
        BQ_UNLOCK(bq);
        free(sentinel, M_TEMP);
        return (flushed);
}

/*
 * Check to see if a block is currently memory resident.
 */
struct buf *
incore(struct bufobj *bo, daddr_t blkno)
{
        return (gbincore_unlocked(bo, blkno));
}

/*
 * Returns true if no I/O is needed to access the
 * associated VM object.  This is like incore except
 * it also hunts around in the VM system for the data.
 */
bool
inmem(struct vnode * vp, daddr_t blkno)
{
        vm_object_t obj;
        vm_offset_t toff, tinc, size;
        vm_page_t m, n;
        vm_ooffset_t off;
        int valid;

        ASSERT_VOP_LOCKED(vp, "inmem");

        if (incore(&vp->v_bufobj, blkno))
                return (true);
        if (vp->v_mount == NULL)
                return (false);
        obj = vp->v_object;
        if (obj == NULL)
                return (false);

        size = PAGE_SIZE;
        if (size > vp->v_mount->mnt_stat.f_iosize)
                size = vp->v_mount->mnt_stat.f_iosize;
        off = (vm_ooffset_t)blkno * (vm_ooffset_t)vp->v_mount->mnt_stat.f_iosize;

        for (toff = 0; toff < vp->v_mount->mnt_stat.f_iosize; toff += tinc) {
                m = vm_page_lookup_unlocked(obj, OFF_TO_IDX(off + toff));
recheck:
                if (m == NULL)
                        return (false);

                tinc = size;
                if (tinc > PAGE_SIZE - ((toff + off) & PAGE_MASK))
                        tinc = PAGE_SIZE - ((toff + off) & PAGE_MASK);
                /*
                 * Consider page validity only if page mapping didn't change
                 * during the check.
                 */
                valid = vm_page_is_valid(m,
                    (vm_offset_t)((toff + off) & PAGE_MASK), tinc);
                n = vm_page_lookup_unlocked(obj, OFF_TO_IDX(off + toff));
                if (m != n) {
                        m = n;
                        goto recheck;
                }
                if (!valid)
                        return (false);
        }
        return (true);
}

/*
 * Set the dirty range for a buffer based on the status of the dirty
 * bits in the pages comprising the buffer.  The range is limited
 * to the size of the buffer.
 *
 * Tell the VM system that the pages associated with this buffer
 * are clean.  This is used for delayed writes where the data is
 * going to go to disk eventually without additional VM intevention.
 *
 * Note that while we only really need to clean through to b_bcount, we
 * just go ahead and clean through to b_bufsize.
 */
static void
vfs_clean_pages_dirty_buf(struct buf *bp)
{
        vm_ooffset_t foff, noff, eoff;
        vm_page_t m;
        int i;

        if ((bp->b_flags & B_VMIO) == 0 || bp->b_bufsize == 0)
                return;

        foff = bp->b_offset;
        KASSERT(bp->b_offset != NOOFFSET,
            ("vfs_clean_pages_dirty_buf: no buffer offset"));

        vfs_busy_pages_acquire(bp);
        vfs_setdirty_range(bp);
        for (i = 0; i < bp->b_npages; i++) {
                noff = (foff + PAGE_SIZE) & ~(off_t)PAGE_MASK;
                eoff = noff;
                if (eoff > bp->b_offset + bp->b_bufsize)
                        eoff = bp->b_offset + bp->b_bufsize;
                m = bp->b_pages[i];
                vfs_page_set_validclean(bp, foff, m);
                /* vm_page_clear_dirty(m, foff & PAGE_MASK, eoff - foff); */
                foff = noff;
        }
        vfs_busy_pages_release(bp);
}

static void
vfs_setdirty_range(struct buf *bp)
{
        vm_offset_t boffset;
        vm_offset_t eoffset;
        int i;

        /*
         * test the pages to see if they have been modified directly
         * by users through the VM system.
         */
        for (i = 0; i < bp->b_npages; i++)
                vm_page_test_dirty(bp->b_pages[i]);

        /*
         * Calculate the encompassing dirty range, boffset and eoffset,
         * (eoffset - boffset) bytes.
         */

        for (i = 0; i < bp->b_npages; i++) {
                if (bp->b_pages[i]->dirty)
                        break;
        }
        boffset = (i << PAGE_SHIFT) - (bp->b_offset & PAGE_MASK);

        for (i = bp->b_npages - 1; i >= 0; --i) {
                if (bp->b_pages[i]->dirty) {
                        break;
                }
        }
        eoffset = ((i + 1) << PAGE_SHIFT) - (bp->b_offset & PAGE_MASK);

        /*
         * Fit it to the buffer.
         */

        if (eoffset > bp->b_bcount)
                eoffset = bp->b_bcount;

        /*
         * If we have a good dirty range, merge with the existing
         * dirty range.
         */

        if (boffset < eoffset) {
                if (bp->b_dirtyoff > boffset)
                        bp->b_dirtyoff = boffset;
                if (bp->b_dirtyend < eoffset)
                        bp->b_dirtyend = eoffset;
        }
}

/*
 * Allocate the KVA mapping for an existing buffer.
 * If an unmapped buffer is provided but a mapped buffer is requested, take
 * also care to properly setup mappings between pages and KVA.
 */
static void
bp_unmapped_get_kva(struct buf *bp, daddr_t blkno, int size, int gbflags)
{
        int bsize, maxsize, need_mapping, need_kva;
        off_t offset;

        need_mapping = bp->b_data == unmapped_buf &&
            (gbflags & GB_UNMAPPED) == 0;
        need_kva = bp->b_kvabase == unmapped_buf &&
            bp->b_data == unmapped_buf &&
            (gbflags & GB_KVAALLOC) != 0;
        if (!need_mapping && !need_kva)
                return;

        BUF_CHECK_UNMAPPED(bp);

        if (need_mapping && bp->b_kvabase != unmapped_buf) {
                /*
                 * Buffer is not mapped, but the KVA was already
                 * reserved at the time of the instantiation.  Use the
                 * allocated space.
                 */
                goto has_addr;
        }

        /*
         * Calculate the amount of the address space we would reserve
         * if the buffer was mapped.
         */
        bsize = vn_isdisk(bp->b_vp) ? DEV_BSIZE : bp->b_bufobj->bo_bsize;
        KASSERT(bsize != 0, ("bsize == 0, check bo->bo_bsize"));
        offset = blkno * bsize;
        maxsize = size + (offset & PAGE_MASK);
        maxsize = imax(maxsize, bsize);

        while (bufkva_alloc(bp, maxsize, gbflags) != 0) {
                if ((gbflags & GB_NOWAIT_BD) != 0) {
                        /*
                         * XXXKIB: defragmentation cannot
                         * succeed, not sure what else to do.
                         */
                        panic("GB_NOWAIT_BD and GB_UNMAPPED %p", bp);
                }
                counter_u64_add(mappingrestarts, 1);
                bufspace_wait(bufdomain(bp), bp->b_vp, gbflags, 0, 0);
        }
has_addr:
        if (need_mapping) {
                /* b_offset is handled by bpmap_qenter. */
                bp->b_data = bp->b_kvabase;
                BUF_CHECK_MAPPED(bp);
                bpmap_qenter(bp);
        }
}

struct buf *
getblk(struct vnode *vp, daddr_t blkno, int size, int slpflag, int slptimeo,
    int flags)
{
        struct buf *bp;
        int error;

        error = getblkx(vp, blkno, blkno, size, slpflag, slptimeo, flags, &bp);
        if (error != 0)
                return (NULL);
        return (bp);
}

/*
 *      getblkx:
 *
 *      Get a block given a specified block and offset into a file/device.
 *      The buffers B_DONE bit will be cleared on return, making it almost
 *      ready for an I/O initiation.  B_INVAL may or may not be set on 
 *      return.  The caller should clear B_INVAL prior to initiating a
 *      READ.
 *
 *      For a non-VMIO buffer, B_CACHE is set to the opposite of B_INVAL for
 *      an existing buffer.
 *
 *      For a VMIO buffer, B_CACHE is modified according to the backing VM.
 *      If getblk()ing a previously 0-sized invalid buffer, B_CACHE is set
 *      and then cleared based on the backing VM.  If the previous buffer is
 *      non-0-sized but invalid, B_CACHE will be cleared.
 *
 *      If getblk() must create a new buffer, the new buffer is returned with
 *      both B_INVAL and B_CACHE clear unless it is a VMIO buffer, in which
 *      case it is returned with B_INVAL clear and B_CACHE set based on the
 *      backing VM.
 *
 *      getblk() also forces a bwrite() for any B_DELWRI buffer whose
 *      B_CACHE bit is clear.
 *
 *      What this means, basically, is that the caller should use B_CACHE to
 *      determine whether the buffer is fully valid or not and should clear
 *      B_INVAL prior to issuing a read.  If the caller intends to validate
 *      the buffer by loading its data area with something, the caller needs
 *      to clear B_INVAL.  If the caller does this without issuing an I/O, 
 *      the caller should set B_CACHE ( as an optimization ), else the caller
 *      should issue the I/O and biodone() will set B_CACHE if the I/O was
 *      a write attempt or if it was a successful read.  If the caller 
 *      intends to issue a READ, the caller must clear B_INVAL and BIO_ERROR
 *      prior to issuing the READ.  biodone() will *not* clear B_INVAL.
 *
 *      The blkno parameter is the logical block being requested. Normally
 *      the mapping of logical block number to disk block address is done
 *      by calling VOP_BMAP(). However, if the mapping is already known, the
 *      disk block address can be passed using the dblkno parameter. If the
 *      disk block address is not known, then the same value should be passed
 *      for blkno and dblkno.
 */
int
getblkx(struct vnode *vp, daddr_t blkno, daddr_t dblkno, int size, int slpflag,
    int slptimeo, int flags, struct buf **bpp)
{
        struct buf *bp;
        struct bufobj *bo;
        daddr_t d_blkno;
        int bsize, error, maxsize, vmio;
        off_t offset;

        CTR3(KTR_BUF, "getblk(%p, %ld, %d)", vp, (long)blkno, size);
        KASSERT((flags & (GB_UNMAPPED | GB_KVAALLOC)) != GB_KVAALLOC,
            ("GB_KVAALLOC only makes sense with GB_UNMAPPED"));
        ASSERT_VOP_LOCKED(vp, "getblk");
        if (size > maxbcachebuf)
                panic("getblk: size(%d) > maxbcachebuf(%d)\n", size,
                    maxbcachebuf);
        if (!unmapped_buf_allowed)
                flags &= ~(GB_UNMAPPED | GB_KVAALLOC);

        bo = &vp->v_bufobj;
        d_blkno = dblkno;

        /* Attempt lockless lookup first. */
        bp = gbincore_unlocked(bo, blkno);
        if (bp == NULL) {
                /*
                 * With GB_NOCREAT we must be sure about not finding the buffer
                 * as it may have been reassigned during unlocked lookup.
                 */
                if ((flags & GB_NOCREAT) != 0)
                        goto loop;
                goto newbuf_unlocked;
        }

        error = BUF_TIMELOCK(bp, LK_EXCLUSIVE | LK_NOWAIT, NULL, "getblku", 0,
            0);
        if (error != 0)
                goto loop;

        /* Verify buf identify has not changed since lookup. */
        if (bp->b_bufobj == bo && bp->b_lblkno == blkno)
                goto foundbuf_fastpath;

        /* It changed, fallback to locked lookup. */
        BUF_UNLOCK_RAW(bp);

loop:
        BO_RLOCK(bo);
        bp = gbincore(bo, blkno);
        if (bp != NULL) {
                int lockflags;

                /*
                 * Buffer is in-core.  If the buffer is not busy nor managed,
                 * it must be on a queue.
                 */
                lockflags = LK_EXCLUSIVE | LK_INTERLOCK |
                    ((flags & GB_LOCK_NOWAIT) ? LK_NOWAIT : LK_SLEEPFAIL);

                error = BUF_TIMELOCK(bp, lockflags,
                    BO_LOCKPTR(bo), "getblk", slpflag, slptimeo);

                /*
                 * If we slept and got the lock we have to restart in case
                 * the buffer changed identities.
                 */
                if (error == ENOLCK)
                        goto loop;
                /* We timed out or were interrupted. */
                else if (error != 0)
                        return (error);

foundbuf_fastpath:
                /* If recursed, assume caller knows the rules. */
                if (BUF_LOCKRECURSED(bp))
                        goto end;

                /*
                 * The buffer is locked.  B_CACHE is cleared if the buffer is 
                 * invalid.  Otherwise, for a non-VMIO buffer, B_CACHE is set
                 * and for a VMIO buffer B_CACHE is adjusted according to the
                 * backing VM cache.
                 */
                if (bp->b_flags & B_INVAL)
                        bp->b_flags &= ~B_CACHE;
                else if ((bp->b_flags & (B_VMIO | B_INVAL)) == 0)
                        bp->b_flags |= B_CACHE;
                if (bp->b_flags & B_MANAGED)
                        MPASS(bp->b_qindex == QUEUE_NONE);
                else
                        bremfree(bp);

                /*
                 * check for size inconsistencies for non-VMIO case.
                 */
                if (bp->b_bcount != size) {
                        if ((bp->b_flags & B_VMIO) == 0 ||
                            (size > bp->b_kvasize)) {
                                if (bp->b_flags & B_DELWRI) {
                                        bp->b_flags |= B_NOCACHE;
                                        bwrite(bp);
                                } else {
                                        if (LIST_EMPTY(&bp->b_dep)) {
                                                bp->b_flags |= B_RELBUF;
                                                brelse(bp);
                                        } else {
                                                bp->b_flags |= B_NOCACHE;
                                                bwrite(bp);
                                        }
                                }
                                goto loop;
                        }
                }

                /*
                 * Handle the case of unmapped buffer which should
                 * become mapped, or the buffer for which KVA
                 * reservation is requested.
                 */
                bp_unmapped_get_kva(bp, blkno, size, flags);

                /*
                 * If the size is inconsistent in the VMIO case, we can resize
                 * the buffer.  This might lead to B_CACHE getting set or
                 * cleared.  If the size has not changed, B_CACHE remains
                 * unchanged from its previous state.
                 */
                allocbuf(bp, size);

                KASSERT(bp->b_offset != NOOFFSET, 
                    ("getblk: no buffer offset"));

                /*
                 * A buffer with B_DELWRI set and B_CACHE clear must
                 * be committed before we can return the buffer in
                 * order to prevent the caller from issuing a read
                 * ( due to B_CACHE not being set ) and overwriting
                 * it.
                 *
                 * Most callers, including NFS and FFS, need this to
                 * operate properly either because they assume they
                 * can issue a read if B_CACHE is not set, or because
                 * ( for example ) an uncached B_DELWRI might loop due 
                 * to softupdates re-dirtying the buffer.  In the latter
                 * case, B_CACHE is set after the first write completes,
                 * preventing further loops.
                 * NOTE!  b*write() sets B_CACHE.  If we cleared B_CACHE
                 * above while extending the buffer, we cannot allow the
                 * buffer to remain with B_CACHE set after the write
                 * completes or it will represent a corrupt state.  To
                 * deal with this we set B_NOCACHE to scrap the buffer
                 * after the write.
                 *
                 * We might be able to do something fancy, like setting
                 * B_CACHE in bwrite() except if B_DELWRI is already set,
                 * so the below call doesn't set B_CACHE, but that gets real
                 * confusing.  This is much easier.
                 */

                if ((bp->b_flags & (B_CACHE|B_DELWRI)) == B_DELWRI) {
                        bp->b_flags |= B_NOCACHE;
                        bwrite(bp);
                        goto loop;
                }
                bp->b_flags &= ~B_DONE;
        } else {
                /*
                 * Buffer is not in-core, create new buffer.  The buffer
                 * returned by getnewbuf() is locked.  Note that the returned
                 * buffer is also considered valid (not marked B_INVAL).
                 */
                BO_RUNLOCK(bo);
newbuf_unlocked:
                /*
                 * If the user does not want us to create the buffer, bail out
                 * here.
                 */
                if (flags & GB_NOCREAT)
                        return (EEXIST);

                bsize = vn_isdisk(vp) ? DEV_BSIZE : bo->bo_bsize;
                KASSERT(bsize != 0, ("bsize == 0, check bo->bo_bsize"));
                offset = blkno * bsize;
                vmio = vp->v_object != NULL;
                if (vmio) {
                        maxsize = size + (offset & PAGE_MASK);
                } else {
                        maxsize = size;
                        /* Do not allow non-VMIO notmapped buffers. */
                        flags &= ~(GB_UNMAPPED | GB_KVAALLOC);
                }
                maxsize = imax(maxsize, bsize);
                if ((flags & GB_NOSPARSE) != 0 && vmio &&
                    !vn_isdisk(vp)) {
                        error = VOP_BMAP(vp, blkno, NULL, &d_blkno, 0, 0);
                        KASSERT(error != EOPNOTSUPP,
                            ("GB_NOSPARSE from fs not supporting bmap, vp %p",
                            vp));
                        if (error != 0)
                                return (error);
                        if (d_blkno == -1)
                                return (EJUSTRETURN);
                }

                bp = getnewbuf(vp, slpflag, slptimeo, maxsize, flags);
                if (bp == NULL) {
                        if (slpflag || slptimeo)
                                return (ETIMEDOUT);
                        /*
                         * XXX This is here until the sleep path is diagnosed
                         * enough to work under very low memory conditions.
                         *
                         * There's an issue on low memory, 4BSD+non-preempt
                         * systems (eg MIPS routers with 32MB RAM) where buffer
                         * exhaustion occurs without sleeping for buffer
                         * reclaimation.  This just sticks in a loop and
                         * constantly attempts to allocate a buffer, which
                         * hits exhaustion and tries to wakeup bufdaemon.
                         * This never happens because we never yield.
                         *
                         * The real solution is to identify and fix these cases
                         * so we aren't effectively busy-waiting in a loop
                         * until the reclaimation path has cycles to run.
                         */
                        kern_yield(PRI_USER);
                        goto loop;
                }

                /*
                 * This code is used to make sure that a buffer is not
                 * created while the getnewbuf routine is blocked.
                 * This can be a problem whether the vnode is locked or not.
                 * If the buffer is created out from under us, we have to
                 * throw away the one we just created.
                 *
                 * Note: this must occur before we associate the buffer
                 * with the vp especially considering limitations in
                 * the splay tree implementation when dealing with duplicate
                 * lblkno's.
                 */
                BO_LOCK(bo);
                if (gbincore(bo, blkno)) {
                        BO_UNLOCK(bo);
                        bp->b_flags |= B_INVAL;
                        bufspace_release(bufdomain(bp), maxsize);
                        brelse(bp);
                        goto loop;
                }

                /*
                 * Insert the buffer into the hash, so that it can
                 * be found by incore.
                 */
                bp->b_lblkno = blkno;
                bp->b_blkno = d_blkno;
                bp->b_offset = offset;
                bgetvp(vp, bp);
                BO_UNLOCK(bo);

                /*
                 * set B_VMIO bit.  allocbuf() the buffer bigger.  Since the
                 * buffer size starts out as 0, B_CACHE will be set by
                 * allocbuf() for the VMIO case prior to it testing the
                 * backing store for validity.
                 */

                if (vmio) {
                        bp->b_flags |= B_VMIO;
                        KASSERT(vp->v_object == bp->b_bufobj->bo_object,
                            ("ARGH! different b_bufobj->bo_object %p %p %p\n",
                            bp, vp->v_object, bp->b_bufobj->bo_object));
                } else {
                        bp->b_flags &= ~B_VMIO;
                        KASSERT(bp->b_bufobj->bo_object == NULL,
                            ("ARGH! has b_bufobj->bo_object %p %p\n",
                            bp, bp->b_bufobj->bo_object));
                        BUF_CHECK_MAPPED(bp);
                }

                allocbuf(bp, size);
                bufspace_release(bufdomain(bp), maxsize);
                bp->b_flags &= ~B_DONE;
        }
        CTR4(KTR_BUF, "getblk(%p, %ld, %d) = %p", vp, (long)blkno, size, bp);
end:
        buf_track(bp, __func__);
        KASSERT(bp->b_bufobj == bo,
            ("bp %p wrong b_bufobj %p should be %p", bp, bp->b_bufobj, bo));
        *bpp = bp;
        return (0);
}

/*
 * Get an empty, disassociated buffer of given size.  The buffer is initially
 * set to B_INVAL.
 */
struct buf *
geteblk(int size, int flags)
{
        struct buf *bp;
        int maxsize;

        maxsize = (size + BKVAMASK) & ~BKVAMASK;
        while ((bp = getnewbuf(NULL, 0, 0, maxsize, flags)) == NULL) {
                if ((flags & GB_NOWAIT_BD) &&
                    (curthread->td_pflags & TDP_BUFNEED) != 0)
                        return (NULL);
        }
        allocbuf(bp, size);
        bufspace_release(bufdomain(bp), maxsize);
        bp->b_flags |= B_INVAL; /* b_dep cleared by getnewbuf() */
        return (bp);
}

/*
 * Truncate the backing store for a non-vmio buffer.
 */
static void
vfs_nonvmio_truncate(struct buf *bp, int newbsize)
{

        if (bp->b_flags & B_MALLOC) {
                /*
                 * malloced buffers are not shrunk
                 */
                if (newbsize == 0) {
                        bufmallocadjust(bp, 0);
                        free(bp->b_data, M_BIOBUF);
                        bp->b_data = bp->b_kvabase;
                        bp->b_flags &= ~B_MALLOC;
                }
                return;
        }
        vm_hold_free_pages(bp, newbsize);
        bufspace_adjust(bp, newbsize);
}

/*
 * Extend the backing for a non-VMIO buffer.
 */
static void
vfs_nonvmio_extend(struct buf *bp, int newbsize)
{
        caddr_t origbuf;
        int origbufsize;

        /*
         * We only use malloced memory on the first allocation.
         * and revert to page-allocated memory when the buffer
         * grows.
         *
         * There is a potential smp race here that could lead
         * to bufmallocspace slightly passing the max.  It
         * is probably extremely rare and not worth worrying
         * over.
         */
        if (bp->b_bufsize == 0 && newbsize <= PAGE_SIZE/2 &&
            bufmallocspace < maxbufmallocspace) {
                bp->b_data = malloc(newbsize, M_BIOBUF, M_WAITOK);
                bp->b_flags |= B_MALLOC;
                bufmallocadjust(bp, newbsize);
                return;
        }

        /*
         * If the buffer is growing on its other-than-first
         * allocation then we revert to the page-allocation
         * scheme.
         */
        origbuf = NULL;
        origbufsize = 0;
        if (bp->b_flags & B_MALLOC) {
                origbuf = bp->b_data;
                origbufsize = bp->b_bufsize;
                bp->b_data = bp->b_kvabase;
                bufmallocadjust(bp, 0);
                bp->b_flags &= ~B_MALLOC;
                newbsize = round_page(newbsize);
        }
        vm_hold_load_pages(bp, (vm_offset_t) bp->b_data + bp->b_bufsize,
            (vm_offset_t) bp->b_data + newbsize);
        if (origbuf != NULL) {
                bcopy(origbuf, bp->b_data, origbufsize);
                free(origbuf, M_BIOBUF);
        }
        bufspace_adjust(bp, newbsize);
}

/*
 * This code constitutes the buffer memory from either anonymous system
 * memory (in the case of non-VMIO operations) or from an associated
 * VM object (in the case of VMIO operations).  This code is able to
 * resize a buffer up or down.
 *
 * Note that this code is tricky, and has many complications to resolve
 * deadlock or inconsistent data situations.  Tread lightly!!! 
 * There are B_CACHE and B_DELWRI interactions that must be dealt with by 
 * the caller.  Calling this code willy nilly can result in the loss of data.
 *
 * allocbuf() only adjusts B_CACHE for VMIO buffers.  getblk() deals with
 * B_CACHE for the non-VMIO case.
 */
int
allocbuf(struct buf *bp, int size)
{
        int newbsize;

        if (bp->b_bcount == size)
                return (1);

        if (bp->b_kvasize != 0 && bp->b_kvasize < size)
                panic("allocbuf: buffer too small");

        newbsize = roundup2(size, DEV_BSIZE);
        if ((bp->b_flags & B_VMIO) == 0) {
                if ((bp->b_flags & B_MALLOC) == 0)
                        newbsize = round_page(newbsize);
                /*
                 * Just get anonymous memory from the kernel.  Don't
                 * mess with B_CACHE.
                 */
                if (newbsize < bp->b_bufsize)
                        vfs_nonvmio_truncate(bp, newbsize);
                else if (newbsize > bp->b_bufsize)
                        vfs_nonvmio_extend(bp, newbsize);
        } else {
                int desiredpages;

                desiredpages = (size == 0) ? 0 :
                    num_pages((bp->b_offset & PAGE_MASK) + newbsize);

                if (bp->b_flags & B_MALLOC)
                        panic("allocbuf: VMIO buffer can't be malloced");
                /*
                 * Set B_CACHE initially if buffer is 0 length or will become
                 * 0-length.
                 */
                if (size == 0 || bp->b_bufsize == 0)
                        bp->b_flags |= B_CACHE;

                if (newbsize < bp->b_bufsize)
                        vfs_vmio_truncate(bp, desiredpages);
                /* XXX This looks as if it should be newbsize > b_bufsize */
                else if (size > bp->b_bcount)
                        vfs_vmio_extend(bp, desiredpages, size);
                bufspace_adjust(bp, newbsize);
        }
        bp->b_bcount = size;            /* requested buffer size. */
        return (1);
}

extern int inflight_transient_maps;

static struct bio_queue nondump_bios;

void
biodone(struct bio *bp)
{
        struct mtx *mtxp;
        void (*done)(struct bio *);
        vm_offset_t start, end;

        biotrack(bp, __func__);

        /*
         * Avoid completing I/O when dumping after a panic since that may
         * result in a deadlock in the filesystem or pager code.  Note that
         * this doesn't affect dumps that were started manually since we aim
         * to keep the system usable after it has been resumed.
         */
        if (__predict_false(dumping && SCHEDULER_STOPPED())) {
                TAILQ_INSERT_HEAD(&nondump_bios, bp, bio_queue);
                return;
        }
        if ((bp->bio_flags & BIO_TRANSIENT_MAPPING) != 0) {
                bp->bio_flags &= ~BIO_TRANSIENT_MAPPING;
                bp->bio_flags |= BIO_UNMAPPED;
                start = trunc_page((vm_offset_t)bp->bio_data);
                end = round_page((vm_offset_t)bp->bio_data + bp->bio_length);
                bp->bio_data = unmapped_buf;
                pmap_qremove(start, atop(end - start));
                vmem_free(transient_arena, start, end - start);
                atomic_add_int(&inflight_transient_maps, -1);
        }
        done = bp->bio_done;
        if (done == NULL) {
                mtxp = mtx_pool_find(mtxpool_sleep, bp);
                mtx_lock(mtxp);
                bp->bio_flags |= BIO_DONE;
                wakeup(bp);
                mtx_unlock(mtxp);
        } else
                done(bp);
}

/*
 * Wait for a BIO to finish.
 */
int
biowait(struct bio *bp, const char *wchan)
{
        struct mtx *mtxp;

        mtxp = mtx_pool_find(mtxpool_sleep, bp);
        mtx_lock(mtxp);
        while ((bp->bio_flags & BIO_DONE) == 0)
                msleep(bp, mtxp, PRIBIO, wchan, 0);
        mtx_unlock(mtxp);
        if (bp->bio_error != 0)
                return (bp->bio_error);
        if (!(bp->bio_flags & BIO_ERROR))
                return (0);
        return (EIO);
}

void
biofinish(struct bio *bp, struct devstat *stat, int error)
{

        if (error) {
                bp->bio_error = error;
                bp->bio_flags |= BIO_ERROR;
        }
        if (stat != NULL)
                devstat_end_transaction_bio(stat, bp);
        biodone(bp);
}

#if defined(BUF_TRACKING) || defined(FULL_BUF_TRACKING)
void
biotrack_buf(struct bio *bp, const char *location)
{

        buf_track(bp->bio_track_bp, location);
}
#endif

/*
 *      bufwait:
 *
 *      Wait for buffer I/O completion, returning error status.  The buffer
 *      is left locked and B_DONE on return.  B_EINTR is converted into an EINTR
 *      error and cleared.
 */
int
bufwait(struct buf *bp)
{
        if (bp->b_iocmd == BIO_READ)
                bwait(bp, PRIBIO, "biord");
        else
                bwait(bp, PRIBIO, "biowr");
        if (bp->b_flags & B_EINTR) {
                bp->b_flags &= ~B_EINTR;
                return (EINTR);
        }
        if (bp->b_ioflags & BIO_ERROR) {
                return (bp->b_error ? bp->b_error : EIO);
        } else {
                return (0);
        }
}

/*
 *      bufdone:
 *
 *      Finish I/O on a buffer, optionally calling a completion function.
 *      This is usually called from an interrupt so process blocking is
 *      not allowed.
 *
 *      biodone is also responsible for setting B_CACHE in a B_VMIO bp.
 *      In a non-VMIO bp, B_CACHE will be set on the next getblk() 
 *      assuming B_INVAL is clear.
 *
 *      For the VMIO case, we set B_CACHE if the op was a read and no
 *      read error occurred, or if the op was a write.  B_CACHE is never
 *      set if the buffer is invalid or otherwise uncacheable.
 *
 *      bufdone does not mess with B_INVAL, allowing the I/O routine or the
 *      initiator to leave B_INVAL set to brelse the buffer out of existence
 *      in the biodone routine.
 */
void
bufdone(struct buf *bp)
{
        struct bufobj *dropobj;
        void    (*biodone)(struct buf *);

        buf_track(bp, __func__);
        CTR3(KTR_BUF, "bufdone(%p) vp %p flags %X", bp, bp->b_vp, bp->b_flags);
        dropobj = NULL;

        KASSERT(!(bp->b_flags & B_DONE), ("biodone: bp %p already done", bp));

        runningbufwakeup(bp);
        if (bp->b_iocmd == BIO_WRITE)
                dropobj = bp->b_bufobj;
        /* call optional completion function if requested */
        if (bp->b_iodone != NULL) {
                biodone = bp->b_iodone;
                bp->b_iodone = NULL;
                (*biodone) (bp);
                if (dropobj)
                        bufobj_wdrop(dropobj);
                return;
        }
        if (bp->b_flags & B_VMIO) {
                /*
                 * Set B_CACHE if the op was a normal read and no error
                 * occurred.  B_CACHE is set for writes in the b*write()
                 * routines.
                 */
                if (bp->b_iocmd == BIO_READ &&
                    !(bp->b_flags & (B_INVAL|B_NOCACHE)) &&
                    !(bp->b_ioflags & BIO_ERROR))
                        bp->b_flags |= B_CACHE;
                vfs_vmio_iodone(bp);
        }
        if (!LIST_EMPTY(&bp->b_dep))
                buf_complete(bp);
        if ((bp->b_flags & B_CKHASH) != 0) {
                KASSERT(bp->b_iocmd == BIO_READ,
                    ("bufdone: b_iocmd %d not BIO_READ", bp->b_iocmd));
                KASSERT(buf_mapped(bp), ("bufdone: bp %p not mapped", bp));
                (*bp->b_ckhashcalc)(bp);
        }
        /*
         * For asynchronous completions, release the buffer now. The brelse
         * will do a wakeup there if necessary - so no need to do a wakeup
         * here in the async case. The sync case always needs to do a wakeup.
         */
        if (bp->b_flags & B_ASYNC) {
                if ((bp->b_flags & (B_NOCACHE | B_INVAL | B_RELBUF)) ||
                    (bp->b_ioflags & BIO_ERROR))
                        brelse(bp);
                else
                        bqrelse(bp);
        } else
                bdone(bp);
        if (dropobj)
                bufobj_wdrop(dropobj);
}

/*
 * This routine is called in lieu of iodone in the case of
 * incomplete I/O.  This keeps the busy status for pages
 * consistent.
 */
void
vfs_unbusy_pages(struct buf *bp)
{
        int i;
        vm_object_t obj;
        vm_page_t m;

        runningbufwakeup(bp);
        if (!(bp->b_flags & B_VMIO))
                return;

        obj = bp->b_bufobj->bo_object;
        for (i = 0; i < bp->b_npages; i++) {
                m = bp->b_pages[i];
                if (m == bogus_page) {
                        m = vm_page_relookup(obj, OFF_TO_IDX(bp->b_offset) + i);
                        if (!m)
                                panic("vfs_unbusy_pages: page missing\n");
                        bp->b_pages[i] = m;
                        if (buf_mapped(bp)) {
                                BUF_CHECK_MAPPED(bp);
                                pmap_qenter(trunc_page((vm_offset_t)bp->b_data),
                                    bp->b_pages, bp->b_npages);
                        } else
                                BUF_CHECK_UNMAPPED(bp);
                }
                vm_page_sunbusy(m);
        }
        vm_object_pip_wakeupn(obj, bp->b_npages);
}

/*
 * vfs_page_set_valid:
 *
 *      Set the valid bits in a page based on the supplied offset.   The
 *      range is restricted to the buffer's size.
 *
 *      This routine is typically called after a read completes.
 */
static void
vfs_page_set_valid(struct buf *bp, vm_ooffset_t off, vm_page_t m)
{
        vm_ooffset_t eoff;

        /*
         * Compute the end offset, eoff, such that [off, eoff) does not span a
         * page boundary and eoff is not greater than the end of the buffer.
         * The end of the buffer, in this case, is our file EOF, not the
         * allocation size of the buffer.
         */
        eoff = (off + PAGE_SIZE) & ~(vm_ooffset_t)PAGE_MASK;
        if (eoff > bp->b_offset + bp->b_bcount)
                eoff = bp->b_offset + bp->b_bcount;

        /*
         * Set valid range.  This is typically the entire buffer and thus the
         * entire page.
         */
        if (eoff > off)
                vm_page_set_valid_range(m, off & PAGE_MASK, eoff - off);
}

/*
 * vfs_page_set_validclean:
 *
 *      Set the valid bits and clear the dirty bits in a page based on the
 *      supplied offset.   The range is restricted to the buffer's size.
 */
static void
vfs_page_set_validclean(struct buf *bp, vm_ooffset_t off, vm_page_t m)
{
        vm_ooffset_t soff, eoff;

        /*
         * Start and end offsets in buffer.  eoff - soff may not cross a
         * page boundary or cross the end of the buffer.  The end of the
         * buffer, in this case, is our file EOF, not the allocation size
         * of the buffer.
         */
        soff = off;
        eoff = (off + PAGE_SIZE) & ~(off_t)PAGE_MASK;
        if (eoff > bp->b_offset + bp->b_bcount)
                eoff = bp->b_offset + bp->b_bcount;

        /*
         * Set valid range.  This is typically the entire buffer and thus the
         * entire page.
         */
        if (eoff > soff) {
                vm_page_set_validclean(
                    m,
                   (vm_offset_t) (soff & PAGE_MASK),
                   (vm_offset_t) (eoff - soff)
                );
        }
}

/*
 * Acquire a shared busy on all pages in the buf.
 */
void
vfs_busy_pages_acquire(struct buf *bp)
{
        int i;

        for (i = 0; i < bp->b_npages; i++)
                vm_page_busy_acquire(bp->b_pages[i], VM_ALLOC_SBUSY);
}

void
vfs_busy_pages_release(struct buf *bp)
{
        int i;

        for (i = 0; i < bp->b_npages; i++)
                vm_page_sunbusy(bp->b_pages[i]);
}

/*
 * This routine is called before a device strategy routine.
 * It is used to tell the VM system that paging I/O is in
 * progress, and treat the pages associated with the buffer
 * almost as being exclusive busy.  Also the object paging_in_progress
 * flag is handled to make sure that the object doesn't become
 * inconsistent.
 *
 * Since I/O has not been initiated yet, certain buffer flags
 * such as BIO_ERROR or B_INVAL may be in an inconsistent state
 * and should be ignored.
 */
void
vfs_busy_pages(struct buf *bp, int clear_modify)
{
        vm_object_t obj;
        vm_ooffset_t foff;
        vm_page_t m;
        int i;
        bool bogus;

        if (!(bp->b_flags & B_VMIO))
                return;

        obj = bp->b_bufobj->bo_object;
        foff = bp->b_offset;
        KASSERT(bp->b_offset != NOOFFSET,
            ("vfs_busy_pages: no buffer offset"));
        if ((bp->b_flags & B_CLUSTER) == 0) {
                vm_object_pip_add(obj, bp->b_npages);
                vfs_busy_pages_acquire(bp);
        }
        if (bp->b_bufsize != 0)
                vfs_setdirty_range(bp);
        bogus = false;
        for (i = 0; i < bp->b_npages; i++) {
                m = bp->b_pages[i];
                vm_page_assert_sbusied(m);

                /*
                 * When readying a buffer for a read ( i.e
                 * clear_modify == 0 ), it is important to do
                 * bogus_page replacement for valid pages in 
                 * partially instantiated buffers.  Partially 
                 * instantiated buffers can, in turn, occur when
                 * reconstituting a buffer from its VM backing store
                 * base.  We only have to do this if B_CACHE is
                 * clear ( which causes the I/O to occur in the
                 * first place ).  The replacement prevents the read
                 * I/O from overwriting potentially dirty VM-backed
                 * pages.  XXX bogus page replacement is, uh, bogus.
                 * It may not work properly with small-block devices.
                 * We need to find a better way.
                 */
                if (clear_modify) {
                        pmap_remove_write(m);
                        vfs_page_set_validclean(bp, foff, m);
                } else if (vm_page_all_valid(m) &&
                    (bp->b_flags & B_CACHE) == 0) {
                        bp->b_pages[i] = bogus_page;
                        bogus = true;
                }
                foff = (foff + PAGE_SIZE) & ~(off_t)PAGE_MASK;
        }
        if (bogus && buf_mapped(bp)) {
                BUF_CHECK_MAPPED(bp);
                pmap_qenter(trunc_page((vm_offset_t)bp->b_data),
                    bp->b_pages, bp->b_npages);
        }
}

/*
 *      vfs_bio_set_valid:
 *
 *      Set the range within the buffer to valid.  The range is
 *      relative to the beginning of the buffer, b_offset.  Note that
 *      b_offset itself may be offset from the beginning of the first
 *      page.
 */
void
vfs_bio_set_valid(struct buf *bp, int base, int size)
{
        int i, n;
        vm_page_t m;

        if (!(bp->b_flags & B_VMIO))
                return;

        /*
         * Fixup base to be relative to beginning of first page.
         * Set initial n to be the maximum number of bytes in the
         * first page that can be validated.
         */
        base += (bp->b_offset & PAGE_MASK);
        n = PAGE_SIZE - (base & PAGE_MASK);

        /*
         * Busy may not be strictly necessary here because the pages are
         * unlikely to be fully valid and the vnode lock will synchronize
         * their access via getpages.  It is grabbed for consistency with
         * other page validation.
         */
        vfs_busy_pages_acquire(bp);
        for (i = base / PAGE_SIZE; size > 0 && i < bp->b_npages; ++i) {
                m = bp->b_pages[i];
                if (n > size)
                        n = size;
                vm_page_set_valid_range(m, base & PAGE_MASK, n);
                base += n;
                size -= n;
                n = PAGE_SIZE;
        }
        vfs_busy_pages_release(bp);
}

/*
 *      vfs_bio_clrbuf:
 *
 *      If the specified buffer is a non-VMIO buffer, clear the entire
 *      buffer.  If the specified buffer is a VMIO buffer, clear and
 *      validate only the previously invalid portions of the buffer.
 *      This routine essentially fakes an I/O, so we need to clear
 *      BIO_ERROR and B_INVAL.
 *
 *      Note that while we only theoretically need to clear through b_bcount,
 *      we go ahead and clear through b_bufsize.
 */
void
vfs_bio_clrbuf(struct buf *bp) 
{
        int i, j, mask, sa, ea, slide;

        if ((bp->b_flags & (B_VMIO | B_MALLOC)) != B_VMIO) {
                clrbuf(bp);
                return;
        }
        bp->b_flags &= ~B_INVAL;
        bp->b_ioflags &= ~BIO_ERROR;
        vfs_busy_pages_acquire(bp);
        sa = bp->b_offset & PAGE_MASK;
        slide = 0;
        for (i = 0; i < bp->b_npages; i++, sa = 0) {
                slide = imin(slide + PAGE_SIZE, bp->b_offset + bp->b_bufsize);
                ea = slide & PAGE_MASK;
                if (ea == 0)
                        ea = PAGE_SIZE;
                if (bp->b_pages[i] == bogus_page)
                        continue;
                j = sa / DEV_BSIZE;
                mask = ((1 << ((ea - sa) / DEV_BSIZE)) - 1) << j;
                if ((bp->b_pages[i]->valid & mask) == mask)
                        continue;
                if ((bp->b_pages[i]->valid & mask) == 0)
                        pmap_zero_page_area(bp->b_pages[i], sa, ea - sa);
                else {
                        for (; sa < ea; sa += DEV_BSIZE, j++) {
                                if ((bp->b_pages[i]->valid & (1 << j)) == 0) {
                                        pmap_zero_page_area(bp->b_pages[i],
                                            sa, DEV_BSIZE);
                                }
                        }
                }
                vm_page_set_valid_range(bp->b_pages[i], j * DEV_BSIZE,
                    roundup2(ea - sa, DEV_BSIZE));
        }
        vfs_busy_pages_release(bp);
        bp->b_resid = 0;
}

void
vfs_bio_bzero_buf(struct buf *bp, int base, int size)
{
        vm_page_t m;
        int i, n;

        if (buf_mapped(bp)) {
                BUF_CHECK_MAPPED(bp);
                bzero(bp->b_data + base, size);
        } else {
                BUF_CHECK_UNMAPPED(bp);
                n = PAGE_SIZE - (base & PAGE_MASK);
                for (i = base / PAGE_SIZE; size > 0 && i < bp->b_npages; ++i) {
                        m = bp->b_pages[i];
                        if (n > size)
                                n = size;
                        pmap_zero_page_area(m, base & PAGE_MASK, n);
                        base += n;
                        size -= n;
                        n = PAGE_SIZE;
                }
        }
}

/*
 * Update buffer flags based on I/O request parameters, optionally releasing the
 * buffer.  If it's VMIO or direct I/O, the buffer pages are released to the VM,
 * where they may be placed on a page queue (VMIO) or freed immediately (direct
 * I/O).  Otherwise the buffer is released to the cache.
 */
static void
b_io_dismiss(struct buf *bp, int ioflag, bool release)
{

        KASSERT((ioflag & IO_NOREUSE) == 0 || (ioflag & IO_VMIO) != 0,
            ("buf %p non-VMIO noreuse", bp));

        if ((ioflag & IO_DIRECT) != 0)
                bp->b_flags |= B_DIRECT;
        if ((ioflag & IO_EXT) != 0)
                bp->b_xflags |= BX_ALTDATA;
        if ((ioflag & (IO_VMIO | IO_DIRECT)) != 0 && LIST_EMPTY(&bp->b_dep)) {
                bp->b_flags |= B_RELBUF;
                if ((ioflag & IO_NOREUSE) != 0)
                        bp->b_flags |= B_NOREUSE;
                if (release)
                        brelse(bp);
        } else if (release)
                bqrelse(bp);
}

void
vfs_bio_brelse(struct buf *bp, int ioflag)
{

        b_io_dismiss(bp, ioflag, true);
}

void
vfs_bio_set_flags(struct buf *bp, int ioflag)
{

        b_io_dismiss(bp, ioflag, false);
}

/*
 * vm_hold_load_pages and vm_hold_free_pages get pages into
 * a buffers address space.  The pages are anonymous and are
 * not associated with a file object.
 */
static void
vm_hold_load_pages(struct buf *bp, vm_offset_t from, vm_offset_t to)
{
        vm_offset_t pg;
        vm_page_t p;
        int index;

        BUF_CHECK_MAPPED(bp);

        to = round_page(to);
        from = round_page(from);
        index = (from - trunc_page((vm_offset_t)bp->b_data)) >> PAGE_SHIFT;
        MPASS((bp->b_flags & B_MAXPHYS) == 0);
        KASSERT(to - from <= maxbcachebuf,
            ("vm_hold_load_pages too large %p %#jx %#jx %u",
            bp, (uintmax_t)from, (uintmax_t)to, maxbcachebuf));

        for (pg = from; pg < to; pg += PAGE_SIZE, index++) {
                /*
                 * note: must allocate system pages since blocking here
                 * could interfere with paging I/O, no matter which
                 * process we are.
                 */
                p = vm_page_alloc(NULL, 0, VM_ALLOC_SYSTEM | VM_ALLOC_NOOBJ |
                    VM_ALLOC_WIRED | VM_ALLOC_COUNT((to - pg) >> PAGE_SHIFT) |
                    VM_ALLOC_WAITOK);
                pmap_qenter(pg, &p, 1);
                bp->b_pages[index] = p;
        }
        bp->b_npages = index;
}

/* Return pages associated with this buf to the vm system */
static void
vm_hold_free_pages(struct buf *bp, int newbsize)
{
        vm_offset_t from;
        vm_page_t p;
        int index, newnpages;

        BUF_CHECK_MAPPED(bp);

        from = round_page((vm_offset_t)bp->b_data + newbsize);
        newnpages = (from - trunc_page((vm_offset_t)bp->b_data)) >> PAGE_SHIFT;
        if (bp->b_npages > newnpages)
                pmap_qremove(from, bp->b_npages - newnpages);
        for (index = newnpages; index < bp->b_npages; index++) {
                p = bp->b_pages[index];
                bp->b_pages[index] = NULL;
                vm_page_unwire_noq(p);
                vm_page_free(p);
        }
        bp->b_npages = newnpages;
}

/*
 * Map an IO request into kernel virtual address space.
 *
 * All requests are (re)mapped into kernel VA space.
 * Notice that we use b_bufsize for the size of the buffer
 * to be mapped.  b_bcount might be modified by the driver.
 *
 * Note that even if the caller determines that the address space should
 * be valid, a race or a smaller-file mapped into a larger space may
 * actually cause vmapbuf() to fail, so all callers of vmapbuf() MUST
 * check the return value.
 *
 * This function only works with pager buffers.
 */
int
vmapbuf(struct buf *bp, void *uaddr, size_t len, int mapbuf)
{
        vm_prot_t prot;
        int pidx;

        MPASS((bp->b_flags & B_MAXPHYS) != 0);
        prot = VM_PROT_READ;
        if (bp->b_iocmd == BIO_READ)
                prot |= VM_PROT_WRITE;  /* Less backwards than it looks */
        pidx = vm_fault_quick_hold_pages(&curproc->p_vmspace->vm_map,
            (vm_offset_t)uaddr, len, prot, bp->b_pages, PBUF_PAGES);
        if (pidx < 0)
                return (-1);
        bp->b_bufsize = len;
        bp->b_npages = pidx;
        bp->b_offset = ((vm_offset_t)uaddr) & PAGE_MASK;
        if (mapbuf || !unmapped_buf_allowed) {
                pmap_qenter((vm_offset_t)bp->b_kvabase, bp->b_pages, pidx);
                bp->b_data = bp->b_kvabase + bp->b_offset;
        } else
                bp->b_data = unmapped_buf;
        return (0);
}

/*
 * Free the io map PTEs associated with this IO operation.
 * We also invalidate the TLB entries and restore the original b_addr.
 *
 * This function only works with pager buffers.
 */
void
vunmapbuf(struct buf *bp)
{
        int npages;

        npages = bp->b_npages;
        if (buf_mapped(bp))
                pmap_qremove(trunc_page((vm_offset_t)bp->b_data), npages);
        vm_page_unhold_pages(bp->b_pages, npages);

        bp->b_data = unmapped_buf;
}

void
bdone(struct buf *bp)
{
        struct mtx *mtxp;

        mtxp = mtx_pool_find(mtxpool_sleep, bp);
        mtx_lock(mtxp);
        bp->b_flags |= B_DONE;
        wakeup(bp);
        mtx_unlock(mtxp);
}

void
bwait(struct buf *bp, u_char pri, const char *wchan)
{
        struct mtx *mtxp;

        mtxp = mtx_pool_find(mtxpool_sleep, bp);
        mtx_lock(mtxp);
        while ((bp->b_flags & B_DONE) == 0)
                msleep(bp, mtxp, pri, wchan, 0);
        mtx_unlock(mtxp);
}

int
bufsync(struct bufobj *bo, int waitfor)
{

        return (VOP_FSYNC(bo2vnode(bo), waitfor, curthread));
}

void
bufstrategy(struct bufobj *bo, struct buf *bp)
{
        int i __unused;
        struct vnode *vp;

        vp = bp->b_vp;
        KASSERT(vp == bo->bo_private, ("Inconsistent vnode bufstrategy"));
        KASSERT(vp->v_type != VCHR && vp->v_type != VBLK,
            ("Wrong vnode in bufstrategy(bp=%p, vp=%p)", bp, vp));
        i = VOP_STRATEGY(vp, bp);
        KASSERT(i == 0, ("VOP_STRATEGY failed bp=%p vp=%p", bp, bp->b_vp));
}

/*
 * Initialize a struct bufobj before use.  Memory is assumed zero filled.
 */
void
bufobj_init(struct bufobj *bo, void *private)
{
        static volatile int bufobj_cleanq;

        bo->bo_domain =
            atomic_fetchadd_int(&bufobj_cleanq, 1) % buf_domains;
        rw_init(BO_LOCKPTR(bo), "bufobj interlock");
        bo->bo_private = private;
        TAILQ_INIT(&bo->bo_clean.bv_hd);
        TAILQ_INIT(&bo->bo_dirty.bv_hd);
}

void
bufobj_wrefl(struct bufobj *bo)
{

        KASSERT(bo != NULL, ("NULL bo in bufobj_wref"));
        ASSERT_BO_WLOCKED(bo);
        bo->bo_numoutput++;
}

void
bufobj_wref(struct bufobj *bo)
{

        KASSERT(bo != NULL, ("NULL bo in bufobj_wref"));
        BO_LOCK(bo);
        bo->bo_numoutput++;
        BO_UNLOCK(bo);
}

void
bufobj_wdrop(struct bufobj *bo)
{

        KASSERT(bo != NULL, ("NULL bo in bufobj_wdrop"));
        BO_LOCK(bo);
        KASSERT(bo->bo_numoutput > 0, ("bufobj_wdrop non-positive count"));
        if ((--bo->bo_numoutput == 0) && (bo->bo_flag & BO_WWAIT)) {
                bo->bo_flag &= ~BO_WWAIT;
                wakeup(&bo->bo_numoutput);
        }
        BO_UNLOCK(bo);
}

int
bufobj_wwait(struct bufobj *bo, int slpflag, int timeo)
{
        int error;

        KASSERT(bo != NULL, ("NULL bo in bufobj_wwait"));
        ASSERT_BO_WLOCKED(bo);
        error = 0;
        while (bo->bo_numoutput) {
                bo->bo_flag |= BO_WWAIT;
                error = msleep(&bo->bo_numoutput, BO_LOCKPTR(bo),
                    slpflag | (PRIBIO + 1), "bo_wwait", timeo);
                if (error)
                        break;
        }
        return (error);
}

/*
 * Set bio_data or bio_ma for struct bio from the struct buf.
 */
void
bdata2bio(struct buf *bp, struct bio *bip)
{

        if (!buf_mapped(bp)) {
                KASSERT(unmapped_buf_allowed, ("unmapped"));
                bip->bio_ma = bp->b_pages;
                bip->bio_ma_n = bp->b_npages;
                bip->bio_data = unmapped_buf;
                bip->bio_ma_offset = (vm_offset_t)bp->b_offset & PAGE_MASK;
                bip->bio_flags |= BIO_UNMAPPED;
                KASSERT(round_page(bip->bio_ma_offset + bip->bio_length) /
                    PAGE_SIZE == bp->b_npages,
                    ("Buffer %p too short: %d %lld %d", bp, bip->bio_ma_offset,
                    (long long)bip->bio_length, bip->bio_ma_n));
        } else {
                bip->bio_data = bp->b_data;
                bip->bio_ma = NULL;
        }
}

/*
 * The MIPS pmap code currently doesn't handle aliased pages.
 * The VIPT caches may not handle page aliasing themselves, leading
 * to data corruption.
 *
 * As such, this code makes a system extremely unhappy if said
 * system doesn't support unaliasing the above situation in hardware.
 * Some "recent" systems (eg some mips24k/mips74k cores) don't enable
 * this feature at build time, so it has to be handled in software.
 *
 * Once the MIPS pmap/cache code grows to support this function on
 * earlier chips, it should be flipped back off.
 */
#ifdef  __mips__
static int buf_pager_relbuf = 1;
#else
static int buf_pager_relbuf = 0;
#endif
SYSCTL_INT(_vfs, OID_AUTO, buf_pager_relbuf, CTLFLAG_RWTUN,
    &buf_pager_relbuf, 0,
    "Make buffer pager release buffers after reading");

/*
 * The buffer pager.  It uses buffer reads to validate pages.
 *
 * In contrast to the generic local pager from vm/vnode_pager.c, this
 * pager correctly and easily handles volumes where the underlying
 * device block size is greater than the machine page size.  The
 * buffer cache transparently extends the requested page run to be
 * aligned at the block boundary, and does the necessary bogus page
 * replacements in the addends to avoid obliterating already valid
 * pages.
 *
 * The only non-trivial issue is that the exclusive busy state for
 * pages, which is assumed by the vm_pager_getpages() interface, is
 * incompatible with the VMIO buffer cache's desire to share-busy the
 * pages.  This function performs a trivial downgrade of the pages'
 * state before reading buffers, and a less trivial upgrade from the
 * shared-busy to excl-busy state after the read.
 */
int
vfs_bio_getpages(struct vnode *vp, vm_page_t *ma, int count,
    int *rbehind, int *rahead, vbg_get_lblkno_t get_lblkno,
    vbg_get_blksize_t get_blksize)
{
        vm_page_t m;
        vm_object_t object;
        struct buf *bp;
        struct mount *mp;
        daddr_t lbn, lbnp;
        vm_ooffset_t la, lb, poff, poffe;
        long bsize;
        int bo_bs, br_flags, error, i, pgsin, pgsin_a, pgsin_b;
        bool redo, lpart;

        object = vp->v_object;
        mp = vp->v_mount;
        error = 0;
        la = IDX_TO_OFF(ma[count - 1]->pindex);
        if (la >= object->un_pager.vnp.vnp_size)
                return (VM_PAGER_BAD);

        /*
         * Change the meaning of la from where the last requested page starts
         * to where it ends, because that's the end of the requested region
         * and the start of the potential read-ahead region.
         */
        la += PAGE_SIZE;
        lpart = la > object->un_pager.vnp.vnp_size;
        bo_bs = get_blksize(vp, get_lblkno(vp, IDX_TO_OFF(ma[0]->pindex)));

        /*
         * Calculate read-ahead, behind and total pages.
         */
        pgsin = count;
        lb = IDX_TO_OFF(ma[0]->pindex);
        pgsin_b = OFF_TO_IDX(lb - rounddown2(lb, bo_bs));
        pgsin += pgsin_b;
        if (rbehind != NULL)
                *rbehind = pgsin_b;
        pgsin_a = OFF_TO_IDX(roundup2(la, bo_bs) - la);
        if (la + IDX_TO_OFF(pgsin_a) >= object->un_pager.vnp.vnp_size)
                pgsin_a = OFF_TO_IDX(roundup2(object->un_pager.vnp.vnp_size,
                    PAGE_SIZE) - la);
        pgsin += pgsin_a;
        if (rahead != NULL)
                *rahead = pgsin_a;
        VM_CNT_INC(v_vnodein);
        VM_CNT_ADD(v_vnodepgsin, pgsin);

        br_flags = (mp != NULL && (mp->mnt_kern_flag & MNTK_UNMAPPED_BUFS)
            != 0) ? GB_UNMAPPED : 0;
again:
        for (i = 0; i < count; i++) {
                if (ma[i] != bogus_page)
                        vm_page_busy_downgrade(ma[i]);
        }

        lbnp = -1;
        for (i = 0; i < count; i++) {
                m = ma[i];
                if (m == bogus_page)
                        continue;

                /*
                 * Pages are shared busy and the object lock is not
                 * owned, which together allow for the pages'
                 * invalidation.  The racy test for validity avoids
                 * useless creation of the buffer for the most typical
                 * case when invalidation is not used in redo or for
                 * parallel read.  The shared->excl upgrade loop at
                 * the end of the function catches the race in a
                 * reliable way (protected by the object lock).
                 */
                if (vm_page_all_valid(m))
                        continue;

                poff = IDX_TO_OFF(m->pindex);
                poffe = MIN(poff + PAGE_SIZE, object->un_pager.vnp.vnp_size);
                for (; poff < poffe; poff += bsize) {
                        lbn = get_lblkno(vp, poff);
                        if (lbn == lbnp)
                                goto next_page;
                        lbnp = lbn;

                        bsize = get_blksize(vp, lbn);
                        error = bread_gb(vp, lbn, bsize, curthread->td_ucred,
                            br_flags, &bp);
                        if (error != 0)
                                goto end_pages;
                        if (bp->b_rcred == curthread->td_ucred) {
                                crfree(bp->b_rcred);
                                bp->b_rcred = NOCRED;
                        }
                        if (LIST_EMPTY(&bp->b_dep)) {
                                /*
                                 * Invalidation clears m->valid, but
                                 * may leave B_CACHE flag if the
                                 * buffer existed at the invalidation
                                 * time.  In this case, recycle the
                                 * buffer to do real read on next
                                 * bread() after redo.
                                 *
                                 * Otherwise B_RELBUF is not strictly
                                 * necessary, enable to reduce buf
                                 * cache pressure.
                                 */
                                if (buf_pager_relbuf ||
                                    !vm_page_all_valid(m))
                                        bp->b_flags |= B_RELBUF;

                                bp->b_flags &= ~B_NOCACHE;
                                brelse(bp);
                        } else {
                                bqrelse(bp);
                        }
                }
                KASSERT(1 /* racy, enable for debugging */ ||
                    vm_page_all_valid(m) || i == count - 1,
                    ("buf %d %p invalid", i, m));
                if (i == count - 1 && lpart) {
                        if (!vm_page_none_valid(m) &&
                            !vm_page_all_valid(m))
                                vm_page_zero_invalid(m, TRUE);
                }
next_page:;
        }
end_pages:

        redo = false;
        for (i = 0; i < count; i++) {
                if (ma[i] == bogus_page)
                        continue;
                if (vm_page_busy_tryupgrade(ma[i]) == 0) {
                        vm_page_sunbusy(ma[i]);
                        ma[i] = vm_page_grab_unlocked(object, ma[i]->pindex,
                            VM_ALLOC_NORMAL);
                }

                /*
                 * Since the pages were only sbusy while neither the
                 * buffer nor the object lock was held by us, or
                 * reallocated while vm_page_grab() slept for busy
                 * relinguish, they could have been invalidated.
                 * Recheck the valid bits and re-read as needed.
                 *
                 * Note that the last page is made fully valid in the
                 * read loop, and partial validity for the page at
                 * index count - 1 could mean that the page was
                 * invalidated or removed, so we must restart for
                 * safety as well.
                 */
                if (!vm_page_all_valid(ma[i]))
                        redo = true;
        }
        if (redo && error == 0)
                goto again;
        return (error != 0 ? VM_PAGER_ERROR : VM_PAGER_OK);
}

#include "opt_ddb.h"
#ifdef DDB
#include <ddb/ddb.h>

/* DDB command to show buffer data */
DB_SHOW_COMMAND(buffer, db_show_buffer)
{
        /* get args */
        struct buf *bp = (struct buf *)addr;
#ifdef FULL_BUF_TRACKING
        uint32_t i, j;
#endif

        if (!have_addr) {
                db_printf("usage: show buffer <addr>\n");
                return;
        }

        db_printf("buf at %p\n", bp);
        db_printf("b_flags = 0x%b, b_xflags=0x%b\n",
            (u_int)bp->b_flags, PRINT_BUF_FLAGS,
            (u_int)bp->b_xflags, PRINT_BUF_XFLAGS);
        db_printf("b_vflags=0x%b b_ioflags0x%b\n",
            (u_int)bp->b_vflags, PRINT_BUF_VFLAGS,
            (u_int)bp->b_ioflags, PRINT_BIO_FLAGS);
        db_printf(
            "b_error = %d, b_bufsize = %ld, b_bcount = %ld, b_resid = %ld\n"
            "b_bufobj = (%p), b_data = %p\n, b_blkno = %jd, b_lblkno = %jd, "
            "b_vp = %p, b_dep = %p\n",
            bp->b_error, bp->b_bufsize, bp->b_bcount, bp->b_resid,
            bp->b_bufobj, bp->b_data, (intmax_t)bp->b_blkno,
            (intmax_t)bp->b_lblkno, bp->b_vp, bp->b_dep.lh_first);
        db_printf("b_kvabase = %p, b_kvasize = %d\n",
            bp->b_kvabase, bp->b_kvasize);
        if (bp->b_npages) {
                int i;
                db_printf("b_npages = %d, pages(OBJ, IDX, PA): ", bp->b_npages);
                for (i = 0; i < bp->b_npages; i++) {
                        vm_page_t m;
                        m = bp->b_pages[i];
                        if (m != NULL)
                                db_printf("(%p, 0x%lx, 0x%lx)", m->object,
                                    (u_long)m->pindex,
                                    (u_long)VM_PAGE_TO_PHYS(m));
                        else
                                db_printf("( ??? )");
                        if ((i + 1) < bp->b_npages)
                                db_printf(",");
                }
                db_printf("\n");
        }
        BUF_LOCKPRINTINFO(bp);
#if defined(FULL_BUF_TRACKING)
        db_printf("b_io_tracking: b_io_tcnt = %u\n", bp->b_io_tcnt);

        i = bp->b_io_tcnt % BUF_TRACKING_SIZE;
        for (j = 1; j <= BUF_TRACKING_SIZE; j++) {
                if (bp->b_io_tracking[BUF_TRACKING_ENTRY(i - j)] == NULL)
                        continue;
                db_printf(" %2u: %s\n", j,
                    bp->b_io_tracking[BUF_TRACKING_ENTRY(i - j)]);
        }
#elif defined(BUF_TRACKING)
        db_printf("b_io_tracking: %s\n", bp->b_io_tracking);
#endif
        db_printf(" ");
}

DB_SHOW_COMMAND(bufqueues, bufqueues)
{
        struct bufdomain *bd;
        struct buf *bp;
        long total;
        int i, j, cnt;

        db_printf("bqempty: %d\n", bqempty.bq_len);

        for (i = 0; i < buf_domains; i++) {
                bd = &bdomain[i];
                db_printf("Buf domain %d\n", i);
                db_printf("\tfreebufs\t%d\n", bd->bd_freebuffers);
                db_printf("\tlofreebufs\t%d\n", bd->bd_lofreebuffers);
                db_printf("\thifreebufs\t%d\n", bd->bd_hifreebuffers);
                db_printf("\n");
                db_printf("\tbufspace\t%ld\n", bd->bd_bufspace);
                db_printf("\tmaxbufspace\t%ld\n", bd->bd_maxbufspace);
                db_printf("\thibufspace\t%ld\n", bd->bd_hibufspace);
                db_printf("\tlobufspace\t%ld\n", bd->bd_lobufspace);
                db_printf("\tbufspacethresh\t%ld\n", bd->bd_bufspacethresh);
                db_printf("\n");
                db_printf("\tnumdirtybuffers\t%d\n", bd->bd_numdirtybuffers);
                db_printf("\tlodirtybuffers\t%d\n", bd->bd_lodirtybuffers);
                db_printf("\thidirtybuffers\t%d\n", bd->bd_hidirtybuffers);
                db_printf("\tdirtybufthresh\t%d\n", bd->bd_dirtybufthresh);
                db_printf("\n");
                total = 0;
                TAILQ_FOREACH(bp, &bd->bd_cleanq->bq_queue, b_freelist)
                        total += bp->b_bufsize;
                db_printf("\tcleanq count\t%d (%ld)\n",
                    bd->bd_cleanq->bq_len, total);
                total = 0;
                TAILQ_FOREACH(bp, &bd->bd_dirtyq.bq_queue, b_freelist)
                        total += bp->b_bufsize;
                db_printf("\tdirtyq count\t%d (%ld)\n",
                    bd->bd_dirtyq.bq_len, total);
                db_printf("\twakeup\t\t%d\n", bd->bd_wanted);
                db_printf("\tlim\t\t%d\n", bd->bd_lim);
                db_printf("\tCPU ");
                for (j = 0; j <= mp_maxid; j++)
                        db_printf("%d, ", bd->bd_subq[j].bq_len);
                db_printf("\n");
                cnt = 0;
                total = 0;
                for (j = 0; j < nbuf; j++) {
                        bp = nbufp(j);
                        if (bp->b_domain == i && BUF_ISLOCKED(bp)) {
                                cnt++;
                                total += bp->b_bufsize;
                        }
                }
                db_printf("\tLocked buffers: %d space %ld\n", cnt, total);
                cnt = 0;
                total = 0;
                for (j = 0; j < nbuf; j++) {
                        bp = nbufp(j);
                        if (bp->b_domain == i) {
                                cnt++;
                                total += bp->b_bufsize;
                        }
                }
                db_printf("\tTotal buffers: %d space %ld\n", cnt, total);
        }
}

DB_SHOW_COMMAND(lockedbufs, lockedbufs)
{
        struct buf *bp;
        int i;

        for (i = 0; i < nbuf; i++) {
                bp = nbufp(i);
                if (BUF_ISLOCKED(bp)) {
                        db_show_buffer((uintptr_t)bp, 1, 0, NULL);
                        db_printf("\n");
                        if (db_pager_quit)
                                break;
                }
        }
}

DB_SHOW_COMMAND(vnodebufs, db_show_vnodebufs)
{
        struct vnode *vp;
        struct buf *bp;

        if (!have_addr) {
                db_printf("usage: show vnodebufs <addr>\n");
                return;
        }
        vp = (struct vnode *)addr;
        db_printf("Clean buffers:\n");
        TAILQ_FOREACH(bp, &vp->v_bufobj.bo_clean.bv_hd, b_bobufs) {
                db_show_buffer((uintptr_t)bp, 1, 0, NULL);
                db_printf("\n");
        }
        db_printf("Dirty buffers:\n");
        TAILQ_FOREACH(bp, &vp->v_bufobj.bo_dirty.bv_hd, b_bobufs) {
                db_show_buffer((uintptr_t)bp, 1, 0, NULL);
                db_printf("\n");
        }
}

DB_COMMAND(countfreebufs, db_coundfreebufs)
{
        struct buf *bp;
        int i, used = 0, nfree = 0;

        if (have_addr) {
                db_printf("usage: countfreebufs\n");
                return;
        }

        for (i = 0; i < nbuf; i++) {
                bp = nbufp(i);
                if (bp->b_qindex == QUEUE_EMPTY)
                        nfree++;
                else
                        used++;
        }

        db_printf("Counted %d free, %d used (%d tot)\n", nfree, used,
            nfree + used);
        db_printf("numfreebuffers is %d\n", numfreebuffers);
}
#endif /* DDB */