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[minix.git] / sys / ufs / ffs / ffs_snapshot.c

/*      $NetBSD: ffs_snapshot.c,v 1.140 2015/06/28 10:04:32 maxv Exp $  */

/*
 * Copyright 2000 Marshall Kirk McKusick. All Rights Reserved.
 *
 * Further information about snapshots can be obtained from:
 *
 *      Marshall Kirk McKusick          http://www.mckusick.com/softdep/
 *      1614 Oxford Street              mckusick@mckusick.com
 *      Berkeley, CA 94709-1608         +1-510-843-9542
 *      USA
 *
 * 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 MARSHALL KIRK MCKUSICK ``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 MARSHALL KIRK MCKUSICK 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.
 *
 *      @(#)ffs_snapshot.c      8.11 (McKusick) 7/23/00
 *
 *      from FreeBSD: ffs_snapshot.c,v 1.79 2004/02/13 02:02:06 kuriyama Exp
 */

#include <sys/cdefs.h>
__KERNEL_RCSID(0, "$NetBSD: ffs_snapshot.c,v 1.140 2015/06/28 10:04:32 maxv Exp $");

#if defined(_KERNEL_OPT)
#include "opt_ffs.h"
#include "opt_quota.h"
#endif

#include <sys/param.h>
#include <sys/kernel.h>
#include <sys/systm.h>
#include <sys/conf.h>
#include <sys/buf.h>
#include <sys/proc.h>
#include <sys/namei.h>
#include <sys/sched.h>
#include <sys/stat.h>
#include <sys/malloc.h>
#include <sys/mount.h>
#include <sys/resource.h>
#include <sys/resourcevar.h>
#include <sys/vnode.h>
#include <sys/kauth.h>
#include <sys/fstrans.h>
#include <sys/wapbl.h>

#include <miscfs/specfs/specdev.h>

#include <ufs/ufs/quota.h>
#include <ufs/ufs/ufsmount.h>
#include <ufs/ufs/inode.h>
#include <ufs/ufs/ufs_extern.h>
#include <ufs/ufs/ufs_bswap.h>
#include <ufs/ufs/ufs_wapbl.h>

#include <ufs/ffs/fs.h>
#include <ufs/ffs/ffs_extern.h>

#include <uvm/uvm.h>

TAILQ_HEAD(inodelst, inode);                    /* List of active snapshots */

struct snap_info {
        kmutex_t si_lock;                       /* Lock this snapinfo */
        kmutex_t si_snaplock;                   /* Snapshot vnode common lock */
        lwp_t *si_owner;                        /* Snaplock owner */
        struct inodelst si_snapshots;           /* List of active snapshots */
        daddr_t *si_snapblklist;                /* Snapshot block hints list */
        uint32_t si_gen;                        /* Incremented on change */
};

#if !defined(FFS_NO_SNAPSHOT)
typedef int (*acctfunc_t)
    (struct vnode *, void *, int, int, struct fs *, daddr_t, int);

static int snapshot_setup(struct mount *, struct vnode *);
static int snapshot_copyfs(struct mount *, struct vnode *, void **);
static int snapshot_expunge(struct mount *, struct vnode *,
    struct fs *, daddr_t *, daddr_t **);
static int snapshot_expunge_snap(struct mount *, struct vnode *,
    struct fs *, daddr_t);
static int snapshot_writefs(struct mount *, struct vnode *, void *);
static int cgaccount(struct vnode *, int, int *);
static int cgaccount1(int, struct vnode *, void *, int);
static int expunge(struct vnode *, struct inode *, struct fs *,
    acctfunc_t, int);
static int indiracct(struct vnode *, struct vnode *, int, daddr_t,
    daddr_t, daddr_t, daddr_t, daddr_t, struct fs *, acctfunc_t, int);
static int fullacct(struct vnode *, void *, int, int, struct fs *,
    daddr_t, int);
static int snapacct(struct vnode *, void *, int, int, struct fs *,
    daddr_t, int);
static int mapacct(struct vnode *, void *, int, int, struct fs *,
    daddr_t, int);
#endif /* !defined(FFS_NO_SNAPSHOT) */

static int ffs_copyonwrite(void *, struct buf *, bool);
static int snapblkaddr(struct vnode *, daddr_t, daddr_t *);
static int rwfsblk(struct vnode *, int, void *, daddr_t);
static int syncsnap(struct vnode *);
static int wrsnapblk(struct vnode *, void *, daddr_t);
#if !defined(FFS_NO_SNAPSHOT)
static int blocks_in_journal(struct fs *);
#endif

static inline bool is_active_snapshot(struct snap_info *, struct inode *);
static inline daddr_t db_get(struct inode *, int);
static inline void db_assign(struct inode *, int, daddr_t);
static inline daddr_t ib_get(struct inode *, int);
static inline daddr_t idb_get(struct inode *, void *, int);
static inline void idb_assign(struct inode *, void *, int, daddr_t);

#ifdef DEBUG
static int snapdebug = 0;
#endif

int
ffs_snapshot_init(struct ufsmount *ump)
{
        struct snap_info *si;

        si = ump->um_snapinfo = kmem_alloc(sizeof(*si), KM_SLEEP);
        if (si == NULL)
                return ENOMEM;

        TAILQ_INIT(&si->si_snapshots);
        mutex_init(&si->si_lock, MUTEX_DEFAULT, IPL_NONE);
        mutex_init(&si->si_snaplock, MUTEX_DEFAULT, IPL_NONE);
        si->si_owner = NULL;
        si->si_gen = 0;
        si->si_snapblklist = NULL;

        return 0;
}

void
ffs_snapshot_fini(struct ufsmount *ump)
{
        struct snap_info *si;

        si = ump->um_snapinfo;
        ump->um_snapinfo = NULL;

        KASSERT(TAILQ_EMPTY(&si->si_snapshots));
        mutex_destroy(&si->si_lock);
        mutex_destroy(&si->si_snaplock);
        KASSERT(si->si_snapblklist == NULL);
        kmem_free(si, sizeof(*si));
}

/*
 * Create a snapshot file and initialize it for the filesystem.
 * Vnode is locked on entry and return.
 */
int
ffs_snapshot(struct mount *mp, struct vnode *vp, struct timespec *ctime)
{
#if defined(FFS_NO_SNAPSHOT)
        return EOPNOTSUPP;
}
#else /* defined(FFS_NO_SNAPSHOT) */
        bool suspended = false;
        int error, redo = 0, snaploc;
        void *sbbuf = NULL;
        daddr_t *snaplist = NULL, snaplistsize = 0;
        struct buf *bp, *nbp;
        struct fs *copy_fs = NULL;
        struct fs *fs = VFSTOUFS(mp)->um_fs;
        struct inode *ip = VTOI(vp);
        struct lwp *l = curlwp;
        struct snap_info *si = VFSTOUFS(mp)->um_snapinfo;
        struct timespec ts;
        struct timeval starttime;
#ifdef DEBUG
        struct timeval endtime;
#endif
        struct vnode *devvp = ip->i_devvp;

        /*
         * If the vnode already is a snapshot, return.
         */
        if ((ip->i_flags & SF_SNAPSHOT)) {
                if ((ip->i_flags & SF_SNAPINVAL))
                        return EINVAL;
                if (ctime) {
                        ctime->tv_sec = DIP(ip, mtime);
                        ctime->tv_nsec = DIP(ip, mtimensec);
                }
                return 0;
        }
        /*
         * Check for free snapshot slot in the superblock.
         */
        for (snaploc = 0; snaploc < FSMAXSNAP; snaploc++)
                if (fs->fs_snapinum[snaploc] == 0)
                        break;
        if (snaploc == FSMAXSNAP)
                return (ENOSPC);
        /*
         * Prepare the vnode to become a snapshot.
         */
        error = snapshot_setup(mp, vp);
        if (error)
                goto out;

        /*
         * Copy all the cylinder group maps. Although the
         * filesystem is still active, we hope that only a few
         * cylinder groups will change between now and when we
         * suspend operations. Thus, we will be able to quickly
         * touch up the few cylinder groups that changed during
         * the suspension period.
         */
        error = cgaccount(vp, 1, NULL);
        if (error)
                goto out;

        /*
         * snapshot is now valid
         */
        ip->i_flags &= ~SF_SNAPINVAL;
        DIP_ASSIGN(ip, flags, ip->i_flags);
        ip->i_flag |= IN_CHANGE | IN_UPDATE;

        /*
         * Ensure that the snapshot is completely on disk.
         * Since we have marked it as a snapshot it is safe to
         * unlock it as no process will be allowed to write to it.
         */
        error = VOP_FSYNC(vp, l->l_cred, FSYNC_WAIT, 0, 0);
        if (error)
                goto out;
        VOP_UNLOCK(vp);
        /*
         * All allocations are done, so we can now suspend the filesystem.
         */
        error = vfs_suspend(vp->v_mount, 0);
        vn_lock(vp, LK_EXCLUSIVE | LK_RETRY);
        if (error)
                goto out;
        suspended = true;
        getmicrotime(&starttime);
        /*
         * First, copy all the cylinder group maps that have changed.
         */
        error = cgaccount(vp, 2, &redo);
        if (error)
                goto out;
        /*
         * Create a copy of the superblock and its summary information.
         */
        error = snapshot_copyfs(mp, vp, &sbbuf);
        if (error)
                goto out;
        copy_fs = (struct fs *)((char *)sbbuf + ffs_blkoff(fs, fs->fs_sblockloc));
        /*
         * Expunge unlinked files from our view.
         */
        error = snapshot_expunge(mp, vp, copy_fs, &snaplistsize, &snaplist);
        if (error)
                goto out;
        /*
         * Record snapshot inode. Since this is the newest snapshot,
         * it must be placed at the end of the list.
         */
        if (ip->i_nlink > 0)
                fs->fs_snapinum[snaploc] = ip->i_number;

        mutex_enter(&si->si_lock);
        if (is_active_snapshot(si, ip))
                panic("ffs_snapshot: %"PRIu64" already on list", ip->i_number);
        TAILQ_INSERT_TAIL(&si->si_snapshots, ip, i_nextsnap);
        if (TAILQ_FIRST(&si->si_snapshots) == ip) {
                /*
                 * If this is the first snapshot on this filesystem, put the
                 * preliminary list in place and establish the cow handler.
                 */
                si->si_snapblklist = snaplist;
                fscow_establish(mp, ffs_copyonwrite, devvp);
        }
        si->si_gen++;
        mutex_exit(&si->si_lock);

        vp->v_vflag |= VV_SYSTEM;
        /*
         * Set the mtime to the time the snapshot has been taken.
         */
        TIMEVAL_TO_TIMESPEC(&starttime, &ts);
        if (ctime)
                *ctime = ts;
        DIP_ASSIGN(ip, mtime, ts.tv_sec);
        DIP_ASSIGN(ip, mtimensec, ts.tv_nsec);
        ip->i_flag |= IN_CHANGE | IN_UPDATE;
        /*
         * Copy allocation information from all snapshots and then
         * expunge them from our view.
         */
        error = snapshot_expunge_snap(mp, vp, copy_fs, snaplistsize);
        if (error)
                goto out;
        /*
         * Write the superblock and its summary information to the snapshot.
         */
        error = snapshot_writefs(mp, vp, sbbuf);
        if (error)
                goto out;
        /*
         * We're nearly done, ensure that the snapshot is completely on disk.
         */
        error = VOP_FSYNC(vp, l->l_cred, FSYNC_WAIT, 0, 0);
        if (error)
                goto out;
        /*
         * Invalidate and free all pages on the snapshot vnode.
         * We will read and write through the buffercache.
         */
        mutex_enter(vp->v_interlock);
        error = VOP_PUTPAGES(vp, 0, 0,
                    PGO_ALLPAGES | PGO_CLEANIT | PGO_SYNCIO | PGO_FREE);
        if (error)
                goto out;
        /*
         * Invalidate short ( < fs_bsize ) buffers.  We will always read
         * full size buffers later.
         */
        mutex_enter(&bufcache_lock);
        KASSERT(LIST_FIRST(&vp->v_dirtyblkhd) == NULL);
        for (bp = LIST_FIRST(&vp->v_cleanblkhd); bp; bp = nbp) {
                nbp = LIST_NEXT(bp, b_vnbufs);
                if (bp->b_bcount == fs->fs_bsize)
                        continue;
                error = bbusy(bp, false, 0, NULL);
                if (error != 0) {
                        if (error == EPASSTHROUGH) {
                                nbp = LIST_FIRST(&vp->v_cleanblkhd);
                                continue;
                        }
                        break;
                }
                brelsel(bp, BC_INVAL | BC_VFLUSH);
        }
        mutex_exit(&bufcache_lock);

out:
        if (sbbuf != NULL) {
                free(copy_fs->fs_csp, M_UFSMNT);
                free(sbbuf, M_UFSMNT);
        }
        if (fs->fs_active != NULL) {
                free(fs->fs_active, M_DEVBUF);
                fs->fs_active = NULL;
        }

        mutex_enter(&si->si_lock);
        if (snaplist != NULL) {
                if (si->si_snapblklist == snaplist)
                        si->si_snapblklist = NULL;
                free(snaplist, M_UFSMNT);
        }
        if (error) {
                fs->fs_snapinum[snaploc] = 0;
        } else {
                /*
                 * As this is the newest list, it is the most inclusive, so
                 * should replace the previous list.
                 */
                si->si_snapblklist = ip->i_snapblklist;
        }
        si->si_gen++;
        mutex_exit(&si->si_lock);

        if (suspended) {
                VOP_UNLOCK(vp);
                vfs_resume(vp->v_mount);
                vn_lock(vp, LK_EXCLUSIVE | LK_RETRY);
#ifdef DEBUG
                getmicrotime(&endtime);
                timersub(&endtime, &starttime, &endtime);
                printf("%s: suspended %lld.%03d sec, redo %d of %d\n",
                    mp->mnt_stat.f_mntonname, (long long)endtime.tv_sec,
                    endtime.tv_usec / 1000, redo, fs->fs_ncg);
#endif
        }
        if (error) {
                if (UFS_WAPBL_BEGIN(mp) == 0) {
                        (void) ffs_truncate(vp, (off_t)0, 0, NOCRED);
                        UFS_WAPBL_END(mp);
                }
        } else if (ip->i_nlink > 0)
                vref(vp);
        return (error);
}

/*
 * Prepare vnode to become a snapshot.
 */
static int
snapshot_setup(struct mount *mp, struct vnode *vp)
{
        int error, n, len, loc, cg;
        daddr_t blkno, numblks;
        struct buf *ibp, *nbp;
        struct fs *fs = VFSTOUFS(mp)->um_fs;
        struct lwp *l = curlwp;
        const int wbreak = blocks_in_journal(fs)/8;
        struct inode *ip = VTOI(vp);

        /*
         * Check mount, readonly reference and owner.
         */
        if (vp->v_mount != mp)
                return EXDEV;
        if (vp->v_writecount != 0)
                return EBUSY;
        error = kauth_authorize_system(l->l_cred, KAUTH_SYSTEM_FS_SNAPSHOT,
            0, mp, vp, NULL);
        if (error)
                return EACCES;

        if (vp->v_size != 0) {
                error = ffs_truncate(vp, 0, 0, NOCRED);
                if (error)
                        return error;
        }

        /* Change inode to snapshot type file. */
        error = UFS_WAPBL_BEGIN(mp);
        if (error)
                return error;
#if defined(QUOTA) || defined(QUOTA2)
        /* shapshot inodes are not accounted in quotas */
        chkiq(ip, -1, l->l_cred, 0);
#endif
        ip->i_flags |= (SF_SNAPSHOT | SF_SNAPINVAL);
        DIP_ASSIGN(ip, flags, ip->i_flags);
        ip->i_flag |= IN_CHANGE | IN_UPDATE;
        ffs_update(vp, NULL, NULL, UPDATE_WAIT);
        UFS_WAPBL_END(mp);

        KASSERT(ip->i_flags & SF_SNAPSHOT);
        /*
         * Write an empty list of preallocated blocks to the end of
         * the snapshot to set size to at least that of the filesystem.
         */
        numblks = howmany(fs->fs_size, fs->fs_frag);
        blkno = 1;
        blkno = ufs_rw64(blkno, UFS_FSNEEDSWAP(fs));
        error = vn_rdwr(UIO_WRITE, vp,
            (void *)&blkno, sizeof(blkno), ffs_lblktosize(fs, (off_t)numblks),
            UIO_SYSSPACE, IO_NODELOCKED|IO_UNIT, l->l_cred, NULL, NULL);
        if (error)
                return error;
        /*
         * Preallocate critical data structures so that we can copy
         * them in without further allocation after we suspend all
         * operations on the filesystem. We would like to just release
         * the allocated buffers without writing them since they will
         * be filled in below once we are ready to go, but this upsets
         * the soft update code, so we go ahead and write the new buffers.
         *
         * Allocate all indirect blocks and mark all of them as not
         * needing to be copied.
         */
        error = UFS_WAPBL_BEGIN(mp);
        if (error)
                return error;
        for (blkno = UFS_NDADDR, n = 0; blkno < numblks; blkno += FFS_NINDIR(fs)) {
                error = ffs_balloc(vp, ffs_lblktosize(fs, (off_t)blkno),
                    fs->fs_bsize, l->l_cred, B_METAONLY, &ibp);
                if (error)
                        goto out;
                brelse(ibp, 0);
                if (wbreak > 0 && (++n % wbreak) == 0) {
                        UFS_WAPBL_END(mp);
                        error = UFS_WAPBL_BEGIN(mp);
                        if (error)
                                return error;
                }
        }
        /*
         * Allocate copies for the superblock and its summary information.
         */
        error = ffs_balloc(vp, fs->fs_sblockloc, fs->fs_sbsize, l->l_cred,
            0, &nbp);
        if (error)
                goto out;
        bawrite(nbp);
        blkno = ffs_fragstoblks(fs, fs->fs_csaddr);
        len = howmany(fs->fs_cssize, fs->fs_bsize);
        for (loc = 0; loc < len; loc++) {
                error = ffs_balloc(vp, ffs_lblktosize(fs, (off_t)(blkno + loc)),
                    fs->fs_bsize, l->l_cred, 0, &nbp);
                if (error)
                        goto out;
                bawrite(nbp);
                if (wbreak > 0 && (++n % wbreak) == 0) {
                        UFS_WAPBL_END(mp);
                        error = UFS_WAPBL_BEGIN(mp);
                        if (error)
                                return error;
                }
        }
        /*
         * Allocate all cylinder group blocks.
         */
        for (cg = 0; cg < fs->fs_ncg; cg++) {
                error = ffs_balloc(vp, ffs_lfragtosize(fs, cgtod(fs, cg)),
                    fs->fs_bsize, l->l_cred, 0, &nbp);
                if (error)
                        goto out;
                bawrite(nbp);
                if (wbreak > 0 && (++n % wbreak) == 0) {
                        UFS_WAPBL_END(mp);
                        error = UFS_WAPBL_BEGIN(mp);
                        if (error)
                                return error;
                }
        }

out:
        UFS_WAPBL_END(mp);
        return error;
}

/*
 * Create a copy of the superblock and its summary information.
 * It is up to the caller to free copyfs and copy_fs->fs_csp.
 */
static int
snapshot_copyfs(struct mount *mp, struct vnode *vp, void **sbbuf)
{
        int error, i, len, loc, size;
        void *space;
        int32_t *lp;
        struct buf *bp;
        struct fs *copyfs, *fs = VFSTOUFS(mp)->um_fs;
        struct vnode *devvp = VTOI(vp)->i_devvp;

        /*
         * Grab a copy of the superblock and its summary information.
         * We delay writing it until the suspension is released below.
         */
        *sbbuf = malloc(fs->fs_bsize, M_UFSMNT, M_WAITOK);
        loc = ffs_blkoff(fs, fs->fs_sblockloc);
        if (loc > 0)
                memset(*sbbuf, 0, loc);
        copyfs = (struct fs *)((char *)(*sbbuf) + loc);
        memcpy(copyfs, fs, fs->fs_sbsize);
        size = fs->fs_bsize < SBLOCKSIZE ? fs->fs_bsize : SBLOCKSIZE;
        if (fs->fs_sbsize < size)
                memset((char *)(*sbbuf) + loc + fs->fs_sbsize, 0, 
                    size - fs->fs_sbsize);
        size = ffs_blkroundup(fs, fs->fs_cssize);
        if (fs->fs_contigsumsize > 0)
                size += fs->fs_ncg * sizeof(int32_t);
        space = malloc(size, M_UFSMNT, M_WAITOK);
        copyfs->fs_csp = space;
        memcpy(copyfs->fs_csp, fs->fs_csp, fs->fs_cssize);
        space = (char *)space + fs->fs_cssize;
        loc = howmany(fs->fs_cssize, fs->fs_fsize);
        i = fs->fs_frag - loc % fs->fs_frag;
        len = (i == fs->fs_frag) ? 0 : i * fs->fs_fsize;
        if (len > 0) {
                if ((error = bread(devvp, FFS_FSBTODB(fs, fs->fs_csaddr + loc),
                    len, 0, &bp)) != 0) {
                        free(copyfs->fs_csp, M_UFSMNT);
                        free(*sbbuf, M_UFSMNT);
                        *sbbuf = NULL;
                        return error;
                }
                memcpy(space, bp->b_data, (u_int)len);
                space = (char *)space + len;
                brelse(bp, BC_INVAL | BC_NOCACHE);
        }
        if (fs->fs_contigsumsize > 0) {
                copyfs->fs_maxcluster = lp = space;
                for (i = 0; i < fs->fs_ncg; i++)
                        *lp++ = fs->fs_contigsumsize;
        }
        if (mp->mnt_wapbl)
                copyfs->fs_flags &= ~FS_DOWAPBL;
        return 0;
}

struct snapshot_expunge_ctx {
        struct vnode *logvp;
        struct lwp *l;
        struct vnode *vp;
        struct fs *copy_fs;
};

static bool
snapshot_expunge_selector(void *cl, struct vnode *xvp)
{
        struct vattr vat;
        struct snapshot_expunge_ctx *c = cl;
        struct inode *xp;

        xp = VTOI(xvp);
        if (xvp->v_type == VNON || VTOI(xvp) == NULL ||
            (xp->i_flags & SF_SNAPSHOT))
                return false;
#ifdef DEBUG
        if (snapdebug)
                vprint("ffs_snapshot: busy vnode", xvp);
#endif

        if (xvp == c->logvp)
                return true;

        if (VOP_GETATTR(xvp, &vat, c->l->l_cred) == 0 &&
            vat.va_nlink > 0)
                return false;

        if (ffs_checkfreefile(c->copy_fs, c->vp, xp->i_number))
                return false;

        return true;
}

/*
 * We must check for active files that have been unlinked (e.g., with a zero
 * link count). We have to expunge all trace of these files from the snapshot
 * so that they are not reclaimed prematurely by fsck or unnecessarily dumped.
 * Note that we skip unlinked snapshot files as they will be handled separately.
 * Calculate the snapshot list size and create a preliminary list.
 */
static int
snapshot_expunge(struct mount *mp, struct vnode *vp, struct fs *copy_fs,
    daddr_t *snaplistsize, daddr_t **snaplist)
{
        int cg, error = 0, len, loc;
        daddr_t blkno, *blkp;
        struct fs *fs = VFSTOUFS(mp)->um_fs;
        struct inode *xp;
        struct lwp *l = curlwp;
        struct vnode *logvp = NULL, *xvp;
        struct vnode_iterator *marker;
        struct snapshot_expunge_ctx ctx;

        *snaplist = NULL;
        /*
         * Get the log inode if any.
         */
        if ((fs->fs_flags & FS_DOWAPBL) &&
            fs->fs_journal_location == UFS_WAPBL_JOURNALLOC_IN_FILESYSTEM) {
                error = VFS_VGET(mp,
                    fs->fs_journallocs[UFS_WAPBL_INFS_INO], &logvp);
                if (error)
                        goto out;
        }
        /*
         * We also calculate the needed size for the snapshot list.
         */
        *snaplistsize = fs->fs_ncg + howmany(fs->fs_cssize, fs->fs_bsize) +
            FSMAXSNAP + 1 /* superblock */ + 1 /* last block */ + 1 /* size */;

        vfs_vnode_iterator_init(mp, &marker);
        ctx.logvp = logvp;
        ctx.l = l;
        ctx.vp = vp;
        ctx.copy_fs = copy_fs;
        while ((xvp = vfs_vnode_iterator_next(marker, snapshot_expunge_selector,
            &ctx)))
        {
                /*
                 * If there is a fragment, clear it here.
                 */
                xp = VTOI(xvp);
                blkno = 0;
                loc = howmany(xp->i_size, fs->fs_bsize) - 1;
                if (loc < UFS_NDADDR) {
                        len = ffs_fragroundup(fs, ffs_blkoff(fs, xp->i_size));
                        if (len > 0 && len < fs->fs_bsize) {
                                error = UFS_WAPBL_BEGIN(mp);
                                if (error) {
                                        vrele(xvp);
                                        vfs_vnode_iterator_destroy(marker);
                                        goto out;
                                }
                                ffs_blkfree_snap(copy_fs, vp, db_get(xp, loc),
                                    len, xp->i_number);
                                blkno = db_get(xp, loc);
                                db_assign(xp, loc, 0);
                                UFS_WAPBL_END(mp);
                        }
                }
                *snaplistsize += 1;
                error = expunge(vp, xp, copy_fs, fullacct, BLK_NOCOPY);
                if (blkno)
                        db_assign(xp, loc, blkno);
                if (!error) {
                        error = UFS_WAPBL_BEGIN(mp);
                        if (!error) {
                                error = ffs_freefile_snap(copy_fs, vp,
                                    xp->i_number, xp->i_mode);
                                UFS_WAPBL_END(mp);
                        }
                }
                vrele(xvp);
                if (error) {
                        vfs_vnode_iterator_destroy(marker);
                        goto out;
                }
        }
        vfs_vnode_iterator_destroy(marker);

        /*
         * Create a preliminary list of preallocated snapshot blocks.
         */
        *snaplist = malloc(*snaplistsize * sizeof(daddr_t), M_UFSMNT, M_WAITOK);
        blkp = &(*snaplist)[1];
        *blkp++ = ffs_lblkno(fs, fs->fs_sblockloc);
        blkno = ffs_fragstoblks(fs, fs->fs_csaddr);
        for (cg = 0; cg < fs->fs_ncg; cg++) {
                if (ffs_fragstoblks(fs, cgtod(fs, cg)) > blkno)
                        break;
                *blkp++ = ffs_fragstoblks(fs, cgtod(fs, cg));
        }
        len = howmany(fs->fs_cssize, fs->fs_bsize);
        for (loc = 0; loc < len; loc++)
                *blkp++ = blkno + loc;
        for (; cg < fs->fs_ncg; cg++)
                *blkp++ = ffs_fragstoblks(fs, cgtod(fs, cg));
        (*snaplist)[0] = blkp - &(*snaplist)[0];

out:
        if (logvp != NULL)
                vput(logvp);
        if (error && *snaplist != NULL) {
                free(*snaplist, M_UFSMNT);
                *snaplist = NULL;
        }

        return error;
}

/*
 * Copy allocation information from all the snapshots in this snapshot and
 * then expunge them from its view. Also, collect the list of allocated
 * blocks in i_snapblklist.
 */
static int
snapshot_expunge_snap(struct mount *mp, struct vnode *vp,
    struct fs *copy_fs, daddr_t snaplistsize)
{
        int error = 0, i;
        daddr_t numblks, *snaplist = NULL;
        struct fs *fs = VFSTOUFS(mp)->um_fs;
        struct inode *ip = VTOI(vp), *xp;
        struct lwp *l = curlwp;
        struct snap_info *si = VFSTOUFS(mp)->um_snapinfo;

        TAILQ_FOREACH(xp, &si->si_snapshots, i_nextsnap) {
                if (xp != ip) {
                        error = expunge(vp, xp, fs, snapacct, BLK_SNAP);
                        if (error)
                                break;
                }
                if (xp->i_nlink != 0)
                        continue;
                error = UFS_WAPBL_BEGIN(mp);
                if (error)
                        break;
                error = ffs_freefile_snap(copy_fs, vp, xp->i_number, xp->i_mode);
                UFS_WAPBL_END(mp);
                if (error)
                        break;
        }
        if (error)
                goto out;
        /*
         * Allocate space for the full list of preallocated snapshot blocks.
         */
        snaplist = malloc(snaplistsize * sizeof(daddr_t), M_UFSMNT, M_WAITOK);
        ip->i_snapblklist = &snaplist[1];
        /*
         * Expunge the blocks used by the snapshots from the set of
         * blocks marked as used in the snapshot bitmaps. Also, collect
         * the list of allocated blocks in i_snapblklist.
         */
        error = expunge(vp, ip, copy_fs, mapacct, BLK_SNAP);
        if (error)
                goto out;
        if (snaplistsize < ip->i_snapblklist - snaplist)
                panic("ffs_snapshot: list too small");
        snaplistsize = ip->i_snapblklist - snaplist;
        snaplist[0] = snaplistsize;
        ip->i_snapblklist = &snaplist[0];
        /*
         * Write out the list of allocated blocks to the end of the snapshot.
         */
        numblks = howmany(fs->fs_size, fs->fs_frag);
        for (i = 0; i < snaplistsize; i++)
                snaplist[i] = ufs_rw64(snaplist[i], UFS_FSNEEDSWAP(fs));
        error = vn_rdwr(UIO_WRITE, vp, (void *)snaplist,
            snaplistsize * sizeof(daddr_t), ffs_lblktosize(fs, (off_t)numblks),
            UIO_SYSSPACE, IO_NODELOCKED | IO_UNIT, l->l_cred, NULL, NULL);
        for (i = 0; i < snaplistsize; i++)
                snaplist[i] = ufs_rw64(snaplist[i], UFS_FSNEEDSWAP(fs));
out:
        if (error && snaplist != NULL) {
                free(snaplist, M_UFSMNT);
                ip->i_snapblklist = NULL;
        }
        return error;
}

/*
 * Write the superblock and its summary information to the snapshot.
 * Make sure, the first UFS_NDADDR blocks get copied to the snapshot.
 */
static int
snapshot_writefs(struct mount *mp, struct vnode *vp, void *sbbuf)
{
        int error, len, loc;
        void *space;
        daddr_t blkno;
        struct buf *bp;
        struct fs *copyfs, *fs = VFSTOUFS(mp)->um_fs;
        struct inode *ip = VTOI(vp);
        struct lwp *l = curlwp;

        copyfs = (struct fs *)((char *)sbbuf + ffs_blkoff(fs, fs->fs_sblockloc));

        /*
         * Write the superblock and its summary information
         * to the snapshot.
         */
        blkno = ffs_fragstoblks(fs, fs->fs_csaddr);
        len = howmany(fs->fs_cssize, fs->fs_bsize);
        space = copyfs->fs_csp;
#ifdef FFS_EI
        if (UFS_FSNEEDSWAP(fs)) {
                ffs_sb_swap(copyfs, copyfs);
                ffs_csum_swap(space, space, fs->fs_cssize);
        }
#endif
        error = UFS_WAPBL_BEGIN(mp);
        if (error)
                return error;
        for (loc = 0; loc < len; loc++) {
                error = bread(vp, blkno + loc, fs->fs_bsize,
                    B_MODIFY, &bp);
                if (error) {
                        break;
                }
                memcpy(bp->b_data, space, fs->fs_bsize);
                space = (char *)space + fs->fs_bsize;
                bawrite(bp);
        }
        if (error)
                goto out;
        error = bread(vp, ffs_lblkno(fs, fs->fs_sblockloc),
            fs->fs_bsize, B_MODIFY, &bp);
        if (error) {
                goto out;
        } else {
                memcpy(bp->b_data, sbbuf, fs->fs_bsize);
                bawrite(bp);
        }
        /*
         * Copy the first UFS_NDADDR blocks to the snapshot so
         * ffs_copyonwrite() and ffs_snapblkfree() will always work on
         * indirect blocks.
         */
        for (loc = 0; loc < UFS_NDADDR; loc++) {
                if (db_get(ip, loc) != 0)
                        continue;
                error = ffs_balloc(vp, ffs_lblktosize(fs, (off_t)loc),
                    fs->fs_bsize, l->l_cred, 0, &bp);
                if (error)
                        break;
                error = rwfsblk(vp, B_READ, bp->b_data, loc);
                if (error) {
                        brelse(bp, 0);
                        break;
                }
                bawrite(bp);
        }

out:
        UFS_WAPBL_END(mp);
        return error;
}

/*
 * Copy all cylinder group maps.
 */
static int
cgaccount(struct vnode *vp, int passno, int *redo)
{
        int cg, error = 0;
        struct buf *nbp;
        struct fs *fs = VTOI(vp)->i_fs;

        if (redo != NULL)
                *redo = 0;
        if (passno == 1)
                fs->fs_active = malloc(howmany(fs->fs_ncg, NBBY),
                    M_DEVBUF, M_WAITOK | M_ZERO);
        for (cg = 0; cg < fs->fs_ncg; cg++) {
                if (passno == 2 && ACTIVECG_ISSET(fs, cg))
                        continue;

                if (redo != NULL)
                        *redo += 1;
                error = UFS_WAPBL_BEGIN(vp->v_mount);
                if (error)
                        return error;
                error = ffs_balloc(vp, ffs_lfragtosize(fs, cgtod(fs, cg)),
                    fs->fs_bsize, curlwp->l_cred, 0, &nbp);
                if (error) {
                        UFS_WAPBL_END(vp->v_mount);
                        break;
                }
                error = cgaccount1(cg, vp, nbp->b_data, passno);
                bawrite(nbp);
                UFS_WAPBL_END(vp->v_mount);
                if (error)
                        break;
        }
        return error;
}

/*
 * Copy a cylinder group map. All the unallocated blocks are marked
 * BLK_NOCOPY so that the snapshot knows that it need not copy them
 * if they are later written. If passno is one, then this is a first
 * pass, so only setting needs to be done. If passno is 2, then this
 * is a revision to a previous pass which must be undone as the
 * replacement pass is done.
 */
static int
cgaccount1(int cg, struct vnode *vp, void *data, int passno)
{
        struct buf *bp, *ibp;
        struct inode *ip;
        struct cg *cgp;
        struct fs *fs;
        struct lwp *l = curlwp;
        daddr_t base, numblks;
        int error, len, loc, ns __unused, indiroff;

        ip = VTOI(vp);
        fs = ip->i_fs;
        ns = UFS_FSNEEDSWAP(fs);
        error = bread(ip->i_devvp, FFS_FSBTODB(fs, cgtod(fs, cg)),
                (int)fs->fs_cgsize, 0, &bp);
        if (error) {
                return (error);
        }
        cgp = (struct cg *)bp->b_data;
        if (!cg_chkmagic(cgp, ns)) {
                brelse(bp, 0);
                return (EIO);
        }
        ACTIVECG_SET(fs, cg);

        memcpy(data, bp->b_data, fs->fs_cgsize);
        brelse(bp, 0);
        if (fs->fs_cgsize < fs->fs_bsize)
                memset((char *)data + fs->fs_cgsize, 0,
                    fs->fs_bsize - fs->fs_cgsize);
        numblks = howmany(fs->fs_size, fs->fs_frag);
        len = howmany(fs->fs_fpg, fs->fs_frag);
        base = cg * fs->fs_fpg / fs->fs_frag;
        if (base + len >= numblks)
                len = numblks - base - 1;
        loc = 0;
        if (base < UFS_NDADDR) {
                for ( ; loc < UFS_NDADDR; loc++) {
                        if (ffs_isblock(fs, cg_blksfree(cgp, ns), loc))
                                db_assign(ip, loc, BLK_NOCOPY);
                        else if (db_get(ip, loc) == BLK_NOCOPY) {
                                if (passno == 2)
                                        db_assign(ip, loc, 0);
                                else if (passno == 1)
                                        panic("ffs_snapshot: lost direct block");
                        }
                }
        }
        if ((error = ffs_balloc(vp, ffs_lblktosize(fs, (off_t)(base + loc)),
            fs->fs_bsize, l->l_cred, B_METAONLY, &ibp)) != 0)
                return (error);
        indiroff = (base + loc - UFS_NDADDR) % FFS_NINDIR(fs);
        for ( ; loc < len; loc++, indiroff++) {
                if (indiroff >= FFS_NINDIR(fs)) {
                        bawrite(ibp);
                        if ((error = ffs_balloc(vp,
                            ffs_lblktosize(fs, (off_t)(base + loc)),
                            fs->fs_bsize, l->l_cred, B_METAONLY, &ibp)) != 0)
                                return (error);
                        indiroff = 0;
                }
                if (ffs_isblock(fs, cg_blksfree(cgp, ns), loc))
                        idb_assign(ip, ibp->b_data, indiroff, BLK_NOCOPY);
                else if (idb_get(ip, ibp->b_data, indiroff) == BLK_NOCOPY) {
                        if (passno == 2)
                                idb_assign(ip, ibp->b_data, indiroff, 0);
                        else if (passno == 1)
                                panic("ffs_snapshot: lost indirect block");
                }
        }
        bdwrite(ibp);
        return (0);
}

/*
 * Before expunging a snapshot inode, note all the
 * blocks that it claims with BLK_SNAP so that fsck will
 * be able to account for those blocks properly and so
 * that this snapshot knows that it need not copy them
 * if the other snapshot holding them is freed.
 */
static int
expunge(struct vnode *snapvp, struct inode *cancelip, struct fs *fs,
    acctfunc_t acctfunc, int expungetype)
{
        int i, error, ns __unused;
        daddr_t lbn, rlbn;
        daddr_t len, blkno, numblks, blksperindir;
        struct ufs1_dinode *dip1;
        struct ufs2_dinode *dip2;
        struct lwp *l = curlwp;
        void *bap;
        struct buf *bp;
        struct mount *mp;

        ns = UFS_FSNEEDSWAP(fs);
        mp = snapvp->v_mount;

        error = UFS_WAPBL_BEGIN(mp);
        if (error)
                return error;
        /*
         * Prepare to expunge the inode. If its inode block has not
         * yet been copied, then allocate and fill the copy.
         */
        lbn = ffs_fragstoblks(fs, ino_to_fsba(fs, cancelip->i_number));
        error = snapblkaddr(snapvp, lbn, &blkno);
        if (error)
                return error;
        if (blkno != 0) {
                error = bread(snapvp, lbn, fs->fs_bsize,
                    B_MODIFY, &bp);
        } else {
                error = ffs_balloc(snapvp, ffs_lblktosize(fs, (off_t)lbn),
                    fs->fs_bsize, l->l_cred, 0, &bp);
                if (! error)
                        error = rwfsblk(snapvp, B_READ, bp->b_data, lbn);
        }
        if (error) {
                UFS_WAPBL_END(mp);
                return error;
        }
        /*
         * Set a snapshot inode to be a zero length file, regular files
         * or unlinked snapshots to be completely unallocated.
         */
        if (fs->fs_magic == FS_UFS1_MAGIC) {
                dip1 = (struct ufs1_dinode *)bp->b_data +
                    ino_to_fsbo(fs, cancelip->i_number);
                if (cancelip->i_flags & SF_SNAPSHOT) {
                        dip1->di_flags =
                            ufs_rw32(ufs_rw32(dip1->di_flags, ns) |
                            SF_SNAPINVAL, ns);
                }
                if (expungetype == BLK_NOCOPY || cancelip->i_nlink == 0)
                        dip1->di_mode = 0;
                dip1->di_size = 0;
                dip1->di_blocks = 0;
                memset(&dip1->di_db[0], 0, (UFS_NDADDR + UFS_NIADDR) * sizeof(int32_t));
        } else {
                dip2 = (struct ufs2_dinode *)bp->b_data +
                    ino_to_fsbo(fs, cancelip->i_number);
                if (cancelip->i_flags & SF_SNAPSHOT) {
                        dip2->di_flags =
                            ufs_rw32(ufs_rw32(dip2->di_flags, ns) |
                            SF_SNAPINVAL, ns);
                }
                if (expungetype == BLK_NOCOPY || cancelip->i_nlink == 0)
                        dip2->di_mode = 0;
                dip2->di_size = 0;
                dip2->di_blocks = 0;
                memset(&dip2->di_db[0], 0, (UFS_NDADDR + UFS_NIADDR) * sizeof(int64_t));
        }
        bdwrite(bp);
        UFS_WAPBL_END(mp);
        /*
         * Now go through and expunge all the blocks in the file
         * using the function requested.
         */
        numblks = howmany(cancelip->i_size, fs->fs_bsize);
        if (fs->fs_magic == FS_UFS1_MAGIC)
                bap = &cancelip->i_ffs1_db[0];
        else
                bap = &cancelip->i_ffs2_db[0];
        error = (*acctfunc)(snapvp, bap, 0, UFS_NDADDR, fs, 0, expungetype);
        if (error)
                return (error);
        if (fs->fs_magic == FS_UFS1_MAGIC)
                bap = &cancelip->i_ffs1_ib[0];
        else
                bap = &cancelip->i_ffs2_ib[0];
        error = (*acctfunc)(snapvp, bap, 0, UFS_NIADDR, fs, -1, expungetype);
        if (error)
                return (error);
        blksperindir = 1;
        lbn = -UFS_NDADDR;
        len = numblks - UFS_NDADDR;
        rlbn = UFS_NDADDR;
        for (i = 0; len > 0 && i < UFS_NIADDR; i++) {
                error = indiracct(snapvp, ITOV(cancelip), i,
                    ib_get(cancelip, i), lbn, rlbn, len,
                    blksperindir, fs, acctfunc, expungetype);
                if (error)
                        return (error);
                blksperindir *= FFS_NINDIR(fs);
                lbn -= blksperindir + 1;
                len -= blksperindir;
                rlbn += blksperindir;
        }
        return (0);
}

/*
 * Descend an indirect block chain for vnode cancelvp accounting for all
 * its indirect blocks in snapvp.
 */
static int
indiracct(struct vnode *snapvp, struct vnode *cancelvp, int level,
    daddr_t blkno, daddr_t lbn, daddr_t rlbn, daddr_t remblks,
    daddr_t blksperindir, struct fs *fs, acctfunc_t acctfunc, int expungetype)
{
        int error, num, i;
        daddr_t subblksperindir;
        struct indir indirs[UFS_NIADDR + 2];
        daddr_t last;
        void *bap;
        struct buf *bp;

        if (blkno == 0) {
                if (expungetype == BLK_NOCOPY)
                        return (0);
                panic("indiracct: missing indir");
        }
        if ((error = ufs_getlbns(cancelvp, rlbn, indirs, &num)) != 0)
                return (error);
        if (lbn != indirs[num - 1 - level].in_lbn || num < 2)
                panic("indiracct: botched params");
        /*
         * We have to expand bread here since it will deadlock looking
         * up the block number for any blocks that are not in the cache.
         */
        error = ffs_getblk(cancelvp, lbn, FFS_FSBTODB(fs, blkno), fs->fs_bsize,
            false, &bp);
        if (error)
                return error;
        if ((bp->b_oflags & (BO_DONE | BO_DELWRI)) == 0 && (error =
            rwfsblk(bp->b_vp, B_READ, bp->b_data, ffs_fragstoblks(fs, blkno)))) {
                brelse(bp, 0);
                return (error);
        }
        /*
         * Account for the block pointers in this indirect block.
         */
        last = howmany(remblks, blksperindir);
        if (last > FFS_NINDIR(fs))
                last = FFS_NINDIR(fs);
        bap = malloc(fs->fs_bsize, M_DEVBUF, M_WAITOK | M_ZERO);
        memcpy((void *)bap, bp->b_data, fs->fs_bsize);
        brelse(bp, 0);
        error = (*acctfunc)(snapvp, bap, 0, last,
            fs, level == 0 ? rlbn : -1, expungetype);
        if (error || level == 0)
                goto out;
        /*
         * Account for the block pointers in each of the indirect blocks
         * in the levels below us.
         */
        subblksperindir = blksperindir / FFS_NINDIR(fs);
        for (lbn++, level--, i = 0; i < last; i++) {
                error = indiracct(snapvp, cancelvp, level,
                    idb_get(VTOI(snapvp), bap, i), lbn, rlbn, remblks,
                    subblksperindir, fs, acctfunc, expungetype);
                if (error)
                        goto out;
                rlbn += blksperindir;
                lbn -= blksperindir;
                remblks -= blksperindir;
        }
out:
        free(bap, M_DEVBUF);
        return (error);
}

/*
 * Do both snap accounting and map accounting.
 */
static int
fullacct(struct vnode *vp, void *bap, int oldblkp, int lastblkp,
    struct fs *fs, daddr_t lblkno,
    int exptype /* BLK_SNAP or BLK_NOCOPY */)
{
        int error;

        if ((error = snapacct(vp, bap, oldblkp, lastblkp, fs, lblkno, exptype)))
                return (error);
        return (mapacct(vp, bap, oldblkp, lastblkp, fs, lblkno, exptype));
}

/*
 * Identify a set of blocks allocated in a snapshot inode.
 */
static int
snapacct(struct vnode *vp, void *bap, int oldblkp, int lastblkp,
    struct fs *fs, daddr_t lblkno,
    int expungetype /* BLK_SNAP or BLK_NOCOPY */)
{
        struct inode *ip = VTOI(vp);
        struct lwp *l = curlwp;
        struct mount *mp = vp->v_mount;
        daddr_t blkno;
        daddr_t lbn;
        struct buf *ibp;
        int error, n;
        const int wbreak = blocks_in_journal(VFSTOUFS(mp)->um_fs)/8;

        error = UFS_WAPBL_BEGIN(mp);
        if (error)
                return error;
        for ( n = 0; oldblkp < lastblkp; oldblkp++) {
                blkno = idb_get(ip, bap, oldblkp);
                if (blkno == 0 || blkno == BLK_NOCOPY || blkno == BLK_SNAP)
                        continue;
                lbn = ffs_fragstoblks(fs, blkno);
                if (lbn < UFS_NDADDR) {
                        blkno = db_get(ip, lbn);
                        ip->i_flag |= IN_CHANGE | IN_UPDATE;
                } else {
                        error = ffs_balloc(vp, ffs_lblktosize(fs, (off_t)lbn),
                            fs->fs_bsize, l->l_cred, B_METAONLY, &ibp);
                        if (error)
                                break;
                        blkno = idb_get(ip, ibp->b_data,
                            (lbn - UFS_NDADDR) % FFS_NINDIR(fs));
                }
                /*
                 * If we are expunging a snapshot vnode and we
                 * find a block marked BLK_NOCOPY, then it is
                 * one that has been allocated to this snapshot after
                 * we took our current snapshot and can be ignored.
                 */
                if (expungetype == BLK_SNAP && blkno == BLK_NOCOPY) {
                        if (lbn >= UFS_NDADDR)
                                brelse(ibp, 0);
                } else {
                        if (blkno != 0)
                                panic("snapacct: bad block");
                        if (lbn < UFS_NDADDR)
                                db_assign(ip, lbn, expungetype);
                        else {
                                idb_assign(ip, ibp->b_data,
                                    (lbn - UFS_NDADDR) % FFS_NINDIR(fs), expungetype);
                                bdwrite(ibp);
                        }
                }
                if (wbreak > 0 && (++n % wbreak) == 0) {
                        UFS_WAPBL_END(mp);
                        error = UFS_WAPBL_BEGIN(mp);
                        if (error)
                                return error;
                }
        }
        UFS_WAPBL_END(mp);
        return error;
}

/*
 * Account for a set of blocks allocated in a snapshot inode.
 */
static int
mapacct(struct vnode *vp, void *bap, int oldblkp, int lastblkp,
    struct fs *fs, daddr_t lblkno, int expungetype)
{
        daddr_t blkno;
        struct inode *ip;
        struct mount *mp = vp->v_mount;
        ino_t inum;
        int acctit, error, n;
        const int wbreak = blocks_in_journal(VFSTOUFS(mp)->um_fs)/8;

        error = UFS_WAPBL_BEGIN(mp);
        if (error)
                return error;
        ip = VTOI(vp);
        inum = ip->i_number;
        if (lblkno == -1)
                acctit = 0;
        else
                acctit = 1;
        for ( n = 0; oldblkp < lastblkp; oldblkp++, lblkno++) {
                blkno = idb_get(ip, bap, oldblkp);
                if (blkno == 0 || blkno == BLK_NOCOPY)
                        continue;
                if (acctit && expungetype == BLK_SNAP && blkno != BLK_SNAP)
                        *ip->i_snapblklist++ = lblkno;
                if (blkno == BLK_SNAP)
                        blkno = ffs_blkstofrags(fs, lblkno);
                ffs_blkfree_snap(fs, vp, blkno, fs->fs_bsize, inum);
                if (wbreak > 0 && (++n % wbreak) == 0) {
                        UFS_WAPBL_END(mp);
                        error = UFS_WAPBL_BEGIN(mp);
                        if (error)
                                return error;
                }
        }
        UFS_WAPBL_END(mp);
        return (0);
}

/*
 * Number of blocks that fit into the journal or zero if not logging.
 */
static int
blocks_in_journal(struct fs *fs)
{
        off_t bpj;

        if ((fs->fs_flags & FS_DOWAPBL) == 0)
                return 0;
        bpj = 1;
        if (fs->fs_journal_version == UFS_WAPBL_VERSION) {
                switch (fs->fs_journal_location) {
                case UFS_WAPBL_JOURNALLOC_END_PARTITION:
                        bpj = (off_t)fs->fs_journallocs[UFS_WAPBL_EPART_BLKSZ]*
                            fs->fs_journallocs[UFS_WAPBL_EPART_COUNT];
                        break;
                case UFS_WAPBL_JOURNALLOC_IN_FILESYSTEM:
                        bpj = (off_t)fs->fs_journallocs[UFS_WAPBL_INFS_BLKSZ]*
                            fs->fs_journallocs[UFS_WAPBL_INFS_COUNT];
                        break;
                }
        }
        bpj /= fs->fs_bsize;
        return (bpj > 0 ? bpj : 1);
}
#endif /* defined(FFS_NO_SNAPSHOT) */

/*
 * Decrement extra reference on snapshot when last name is removed.
 * It will not be freed until the last open reference goes away.
 */
void
ffs_snapgone(struct vnode *vp)
{
        struct inode *xp, *ip = VTOI(vp);
        struct mount *mp = spec_node_getmountedfs(ip->i_devvp);
        struct fs *fs;
        struct snap_info *si;
        int snaploc;

        si = VFSTOUFS(mp)->um_snapinfo;

        /*
         * Find snapshot in incore list.
         */
        mutex_enter(&si->si_lock);
        TAILQ_FOREACH(xp, &si->si_snapshots, i_nextsnap)
                if (xp == ip)
                        break;
        mutex_exit(&si->si_lock);
        if (xp != NULL)
                vrele(ITOV(ip));
#ifdef DEBUG
        else if (snapdebug)
                printf("ffs_snapgone: lost snapshot vnode %llu\n",
                    (unsigned long long)ip->i_number);
#endif
        /*
         * Delete snapshot inode from superblock. Keep list dense.
         */
        mutex_enter(&si->si_lock);
        fs = ip->i_fs;
        for (snaploc = 0; snaploc < FSMAXSNAP; snaploc++)
                if (fs->fs_snapinum[snaploc] == ip->i_number)
                        break;
        if (snaploc < FSMAXSNAP) {
                for (snaploc++; snaploc < FSMAXSNAP; snaploc++) {
                        if (fs->fs_snapinum[snaploc] == 0)
                                break;
                        fs->fs_snapinum[snaploc - 1] = fs->fs_snapinum[snaploc];
                }
                fs->fs_snapinum[snaploc - 1] = 0;
        }
        si->si_gen++;
        mutex_exit(&si->si_lock);
}

/*
 * Prepare a snapshot file for being removed.
 */
void
ffs_snapremove(struct vnode *vp)
{
        struct inode *ip = VTOI(vp), *xp;
        struct vnode *devvp = ip->i_devvp;
        struct fs *fs = ip->i_fs;
        struct mount *mp = spec_node_getmountedfs(devvp);
        struct buf *ibp;
        struct snap_info *si;
        struct lwp *l = curlwp;
        daddr_t numblks, blkno, dblk;
        int error, loc, last;

        si = VFSTOUFS(mp)->um_snapinfo;
        /*
         * If active, delete from incore list (this snapshot may
         * already have been in the process of being deleted, so
         * would not have been active).
         *
         * Clear copy-on-write flag if last snapshot.
         */
        mutex_enter(&si->si_snaplock);
        mutex_enter(&si->si_lock);
        if (is_active_snapshot(si, ip)) {
                TAILQ_REMOVE(&si->si_snapshots, ip, i_nextsnap);
                if (TAILQ_FIRST(&si->si_snapshots) != 0) {
                        /* Roll back the list of preallocated blocks. */
                        xp = TAILQ_LAST(&si->si_snapshots, inodelst);
                        si->si_snapblklist = xp->i_snapblklist;
                        si->si_gen++;
                        mutex_exit(&si->si_lock);
                        mutex_exit(&si->si_snaplock);
                } else {
                        si->si_snapblklist = 0;
                        si->si_gen++;
                        mutex_exit(&si->si_lock);
                        mutex_exit(&si->si_snaplock);
                        fscow_disestablish(mp, ffs_copyonwrite, devvp);
                }
                if (ip->i_snapblklist != NULL) {
                        free(ip->i_snapblklist, M_UFSMNT);
                        ip->i_snapblklist = NULL;
                }
        } else {
                mutex_exit(&si->si_lock);
                mutex_exit(&si->si_snaplock);
        }
        /*
         * Clear all BLK_NOCOPY fields. Pass any block claims to other
         * snapshots that want them (see ffs_snapblkfree below).
         */
        for (blkno = 1; blkno < UFS_NDADDR; blkno++) {
                dblk = db_get(ip, blkno);
                if (dblk == BLK_NOCOPY || dblk == BLK_SNAP)
                        db_assign(ip, blkno, 0);
                else if ((dblk == ffs_blkstofrags(fs, blkno) &&
                     ffs_snapblkfree(fs, ip->i_devvp, dblk, fs->fs_bsize,
                     ip->i_number))) {
                        DIP_ADD(ip, blocks, -btodb(fs->fs_bsize));
                        db_assign(ip, blkno, 0);
                }
        }
        numblks = howmany(ip->i_size, fs->fs_bsize);
        for (blkno = UFS_NDADDR; blkno < numblks; blkno += FFS_NINDIR(fs)) {
                error = ffs_balloc(vp, ffs_lblktosize(fs, (off_t)blkno),
                    fs->fs_bsize, l->l_cred, B_METAONLY, &ibp);
                if (error)
                        continue;
                if (fs->fs_size - blkno > FFS_NINDIR(fs))
                        last = FFS_NINDIR(fs);
                else
                        last = fs->fs_size - blkno;
                for (loc = 0; loc < last; loc++) {
                        dblk = idb_get(ip, ibp->b_data, loc);
                        if (dblk == BLK_NOCOPY || dblk == BLK_SNAP)
                                idb_assign(ip, ibp->b_data, loc, 0);
                        else if (dblk == ffs_blkstofrags(fs, blkno) &&
                            ffs_snapblkfree(fs, ip->i_devvp, dblk,
                            fs->fs_bsize, ip->i_number)) {
                                DIP_ADD(ip, blocks, -btodb(fs->fs_bsize));
                                idb_assign(ip, ibp->b_data, loc, 0);
                        }
                }
                bawrite(ibp);
                UFS_WAPBL_END(mp);
                error = UFS_WAPBL_BEGIN(mp);
                KASSERT(error == 0);
        }
        /*
         * Clear snapshot flag and drop reference.
         */
        ip->i_flags &= ~(SF_SNAPSHOT | SF_SNAPINVAL);
        DIP_ASSIGN(ip, flags, ip->i_flags);
        ip->i_flag |= IN_CHANGE | IN_UPDATE;
#if defined(QUOTA) || defined(QUOTA2)
        chkdq(ip, DIP(ip, blocks), l->l_cred, FORCE);
        chkiq(ip, 1, l->l_cred, FORCE);
#endif
}

/*
 * Notification that a block is being freed. Return zero if the free
 * should be allowed to proceed. Return non-zero if the snapshot file
 * wants to claim the block. The block will be claimed if it is an
 * uncopied part of one of the snapshots. It will be freed if it is
 * either a BLK_NOCOPY or has already been copied in all of the snapshots.
 * If a fragment is being freed, then all snapshots that care about
 * it must make a copy since a snapshot file can only claim full sized
 * blocks. Note that if more than one snapshot file maps the block,
 * we can pick one at random to claim it. Since none of the snapshots
 * can change, we are assurred that they will all see the same unmodified
 * image. When deleting a snapshot file (see ffs_snapremove above), we
 * must push any of these claimed blocks to one of the other snapshots
 * that maps it. These claimed blocks are easily identified as they will
 * have a block number equal to their logical block number within the
 * snapshot. A copied block can never have this property because they
 * must always have been allocated from a BLK_NOCOPY location.
 */
int
ffs_snapblkfree(struct fs *fs, struct vnode *devvp, daddr_t bno,
    long size, ino_t inum)
{
        struct mount *mp = spec_node_getmountedfs(devvp);
        struct buf *ibp;
        struct inode *ip;
        struct vnode *vp = NULL;
        struct snap_info *si;
        void *saved_data = NULL;
        daddr_t lbn;
        daddr_t blkno;
        uint32_t gen;
        int indiroff = 0, error = 0, claimedblk = 0;

        si = VFSTOUFS(mp)->um_snapinfo;
        lbn = ffs_fragstoblks(fs, bno);
        mutex_enter(&si->si_snaplock);
        mutex_enter(&si->si_lock);
        si->si_owner = curlwp;
                
retry:
        gen = si->si_gen;
        TAILQ_FOREACH(ip, &si->si_snapshots, i_nextsnap) {
                vp = ITOV(ip);
                /*
                 * Lookup block being written.
                 */
                if (lbn < UFS_NDADDR) {
                        blkno = db_get(ip, lbn);
                } else {
                        mutex_exit(&si->si_lock);
                        error = ffs_balloc(vp, ffs_lblktosize(fs, (off_t)lbn),
                            fs->fs_bsize, FSCRED, B_METAONLY, &ibp);
                        if (error) {
                                mutex_enter(&si->si_lock);
                                break;
                        }
                        indiroff = (lbn - UFS_NDADDR) % FFS_NINDIR(fs);
                        blkno = idb_get(ip, ibp->b_data, indiroff);
                        mutex_enter(&si->si_lock);
                        if (gen != si->si_gen) {
                                brelse(ibp, 0);
                                goto retry;
                        }
                }
                /*
                 * Check to see if block needs to be copied.
                 */
                if (blkno == 0) {
                        /*
                         * A block that we map is being freed. If it has not
                         * been claimed yet, we will claim or copy it (below).
                         */
                        claimedblk = 1;
                } else if (blkno == BLK_SNAP) {
                        /*
                         * No previous snapshot claimed the block,
                         * so it will be freed and become a BLK_NOCOPY
                         * (don't care) for us.
                         */
                        if (claimedblk)
                                panic("snapblkfree: inconsistent block type");
                        if (lbn < UFS_NDADDR) {
                                db_assign(ip, lbn, BLK_NOCOPY);
                                ip->i_flag |= IN_CHANGE | IN_UPDATE;
                        } else {
                                idb_assign(ip, ibp->b_data, indiroff,
                                    BLK_NOCOPY);
                                mutex_exit(&si->si_lock);
                                if (ip->i_nlink > 0)
                                        bwrite(ibp);
                                else
                                        bdwrite(ibp);
                                mutex_enter(&si->si_lock);
                                if (gen != si->si_gen)
                                        goto retry;
                        }
                        continue;
                } else /* BLK_NOCOPY or default */ {
                        /*
                         * If the snapshot has already copied the block
                         * (default), or does not care about the block,
                         * it is not needed.
                         */
                        if (lbn >= UFS_NDADDR)
                                brelse(ibp, 0);
                        continue;
                }
                /*
                 * If this is a full size block, we will just grab it
                 * and assign it to the snapshot inode. Otherwise we
                 * will proceed to copy it. See explanation for this
                 * routine as to why only a single snapshot needs to
                 * claim this block.
                 */
                if (size == fs->fs_bsize) {
#ifdef DEBUG
                        if (snapdebug)
                                printf("%s %llu lbn %" PRId64
                                    "from inum %llu\n",
                                    "Grabonremove: snapino",
                                    (unsigned long long)ip->i_number,
                                    lbn, (unsigned long long)inum);
#endif
                        mutex_exit(&si->si_lock);
                        if (lbn < UFS_NDADDR) {
                                db_assign(ip, lbn, bno);
                        } else {
                                idb_assign(ip, ibp->b_data, indiroff, bno);
                                if (ip->i_nlink > 0)
                                        bwrite(ibp);
                                else
                                        bdwrite(ibp);
                        }
                        DIP_ADD(ip, blocks, btodb(size));
                        ip->i_flag |= IN_CHANGE | IN_UPDATE;
                        if (ip->i_nlink > 0 && mp->mnt_wapbl)
                                error = syncsnap(vp);
                        else
                                error = 0;
                        mutex_enter(&si->si_lock);
                        si->si_owner = NULL;
                        mutex_exit(&si->si_lock);
                        mutex_exit(&si->si_snaplock);
                        return (error == 0);
                }
                if (lbn >= UFS_NDADDR)
                        brelse(ibp, 0);
#ifdef DEBUG
                if (snapdebug)
                        printf("%s%llu lbn %" PRId64 " %s %llu size %ld\n",
                            "Copyonremove: snapino ",
                            (unsigned long long)ip->i_number,
                            lbn, "for inum", (unsigned long long)inum, size);
#endif
                /*
                 * If we have already read the old block contents, then
                 * simply copy them to the new block. Note that we need
                 * to synchronously write snapshots that have not been
                 * unlinked, and hence will be visible after a crash,
                 * to ensure their integrity.
                 */
                mutex_exit(&si->si_lock);
                if (saved_data == NULL) {
                        saved_data = malloc(fs->fs_bsize, M_UFSMNT, M_WAITOK);
                        error = rwfsblk(vp, B_READ, saved_data, lbn);
                        if (error) {
                                free(saved_data, M_UFSMNT);
                                saved_data = NULL;
                                mutex_enter(&si->si_lock);
                                break;
                        }
                }
                error = wrsnapblk(vp, saved_data, lbn);
                if (error == 0 && ip->i_nlink > 0 && mp->mnt_wapbl)
                        error = syncsnap(vp);
                mutex_enter(&si->si_lock);
                if (error)
                        break;
                if (gen != si->si_gen)
                        goto retry;
        }
        si->si_owner = NULL;
        mutex_exit(&si->si_lock);
        mutex_exit(&si->si_snaplock);
        if (saved_data)
                free(saved_data, M_UFSMNT);
        /*
         * If we have been unable to allocate a block in which to do
         * the copy, then return non-zero so that the fragment will
         * not be freed. Although space will be lost, the snapshot
         * will stay consistent.
         */
        return (error);
}

/*
 * Associate snapshot files when mounting.
 */
void
ffs_snapshot_mount(struct mount *mp)
{
        struct vnode *devvp = VFSTOUFS(mp)->um_devvp;
        struct fs *fs = VFSTOUFS(mp)->um_fs;
        struct lwp *l = curlwp;
        struct vnode *vp;
        struct inode *ip, *xp;
        struct snap_info *si;
        daddr_t snaplistsize, *snapblklist;
        int i, error, ns __unused, snaploc, loc;

        /*
         * No persistent snapshots on apple ufs file systems.
         */
        if (UFS_MPISAPPLEUFS(VFSTOUFS(mp)))
                return;

        si = VFSTOUFS(mp)->um_snapinfo;
        ns = UFS_FSNEEDSWAP(fs);
        /*
         * XXX The following needs to be set before ffs_truncate or
         * VOP_READ can be called.
         */
        mp->mnt_stat.f_iosize = fs->fs_bsize;
        /*
         * Process each snapshot listed in the superblock.
         */
        vp = NULL;
        mutex_enter(&si->si_lock);
        for (snaploc = 0; snaploc < FSMAXSNAP; snaploc++) {
                if (fs->fs_snapinum[snaploc] == 0)
                        break;
                if ((error = VFS_VGET(mp, fs->fs_snapinum[snaploc],
                    &vp)) != 0) {
                        printf("ffs_snapshot_mount: vget failed %d\n", error);
                        continue;
                }
                ip = VTOI(vp);
                if ((ip->i_flags & (SF_SNAPSHOT | SF_SNAPINVAL)) !=
                    SF_SNAPSHOT) {
                        printf("ffs_snapshot_mount: non-snapshot inode %d\n",
                            fs->fs_snapinum[snaploc]);
                        vput(vp);
                        vp = NULL;
                        for (loc = snaploc + 1; loc < FSMAXSNAP; loc++) {
                                if (fs->fs_snapinum[loc] == 0)
                                        break;
                                fs->fs_snapinum[loc - 1] = fs->fs_snapinum[loc];
                        }
                        fs->fs_snapinum[loc - 1] = 0;
                        snaploc--;
                        continue;
                }

                /*
                 * Read the block hints list. Use an empty list on
                 * read errors.
                 */
                error = vn_rdwr(UIO_READ, vp,
                    (void *)&snaplistsize, sizeof(snaplistsize),
                    ffs_lblktosize(fs, howmany(fs->fs_size, fs->fs_frag)),
                    UIO_SYSSPACE, IO_NODELOCKED|IO_UNIT|IO_ALTSEMANTICS,
                    l->l_cred, NULL, NULL);
                if (error) {
                        printf("ffs_snapshot_mount: read_1 failed %d\n", error);
                        snaplistsize = 1;
                } else
                        snaplistsize = ufs_rw64(snaplistsize, ns);
                snapblklist = malloc(
                    snaplistsize * sizeof(daddr_t), M_UFSMNT, M_WAITOK);
                if (error)
                        snapblklist[0] = 1;
                else {
                        error = vn_rdwr(UIO_READ, vp, (void *)snapblklist,
                            snaplistsize * sizeof(daddr_t),
                            ffs_lblktosize(fs, howmany(fs->fs_size, fs->fs_frag)),
                            UIO_SYSSPACE, IO_NODELOCKED|IO_UNIT|IO_ALTSEMANTICS,
                            l->l_cred, NULL, NULL);
                        for (i = 0; i < snaplistsize; i++)
                                snapblklist[i] = ufs_rw64(snapblklist[i], ns);
                        if (error) {
                                printf("ffs_snapshot_mount: read_2 failed %d\n",
                                    error);
                                snapblklist[0] = 1;
                        }
                }
                ip->i_snapblklist = &snapblklist[0];

                /*
                 * Link it onto the active snapshot list.
                 */
                if (is_active_snapshot(si, ip))
                        panic("ffs_snapshot_mount: %"PRIu64" already on list",
                            ip->i_number);
                else
                        TAILQ_INSERT_TAIL(&si->si_snapshots, ip, i_nextsnap);
                vp->v_vflag |= VV_SYSTEM;
                VOP_UNLOCK(vp);
        }
        /*
         * No usable snapshots found.
         */
        if (vp == NULL) {
                mutex_exit(&si->si_lock);
                return;
        }
        /*
         * Attach the block hints list. We always want to
         * use the list from the newest snapshot.
        */
        xp = TAILQ_LAST(&si->si_snapshots, inodelst);
        si->si_snapblklist = xp->i_snapblklist;
        fscow_establish(mp, ffs_copyonwrite, devvp);
        si->si_gen++;
        mutex_exit(&si->si_lock);
}

/*
 * Disassociate snapshot files when unmounting.
 */
void
ffs_snapshot_unmount(struct mount *mp)
{
        struct vnode *devvp = VFSTOUFS(mp)->um_devvp;
        struct inode *xp;
        struct vnode *vp = NULL;
        struct snap_info *si;

        si = VFSTOUFS(mp)->um_snapinfo;
        mutex_enter(&si->si_lock);
        while ((xp = TAILQ_FIRST(&si->si_snapshots)) != 0) {
                vp = ITOV(xp);
                TAILQ_REMOVE(&si->si_snapshots, xp, i_nextsnap);
                if (xp->i_snapblklist == si->si_snapblklist)
                        si->si_snapblklist = NULL;
                free(xp->i_snapblklist, M_UFSMNT);
                if (xp->i_nlink > 0) {
                        si->si_gen++;
                        mutex_exit(&si->si_lock);
                        vrele(vp);
                        mutex_enter(&si->si_lock);
                }
        }
        si->si_gen++;
        mutex_exit(&si->si_lock);
        if (vp)
                fscow_disestablish(mp, ffs_copyonwrite, devvp);
}

/*
 * Check for need to copy block that is about to be written,
 * copying the block if necessary.
 */
static int
ffs_copyonwrite(void *v, struct buf *bp, bool data_valid)
{
        struct fs *fs;
        struct inode *ip;
        struct vnode *devvp = v, *vp = NULL;
        struct mount *mp = spec_node_getmountedfs(devvp);
        struct snap_info *si;
        void *saved_data = NULL;
        daddr_t lbn, blkno, *snapblklist;
        uint32_t gen;
        int lower, upper, mid, snapshot_locked = 0, error = 0;

        /*
         * Check for valid snapshots.
         */
        si = VFSTOUFS(mp)->um_snapinfo;
        mutex_enter(&si->si_lock);
        ip = TAILQ_FIRST(&si->si_snapshots);
        if (ip == NULL) {
                mutex_exit(&si->si_lock);
                return 0;
        }
        /*
         * First check to see if it is after the file system,
         * in the journal or in the preallocated list.
         * By doing these checks we avoid several potential deadlocks.
         */
        fs = ip->i_fs;
        lbn = ffs_fragstoblks(fs, FFS_DBTOFSB(fs, bp->b_blkno));
        if (bp->b_blkno >= FFS_FSBTODB(fs, fs->fs_size)) {
                mutex_exit(&si->si_lock);
                return 0;
        }
        if ((fs->fs_flags & FS_DOWAPBL) &&
            fs->fs_journal_location == UFS_WAPBL_JOURNALLOC_IN_FILESYSTEM) {
                off_t blk_off, log_start, log_end;

                log_start = (off_t)fs->fs_journallocs[UFS_WAPBL_INFS_ADDR] *
                    fs->fs_journallocs[UFS_WAPBL_INFS_BLKSZ];
                log_end = log_start + fs->fs_journallocs[UFS_WAPBL_INFS_COUNT] *
                    fs->fs_journallocs[UFS_WAPBL_INFS_BLKSZ];
                blk_off = dbtob(bp->b_blkno);
                if (blk_off >= log_start && blk_off < log_end) {
                        mutex_exit(&si->si_lock);
                        return 0;
                }
        }
        snapblklist = si->si_snapblklist;
        upper = (snapblklist != NULL ? snapblklist[0] - 1 : 0);
        lower = 1;
        while (lower <= upper) {
                mid = (lower + upper) / 2;
                if (snapblklist[mid] == lbn)
                        break;
                if (snapblklist[mid] < lbn)
                        lower = mid + 1;
                else
                        upper = mid - 1;
        }
        if (lower <= upper) {
                mutex_exit(&si->si_lock);
                return 0;
        }
        /*
         * Not in the precomputed list, so check the snapshots.
         */
         if (si->si_owner != curlwp) {
                if (!mutex_tryenter(&si->si_snaplock)) {
                        mutex_exit(&si->si_lock);
                        mutex_enter(&si->si_snaplock);
                        mutex_enter(&si->si_lock);
                }
                si->si_owner = curlwp;
                snapshot_locked = 1;
         }
         if (data_valid && bp->b_bcount == fs->fs_bsize)
                saved_data = bp->b_data;
retry:
        gen = si->si_gen;
        TAILQ_FOREACH(ip, &si->si_snapshots, i_nextsnap) {
                vp = ITOV(ip);
                /*
                 * We ensure that everything of our own that needs to be
                 * copied will be done at the time that ffs_snapshot is
                 * called. Thus we can skip the check here which can
                 * deadlock in doing the lookup in ffs_balloc.
                 */
                if (bp->b_vp == vp)
                        continue;
                /*
                 * Check to see if block needs to be copied.
                 */
                if (lbn < UFS_NDADDR) {
                        blkno = db_get(ip, lbn);
                } else {
                        mutex_exit(&si->si_lock);
                        blkno = 0; /* XXX: GCC */
                        if ((error = snapblkaddr(vp, lbn, &blkno)) != 0) {
                                mutex_enter(&si->si_lock);
                                break;
                        }
                        mutex_enter(&si->si_lock);
                        if (gen != si->si_gen)
                                goto retry;
                }
#ifdef DIAGNOSTIC
                if (blkno == BLK_SNAP && bp->b_lblkno >= 0)
                        panic("ffs_copyonwrite: bad copy block");
#endif
                if (blkno != 0)
                        continue;

                if (curlwp == uvm.pagedaemon_lwp) {
                        error = ENOMEM;
                        break;
                }
                /* Only one level of recursion allowed. */
                KASSERT(snapshot_locked);
                /*
                 * Allocate the block into which to do the copy. Since
                 * multiple processes may all try to copy the same block,
                 * we have to recheck our need to do a copy if we sleep
                 * waiting for the lock.
                 *
                 * Because all snapshots on a filesystem share a single
                 * lock, we ensure that we will never be in competition
                 * with another process to allocate a block.
                 */
#ifdef DEBUG
                if (snapdebug) {
                        printf("Copyonwrite: snapino %llu lbn %" PRId64 " for ",
                            (unsigned long long)ip->i_number, lbn);
                        if (bp->b_vp == devvp)
                                printf("fs metadata");
                        else
                                printf("inum %llu", (unsigned long long)
                                    VTOI(bp->b_vp)->i_number);
                        printf(" lblkno %" PRId64 "\n", bp->b_lblkno);
                }
#endif
                /*
                 * If we have already read the old block contents, then
                 * simply copy them to the new block. Note that we need
                 * to synchronously write snapshots that have not been
                 * unlinked, and hence will be visible after a crash,
                 * to ensure their integrity.
                 */
                mutex_exit(&si->si_lock);
                if (saved_data == NULL) {
                        saved_data = malloc(fs->fs_bsize, M_UFSMNT, M_WAITOK);
                        error = rwfsblk(vp, B_READ, saved_data, lbn);
                        if (error) {
                                free(saved_data, M_UFSMNT);
                                saved_data = NULL;
                                mutex_enter(&si->si_lock);
                                break;
                        }
                }
                error = wrsnapblk(vp, saved_data, lbn);
                if (error == 0 && ip->i_nlink > 0 && mp->mnt_wapbl)
                        error = syncsnap(vp);
                mutex_enter(&si->si_lock);
                if (error)
                        break;
                if (gen != si->si_gen)
                        goto retry;
        }
        /*
         * Note that we need to synchronously write snapshots that
         * have not been unlinked, and hence will be visible after
         * a crash, to ensure their integrity.
         */
        if (snapshot_locked) {
                si->si_owner = NULL;
                mutex_exit(&si->si_lock);
                mutex_exit(&si->si_snaplock);
        } else
                mutex_exit(&si->si_lock);
        if (saved_data && saved_data != bp->b_data)
                free(saved_data, M_UFSMNT);
        return error;
}

/*
 * Read from a snapshot.
 */
int
ffs_snapshot_read(struct vnode *vp, struct uio *uio, int ioflag)
{
        struct inode *ip = VTOI(vp);
        struct fs *fs = ip->i_fs;
        struct snap_info *si = VFSTOUFS(vp->v_mount)->um_snapinfo;
        struct buf *bp;
        daddr_t lbn, nextlbn;
        off_t fsbytes, bytesinfile;
        long size, xfersize, blkoffset;
        int error;

        fstrans_start(vp->v_mount, FSTRANS_SHARED);
        mutex_enter(&si->si_snaplock);

        if (ioflag & IO_ALTSEMANTICS)
                fsbytes = ip->i_size;
        else
                fsbytes = ffs_lfragtosize(fs, fs->fs_size);
        for (error = 0, bp = NULL; uio->uio_resid > 0; bp = NULL) {
                bytesinfile = fsbytes - uio->uio_offset;
                if (bytesinfile <= 0)
                        break;
                lbn = ffs_lblkno(fs, uio->uio_offset);
                nextlbn = lbn + 1;
                size = fs->fs_bsize;
                blkoffset = ffs_blkoff(fs, uio->uio_offset);
                xfersize = MIN(MIN(fs->fs_bsize - blkoffset, uio->uio_resid),
                    bytesinfile);

                if (ffs_lblktosize(fs, nextlbn + 1) >= fsbytes) {
                        if (ffs_lblktosize(fs, lbn) + size > fsbytes)
                                size = ffs_fragroundup(fs,
                                    fsbytes - ffs_lblktosize(fs, lbn));
                        error = bread(vp, lbn, size, 0, &bp);
                } else {
                        int nextsize = fs->fs_bsize;
                        error = breadn(vp, lbn,
                            size, &nextlbn, &nextsize, 1, 0, &bp);
                }
                if (error)
                        break;

                /*
                 * We should only get non-zero b_resid when an I/O error
                 * has occurred, which should cause us to break above.
                 * However, if the short read did not cause an error,
                 * then we want to ensure that we do not uiomove bad
                 * or uninitialized data.
                 */
                size -= bp->b_resid;
                if (size < blkoffset + xfersize) {
                        xfersize = size - blkoffset;
                        if (xfersize <= 0)
                                break;
                }
                error = uiomove((char *)bp->b_data + blkoffset, xfersize, uio);
                if (error)
                        break;
                brelse(bp, BC_AGE);
        }
        if (bp != NULL)
                brelse(bp, BC_AGE);

        mutex_exit(&si->si_snaplock);
        fstrans_done(vp->v_mount);
        return error;
}

/*
 * Lookup a snapshots data block address.
 * Simpler than UFS_BALLOC() as we know all metadata is already allocated
 * and safe even for the pagedaemon where we cannot bread().
 */
static int
snapblkaddr(struct vnode *vp, daddr_t lbn, daddr_t *res)
{
        struct indir indirs[UFS_NIADDR + 2];
        struct inode *ip = VTOI(vp);
        struct fs *fs = ip->i_fs;
        struct buf *bp;
        int error, num;

        KASSERT(lbn >= 0);

        if (lbn < UFS_NDADDR) {
                *res = db_get(ip, lbn);
                return 0;
        }
        if ((error = ufs_getlbns(vp, lbn, indirs, &num)) != 0)
                return error;
        if (curlwp == uvm.pagedaemon_lwp) {
                mutex_enter(&bufcache_lock);
                bp = incore(vp, indirs[num-1].in_lbn);
                if (bp && (bp->b_oflags & (BO_DONE | BO_DELWRI))) {
                        *res = idb_get(ip, bp->b_data, indirs[num-1].in_off);
                        error = 0;
                } else
                        error = ENOMEM;
                mutex_exit(&bufcache_lock);
                return error;
        }
        error = bread(vp, indirs[num-1].in_lbn, fs->fs_bsize, 0, &bp);
        if (error == 0) {
                *res = idb_get(ip, bp->b_data, indirs[num-1].in_off);
                brelse(bp, 0);
        }

        return error;
}

/*
 * Read or write the specified block of the filesystem vp resides on
 * from or to the disk bypassing the buffer cache.
 */
static int
rwfsblk(struct vnode *vp, int flags, void *data, daddr_t lbn)
{
        int error;
        struct inode *ip = VTOI(vp);
        struct fs *fs = ip->i_fs;
        struct buf *nbp;

        nbp = getiobuf(NULL, true);
        nbp->b_flags = flags;
        nbp->b_bcount = nbp->b_bufsize = fs->fs_bsize;
        nbp->b_error = 0;
        nbp->b_data = data;
        nbp->b_blkno = nbp->b_rawblkno = FFS_FSBTODB(fs, ffs_blkstofrags(fs, lbn));
        nbp->b_proc = NULL;
        nbp->b_dev = ip->i_devvp->v_rdev;
        SET(nbp->b_cflags, BC_BUSY);    /* mark buffer busy */

        bdev_strategy(nbp);

        error = biowait(nbp);

        putiobuf(nbp);

        return error;
}

/*
 * Write all dirty buffers to disk and invalidate them.
 */
static int
syncsnap(struct vnode *vp)
{
        int error;
        buf_t *bp;
        struct fs *fs = VTOI(vp)->i_fs;

        mutex_enter(&bufcache_lock);
        while ((bp = LIST_FIRST(&vp->v_dirtyblkhd))) {
                error = bbusy(bp, false, 0, NULL);
                if (error == EPASSTHROUGH)
                        continue;
                else if (error != 0) {
                        mutex_exit(&bufcache_lock);
                        return error;
                }
                KASSERT(bp->b_bcount == fs->fs_bsize);
                mutex_exit(&bufcache_lock);
                error = rwfsblk(vp, B_WRITE, bp->b_data,
                    ffs_fragstoblks(fs, FFS_DBTOFSB(fs, bp->b_blkno)));
                brelse(bp, BC_INVAL | BC_VFLUSH);
                if (error)
                        return error;
                mutex_enter(&bufcache_lock);
        }
        mutex_exit(&bufcache_lock);

        return 0;
}

/*
 * Write the specified block to a snapshot.
 */
static int
wrsnapblk(struct vnode *vp, void *data, daddr_t lbn)
{
        struct inode *ip = VTOI(vp);
        struct fs *fs = ip->i_fs;
        struct buf *bp;
        int error;

        error = ffs_balloc(vp, ffs_lblktosize(fs, (off_t)lbn), fs->fs_bsize,
            FSCRED, (ip->i_nlink > 0 ? B_SYNC : 0), &bp);
        if (error)
                return error;
        memcpy(bp->b_data, data, fs->fs_bsize);
        if (ip->i_nlink > 0)
                error = bwrite(bp);
        else
                bawrite(bp);

        return error;
}

/*
 * Check if this inode is present on the active snapshot list.
 * Must be called with snapinfo locked.
 */
static inline bool
is_active_snapshot(struct snap_info *si, struct inode *ip)
{
        struct inode *xp;

        KASSERT(mutex_owned(&si->si_lock));

        TAILQ_FOREACH(xp, &si->si_snapshots, i_nextsnap)
                if (xp == ip)
                        return true;
        return false;
}

/*
 * Get/Put direct block from inode or buffer containing disk addresses. Take
 * care for fs type (UFS1/UFS2) and byte swapping. These functions should go
 * into a global include.
 */
static inline daddr_t
db_get(struct inode *ip, int loc)
{
        if (ip->i_ump->um_fstype == UFS1)
                return ufs_rw32(ip->i_ffs1_db[loc], UFS_IPNEEDSWAP(ip));
        else
                return ufs_rw64(ip->i_ffs2_db[loc], UFS_IPNEEDSWAP(ip));
}

static inline void
db_assign(struct inode *ip, int loc, daddr_t val)
{
        if (ip->i_ump->um_fstype == UFS1)
                ip->i_ffs1_db[loc] = ufs_rw32(val, UFS_IPNEEDSWAP(ip));
        else
                ip->i_ffs2_db[loc] = ufs_rw64(val, UFS_IPNEEDSWAP(ip));
}

__unused static inline daddr_t
ib_get(struct inode *ip, int loc)
{
        if (ip->i_ump->um_fstype == UFS1)
                return ufs_rw32(ip->i_ffs1_ib[loc], UFS_IPNEEDSWAP(ip));
        else
                return ufs_rw64(ip->i_ffs2_ib[loc], UFS_IPNEEDSWAP(ip));
}

static inline daddr_t
idb_get(struct inode *ip, void *bf, int loc)
{
        if (ip->i_ump->um_fstype == UFS1)
                return ufs_rw32(((int32_t *)(bf))[loc], UFS_IPNEEDSWAP(ip));
        else
                return ufs_rw64(((int64_t *)(bf))[loc], UFS_IPNEEDSWAP(ip));
}

static inline void
idb_assign(struct inode *ip, void *bf, int loc, daddr_t val)
{
        if (ip->i_ump->um_fstype == UFS1)
                ((int32_t *)(bf))[loc] = ufs_rw32(val, UFS_IPNEEDSWAP(ip));
        else
                ((int64_t *)(bf))[loc] = ufs_rw64(val, UFS_IPNEEDSWAP(ip));
}