text: /*
   * 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
   */
  
  #include <sys/cdefs.h>
  __FBSDID("$FreeBSD: src/sys/ufs/ffs/ffs_snapshot.c,v 1.76 2003/11/13 03:56:32 alc Exp $");
  
  #include <sys/param.h>
  #include <sys/kernel.h>
  #include <sys/systm.h>
  #include <sys/conf.h>
  #include <sys/bio.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 <ufs/ufs/extattr.h>
  #include <ufs/ufs/quota.h>
  #include <ufs/ufs/ufsmount.h>
  #include <ufs/ufs/inode.h>
  #include <ufs/ufs/ufs_extern.h>
  
  #include <ufs/ffs/fs.h>
  #include <ufs/ffs/ffs_extern.h>
  
  #define KERNCRED thread0.td_ucred
  #define DEBUG 1
  
  static int cgaccount(int, struct vnode *, struct buf *, int);
  static int expunge_ufs1(struct vnode *, struct inode *, struct fs *,
      int (*)(struct vnode *, ufs1_daddr_t *, ufs1_daddr_t *, struct fs *,
      ufs_lbn_t, int), int);
  static int indiracct_ufs1(struct vnode *, struct vnode *, int,
      ufs1_daddr_t, ufs_lbn_t, ufs_lbn_t, ufs_lbn_t, ufs_lbn_t, struct fs *,
      int (*)(struct vnode *, ufs1_daddr_t *, ufs1_daddr_t *, struct fs *,
      ufs_lbn_t, int), int);
  static int fullacct_ufs1(struct vnode *, ufs1_daddr_t *, ufs1_daddr_t *,
      struct fs *, ufs_lbn_t, int);
  static int snapacct_ufs1(struct vnode *, ufs1_daddr_t *, ufs1_daddr_t *,
      struct fs *, ufs_lbn_t, int);
  static int mapacct_ufs1(struct vnode *, ufs1_daddr_t *, ufs1_daddr_t *,
      struct fs *, ufs_lbn_t, int);
  static int expunge_ufs2(struct vnode *, struct inode *, struct fs *,
      int (*)(struct vnode *, ufs2_daddr_t *, ufs2_daddr_t *, struct fs *,
      ufs_lbn_t, int), int);
  static int indiracct_ufs2(struct vnode *, struct vnode *, int,
      ufs2_daddr_t, ufs_lbn_t, ufs_lbn_t, ufs_lbn_t, ufs_lbn_t, struct fs *,
      int (*)(struct vnode *, ufs2_daddr_t *, ufs2_daddr_t *, struct fs *,
      ufs_lbn_t, int), int);
  static int fullacct_ufs2(struct vnode *, ufs2_daddr_t *, ufs2_daddr_t *,
      struct fs *, ufs_lbn_t, int);
  static int snapacct_ufs2(struct vnode *, ufs2_daddr_t *, ufs2_daddr_t *,
      struct fs *, ufs_lbn_t, int);
  static int mapacct_ufs2(struct vnode *, ufs2_daddr_t *, ufs2_daddr_t *,
      struct fs *, ufs_lbn_t, int);
  static int ffs_copyonwrite(struct vnode *, struct buf *);
  static int readblock(struct buf *, ufs2_daddr_t);
  
  /*
   * To ensure the consistency of snapshots across crashes, we must
   * synchronously write out copied blocks before allowing the
   * originals to be modified. Because of the rather severe speed
   * penalty that this imposes, the following flag allows this
   * crash persistence to be disabled.
   */
  int dopersistence = 0;
  
  #ifdef DEBUG
  #include <sys/sysctl.h>
  SYSCTL_INT(_debug, OID_AUTO, dopersistence, CTLFLAG_RW, &dopersistence, 0, "");
  static int snapdebug = 0;
  SYSCTL_INT(_debug, OID_AUTO, snapdebug, CTLFLAG_RW, &snapdebug, 0, "");
  int collectsnapstats = 0;
  SYSCTL_INT(_debug, OID_AUTO, collectsnapstats, CTLFLAG_RW, &collectsnapstats,
  	0, "");
  #endif /* DEBUG */
  
  /*
   * Create a snapshot file and initialize it for the filesystem.
   */
  int
  ffs_snapshot(mp, snapfile)
  	struct mount *mp;
  	char *snapfile;
  {
  	ufs2_daddr_t numblks, blkno, *blkp, *snapblklist;
  	int error, cg, snaploc;
  	int i, size, len, loc;
  	int flag = mp->mnt_flag;
  	struct timespec starttime = {0, 0}, endtime;
  	char saved_nice = 0;
  	long redo = 0, snaplistsize = 0;
  	int32_t *lp;
  	void *space;
  	struct fs *copy_fs = NULL, *fs = VFSTOUFS(mp)->um_fs;
  	struct snaphead *snaphead;
  	struct thread *td = curthread;
  	struct inode *ip, *xp;
  	struct buf *bp, *nbp, *ibp, *sbp = NULL;
  	struct nameidata nd;
  	struct mount *wrtmp;
  	struct vattr vat;
  	struct vnode *vp, *xvp, *nvp, *devvp;
  	struct uio auio;
  	struct iovec aiov;
  
  	/*
  	 * Need to serialize access to snapshot code per filesystem.
  	 */
  	/*
  	 * Assign a snapshot slot in the superblock.
  	 */
  	for (snaploc = 0; snaploc < FSMAXSNAP; snaploc++)
  		if (fs->fs_snapinum[snaploc] == 0)
  			break;
  	if (snaploc == FSMAXSNAP)
  		return (ENOSPC);
  	/*
  	 * Create the snapshot file.
  	 */
  restart:
  	NDINIT(&nd, CREATE, LOCKPARENT | LOCKLEAF, UIO_USERSPACE, snapfile, td);
  	if ((error = namei(&nd)) != 0)
  		return (error);
  	if (nd.ni_vp != NULL) {
  		vput(nd.ni_vp);
  		error = EEXIST;
  	}
  	if (nd.ni_dvp->v_mount != mp)
  		error = EXDEV;
  	if (error) {
  		NDFREE(&nd, NDF_ONLY_PNBUF);
  		if (nd.ni_dvp == nd.ni_vp)
  			vrele(nd.ni_dvp);
  		else
  			vput(nd.ni_dvp);
  		return (error);
  	}
  	VATTR_NULL(&vat);
  	vat.va_type = VREG;
  	vat.va_mode = S_IRUSR;
  	vat.va_vaflags |= VA_EXCLUSIVE;
  	if (VOP_GETWRITEMOUNT(nd.ni_dvp, &wrtmp))
  		wrtmp = NULL;
  	if (wrtmp != mp)
  		panic("ffs_snapshot: mount mismatch");
  	if (vn_start_write(NULL, &wrtmp, V_NOWAIT) != 0) {
  		NDFREE(&nd, NDF_ONLY_PNBUF);
  		vput(nd.ni_dvp);
  		if ((error = vn_start_write(NULL, &wrtmp,
  		    V_XSLEEP | PCATCH)) != 0)
  			return (error);
  		goto restart;
  	}
  	VOP_LEASE(nd.ni_dvp, td, KERNCRED, LEASE_WRITE);
  	error = VOP_CREATE(nd.ni_dvp, &nd.ni_vp, &nd.ni_cnd, &vat);
  	vput(nd.ni_dvp);
  	if (error) {
  		NDFREE(&nd, NDF_ONLY_PNBUF);
  		vn_finished_write(wrtmp);
  		return (error);
  	}
  	vp = nd.ni_vp;
  	ip = VTOI(vp);
  	devvp = ip->i_devvp;
  	/*
  	 * Allocate and copy the last block contents so as to be able
  	 * to set size to that of the filesystem.
  	 */
  	numblks = howmany(fs->fs_size, fs->fs_frag);
  	error = UFS_BALLOC(vp, lblktosize(fs, (off_t)(numblks - 1)),
  	    fs->fs_bsize, KERNCRED, BA_CLRBUF, &bp);
  	if (error)
  		goto out;
  	ip->i_size = lblktosize(fs, (off_t)numblks);
  	DIP(ip, i_size) = ip->i_size;
  	ip->i_flag |= IN_CHANGE | IN_UPDATE;
  	if ((error = readblock(bp, numblks - 1)) != 0)
  		goto out;
  	bawrite(bp);
  	/*
  	 * 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.
  	 */
  	for (blkno = NDADDR; blkno < numblks; blkno += NINDIR(fs)) {
  		error = UFS_BALLOC(vp, lblktosize(fs, (off_t)blkno),
  		    fs->fs_bsize, td->td_ucred, BA_METAONLY, &ibp);
  		if (error)
  			goto out;
  		bawrite(ibp);
  	}
  	/*
  	 * Allocate copies for the superblock and its summary information.
  	 */
  	error = UFS_BALLOC(vp, fs->fs_sblockloc, fs->fs_sbsize, KERNCRED,
  	    0, &nbp);
  	if (error)
  		goto out;
  	bawrite(nbp);
  	blkno = fragstoblks(fs, fs->fs_csaddr);
  	len = howmany(fs->fs_cssize, fs->fs_bsize);
  	for (loc = 0; loc < len; loc++) {
  		error = UFS_BALLOC(vp, lblktosize(fs, (off_t)(blkno + loc)),
  		    fs->fs_bsize, KERNCRED, 0, &nbp);
  		if (error)
  			goto out;
  		bawrite(nbp);
  	}
  	/*
  	 * Allocate all cylinder group blocks.
  	 */
  	for (cg = 0; cg < fs->fs_ncg; cg++) {
  		error = UFS_BALLOC(vp, lfragtosize(fs, cgtod(fs, cg)),
  		    fs->fs_bsize, KERNCRED, 0, &nbp);
  		if (error)
  			goto out;
  		bawrite(nbp);
  	}
  	/*
  	 * 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.
  	 */
  	len = howmany(fs->fs_ncg, NBBY);
  	MALLOC(fs->fs_active, int *, len, M_DEVBUF, M_WAITOK);
  	bzero(fs->fs_active, len);
  	for (cg = 0; cg < fs->fs_ncg; cg++) {
  		error = UFS_BALLOC(vp, lfragtosize(fs, cgtod(fs, cg)),
  		    fs->fs_bsize, KERNCRED, 0, &nbp);
  		if (error)
  			goto out;
  		error = cgaccount(cg, vp, nbp, 1);
  		bawrite(nbp);
  		if (error)
  			goto out;
  	}
  	/*
  	 * Change inode to snapshot type file.
  	 */
  	ip->i_flags |= SF_SNAPSHOT;
  	DIP(ip, i_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.
  	 */
  	if ((error = VOP_FSYNC(vp, KERNCRED, MNT_WAIT, td)) != 0)
  		goto out;
  	VOP_UNLOCK(vp, 0, td);
  	/*
  	 * All allocations are done, so we can now snapshot the system.
  	 *
  	 * Recind nice scheduling while running with the filesystem suspended.
  	 */
  	if (td->td_ksegrp->kg_nice > 0) {
  		PROC_LOCK(td->td_proc);
  		mtx_lock_spin(&sched_lock);
  		saved_nice = td->td_ksegrp->kg_nice;
  		sched_nice(td->td_ksegrp, 0);
  		mtx_unlock_spin(&sched_lock);
  		PROC_UNLOCK(td->td_proc);
  	}
  	/*
  	 * Suspend operation on filesystem.
  	 */
  	for (;;) {
  		vn_finished_write(wrtmp);
  		if ((error = vfs_write_suspend(vp->v_mount)) != 0) {
  			vn_start_write(NULL, &wrtmp, V_WAIT);
  			vn_lock(vp, LK_EXCLUSIVE | LK_RETRY, td);
  			goto out;
  		}
  		if (mp->mnt_kern_flag & MNTK_SUSPENDED)
  			break;
  		vn_start_write(NULL, &wrtmp, V_WAIT);
  	}
  	vn_lock(vp, LK_EXCLUSIVE | LK_RETRY, td);
  	if (collectsnapstats)
  		nanotime(&starttime);
  	/*
  	 * First, copy all the cylinder group maps that have changed.
  	 */
  	for (cg = 0; cg < fs->fs_ncg; cg++) {
  		if ((ACTIVECGNUM(fs, cg) & ACTIVECGOFF(cg)) != 0)
  			continue;
  		redo++;
  		error = UFS_BALLOC(vp, lfragtosize(fs, cgtod(fs, cg)),
  		    fs->fs_bsize, KERNCRED, 0, &nbp);
  		if (error)
  			goto out1;
  		error = cgaccount(cg, vp, nbp, 2);
  		bawrite(nbp);
  		if (error)
  			goto out1;
  	}
  	/*
  	 * Grab a copy of the superblock and its summary information.
  	 * We delay writing it until the suspension is released below.
  	 */
  	error = bread(vp, lblkno(fs, fs->fs_sblockloc), fs->fs_bsize,
  	    KERNCRED, &sbp);
  	if (error) {
  		brelse(sbp);
  		sbp = NULL;
  		goto out1;
  	}
  	loc = blkoff(fs, fs->fs_sblockloc);
  	copy_fs = (struct fs *)(sbp->b_data + loc);
  	bcopy(fs, copy_fs, fs->fs_sbsize);
  	if ((fs->fs_flags & (FS_UNCLEAN | FS_NEEDSFSCK)) == 0)
  		copy_fs->fs_clean = 1;
  	size = fs->fs_bsize < SBLOCKSIZE ? fs->fs_bsize : SBLOCKSIZE;
  	if (fs->fs_sbsize < size)
  		bzero(&sbp->b_data[loc + fs->fs_sbsize], size - fs->fs_sbsize);
  	size = blkroundup(fs, fs->fs_cssize);
  	if (fs->fs_contigsumsize > 0)
  		size += fs->fs_ncg * sizeof(int32_t);
  	space = malloc((u_long)size, M_UFSMNT, M_WAITOK);
  	copy_fs->fs_csp = space;
  	bcopy(fs->fs_csp, copy_fs->fs_csp, fs->fs_cssize);
  	(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, fsbtodb(fs, fs->fs_csaddr + loc),
  		    len, KERNCRED, &bp)) != 0) {
  			brelse(bp);
  			free(copy_fs->fs_csp, M_UFSMNT);
  			bawrite(sbp);
  			sbp = NULL;
  			goto out1;
  		}
  		bcopy(bp->b_data, space, (u_int)len);
  		(char *)space += len;
  		bp->b_flags |= B_INVAL | B_NOCACHE;
  		brelse(bp);
  	}
  	if (fs->fs_contigsumsize > 0) {
  		copy_fs->fs_maxcluster = lp = space;
  		for (i = 0; i < fs->fs_ncg; i++)
  			*lp++ = fs->fs_contigsumsize;
  	}
  	/*
  	 * 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.
  	 * We turn off the MNTK_SUSPENDED flag to avoid a panic from
  	 * spec_strategy about writing on a suspended filesystem.
  	 * Note that we skip unlinked snapshot files as they will
  	 * be handled separately below.
  	 *
  	 * 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 */;
  	mp->mnt_kern_flag &= ~MNTK_SUSPENDED;
  	MNT_ILOCK(mp);
  loop:
  	for (xvp = TAILQ_FIRST(&mp->mnt_nvnodelist); xvp; xvp = nvp) {
  		/*
  		 * Make sure this vnode wasn't reclaimed in getnewvnode().
  		 * Start over if it has (it won't be on the list anymore).
  		 */
  		if (xvp->v_mount != mp)
  			goto loop;
  		nvp = TAILQ_NEXT(xvp, v_nmntvnodes);
  		VI_LOCK(xvp);
  		MNT_IUNLOCK(mp);
  		if ((xvp->v_iflag & VI_XLOCK) ||
  		    xvp->v_usecount == 0 || xvp->v_type == VNON ||
  		    (VTOI(xvp)->i_flags & SF_SNAPSHOT)) {
  			VI_UNLOCK(xvp);
  			MNT_ILOCK(mp);
  			continue;
  		}
  		if (snapdebug)
  			vprint("ffs_snapshot: busy vnode", xvp);
  		if (vn_lock(xvp, LK_EXCLUSIVE | LK_INTERLOCK, td) != 0) {
  			MNT_ILOCK(mp);
  			goto loop;
  		}
  		if (VOP_GETATTR(xvp, &vat, td->td_ucred, td) == 0 &&
  		    vat.va_nlink > 0) {
  			VOP_UNLOCK(xvp, 0, td);
  			MNT_ILOCK(mp);
  			continue;
  		}
  		xp = VTOI(xvp);
  		if (ffs_checkfreefile(copy_fs, vp, xp->i_number)) {
  			VOP_UNLOCK(xvp, 0, td);
  			MNT_ILOCK(mp);
  			continue;
  		}
  		/*
  		 * If there is a fragment, clear it here.
  		 */
  		blkno = 0;
  		loc = howmany(xp->i_size, fs->fs_bsize) - 1;
  		if (loc < NDADDR) {
  			len = fragroundup(fs, blkoff(fs, xp->i_size));
  			if (len < fs->fs_bsize) {
  				ffs_blkfree(copy_fs, vp, DIP(xp, i_db[loc]),
  				    len, xp->i_number);
  				blkno = DIP(xp, i_db[loc]);
  				DIP(xp, i_db[loc]) = 0;
  			}
  		}
  		snaplistsize += 1;
  		if (xp->i_ump->um_fstype == UFS1)
  			error = expunge_ufs1(vp, xp, copy_fs, fullacct_ufs1,
  			    BLK_NOCOPY);
  		else
  			error = expunge_ufs2(vp, xp, copy_fs, fullacct_ufs2,
  			    BLK_NOCOPY);
  		if (blkno)
  			DIP(xp, i_db[loc]) = blkno;
  		if (!error)
  			error = ffs_freefile(copy_fs, vp, xp->i_number,
  			    xp->i_mode);
  		VOP_UNLOCK(xvp, 0, td);
  		if (error) {
  			free(copy_fs->fs_csp, M_UFSMNT);
  			bawrite(sbp);
  			sbp = NULL;
  			goto out1;
  		}
  		MNT_ILOCK(mp);
  	}
  	MNT_IUNLOCK(mp);
  	/*
  	 * If there already exist snapshots on this filesystem, grab a
  	 * reference to their shared lock. If this is the first snapshot
  	 * on this filesystem, we need to allocate a lock for the snapshots
  	 * to share. In either case, acquire the snapshot lock and give
  	 * up our original private lock.
  	 */
  	VI_LOCK(devvp);
  	snaphead = &devvp->v_rdev->si_snapshots;
  	if ((xp = TAILQ_FIRST(snaphead)) != NULL) {
  		VI_LOCK(vp);
  		vp->v_vnlock = ITOV(xp)->v_vnlock;
  		VI_UNLOCK(devvp);
  	} else {
  		struct lock *lkp;
  
  		VI_UNLOCK(devvp);
  		MALLOC(lkp, struct lock *, sizeof(struct lock), M_UFSMNT,
  		    M_WAITOK);
  		lockinit(lkp, PVFS, "snaplk", VLKTIMEOUT,
  		    LK_CANRECURSE | LK_NOPAUSE);
  		VI_LOCK(vp);
  		vp->v_vnlock = lkp;
  	}
  	vn_lock(vp, LK_INTERLOCK | LK_EXCLUSIVE | LK_RETRY, td);
  	transferlockers(&vp->v_lock, vp->v_vnlock);
  	lockmgr(&vp->v_lock, LK_RELEASE, NULL, td);
  	/*
  	 * If this is the first snapshot on this filesystem, then we need
  	 * to allocate the space for the list of preallocated snapshot blocks.
  	 * This list will be refined below, but this preliminary one will
  	 * keep us out of deadlock until the full one is ready.
  	 */
  	if (xp == NULL) {
  		MALLOC(snapblklist, daddr_t *, snaplistsize * sizeof(daddr_t),
  		    M_UFSMNT, M_WAITOK);
  		blkp = &snapblklist[1];
  		*blkp++ = lblkno(fs, fs->fs_sblockloc);
  		blkno = fragstoblks(fs, fs->fs_csaddr);
  		for (cg = 0; cg < fs->fs_ncg; cg++) {
  			if (fragstoblks(fs, cgtod(fs, cg) > blkno))
  				break;
  			*blkp++ = 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++ = fragstoblks(fs, cgtod(fs, cg));
  		snapblklist[0] = blkp - snapblklist;
  		VI_LOCK(devvp);
  		if (devvp->v_rdev->si_snapblklist != NULL)
  			panic("ffs_snapshot: non-empty list");
  		devvp->v_rdev->si_snapblklist = snapblklist;
  		devvp->v_rdev->si_snaplistsize = blkp - snapblklist;
  		VI_UNLOCK(devvp);
  	}
  	/*
  	 * Record snapshot inode. Since this is the newest snapshot,
  	 * it must be placed at the end of the list.
  	 */
  	VI_LOCK(devvp);
  	fs->fs_snapinum[snaploc] = ip->i_number;
  	if (ip->i_nextsnap.tqe_prev != 0)
  		panic("ffs_snapshot: %d already on list", ip->i_number);
  	TAILQ_INSERT_TAIL(snaphead, ip, i_nextsnap);
  	devvp->v_rdev->si_copyonwrite = ffs_copyonwrite;
  	devvp->v_vflag |= VV_COPYONWRITE;
  	VI_UNLOCK(devvp);
  	ASSERT_VOP_LOCKED(vp, "ffs_snapshot vp");
  	vp->v_vflag |= VV_SYSTEM;
  out1:
  	/*
  	 * Resume operation on filesystem.
  	 */
  	vfs_write_resume(vp->v_mount);
  	vn_start_write(NULL, &wrtmp, V_WAIT);
  	if (collectsnapstats && starttime.tv_sec > 0) {
  		nanotime(&endtime);
  		timespecsub(&endtime, &starttime);
  		printf("%s: suspended %ld.%03ld sec, redo %ld of %d\n",
  		    vp->v_mount->mnt_stat.f_mntonname, (long)endtime.tv_sec,
  		    endtime.tv_nsec / 1000000, redo, fs->fs_ncg);
  	}
  	if (sbp == NULL)
  		goto out;
  	/*
  	 * Copy allocation information from all the snapshots in
  	 * this snapshot and then expunge them from its view.
  	 */
  	snaphead = &devvp->v_rdev->si_snapshots;
  	TAILQ_FOREACH(xp, snaphead, i_nextsnap) {
  		if (xp == ip)
  			break;
  		if (xp->i_ump->um_fstype == UFS1)
  			error = expunge_ufs1(vp, xp, fs, snapacct_ufs1,
  			    BLK_SNAP);
  		else
  			error = expunge_ufs2(vp, xp, fs, snapacct_ufs2,
  			    BLK_SNAP);
  		if (error) {
  			fs->fs_snapinum[snaploc] = 0;
  			goto done;
  		}
  	}
  	/*
  	 * Allocate space for the full list of preallocated snapshot blocks.
  	 */
  	MALLOC(snapblklist, daddr_t *, snaplistsize * sizeof(daddr_t),
  	    M_UFSMNT, M_WAITOK);
  	ip->i_snapblklist = &snapblklist[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.
  	 */
  	if (ip->i_ump->um_fstype == UFS1)
  		error = expunge_ufs1(vp, ip, copy_fs, mapacct_ufs1, BLK_SNAP);
  	else
  		error = expunge_ufs2(vp, ip, copy_fs, mapacct_ufs2, BLK_SNAP);
  	if (error) {
  		fs->fs_snapinum[snaploc] = 0;
  		FREE(snapblklist, M_UFSMNT);
  		goto done;
  	}
  	if (snaplistsize < ip->i_snapblklist - snapblklist)
  		panic("ffs_snapshot: list too small");
  	snaplistsize = ip->i_snapblklist - snapblklist;
  	snapblklist[0] = snaplistsize;
  	ip->i_snapblklist = 0;
  	/*
  	 * Write out the list of allocated blocks to the end of the snapshot.
  	 */
  	auio.uio_iov = &aiov;
  	auio.uio_iovcnt = 1;
  	aiov.iov_base = (void *)snapblklist;
  	aiov.iov_len = snaplistsize * sizeof(daddr_t);
  	auio.uio_resid = aiov.iov_len;;
  	auio.uio_offset = ip->i_size;
  	auio.uio_segflg = UIO_SYSSPACE;
  	auio.uio_rw = UIO_WRITE;
  	auio.uio_td = td;
  	if ((error = VOP_WRITE(vp, &auio, IO_UNIT, td->td_ucred)) != 0) {
  		fs->fs_snapinum[snaploc] = 0;
  		FREE(snapblklist, M_UFSMNT);
  		goto done;
  	}
  	/*
  	 * Write the superblock and its summary information
  	 * to the snapshot.
  	 */
  	blkno = fragstoblks(fs, fs->fs_csaddr);
  	len = howmany(fs->fs_cssize, fs->fs_bsize);
  	space = copy_fs->fs_csp;
  	for (loc = 0; loc < len; loc++) {
  		error = bread(vp, blkno + loc, fs->fs_bsize, KERNCRED, &nbp);
  		if (error) {
  			brelse(nbp);
  			fs->fs_snapinum[snaploc] = 0;
  			FREE(snapblklist, M_UFSMNT);
  			goto done;
  		}
  		bcopy(space, nbp->b_data, fs->fs_bsize);
  		space = (char *)space + fs->fs_bsize;
  		bawrite(nbp);
  	}
  	/*
  	 * As this is the newest list, it is the most inclusive, so
  	 * should replace the previous list.
  	 */
  	VI_LOCK(devvp);
  	space = devvp->v_rdev->si_snapblklist;
  	devvp->v_rdev->si_snapblklist = snapblklist;
  	devvp->v_rdev->si_snaplistsize = snaplistsize;
  	VI_UNLOCK(devvp);
  	if (space != NULL)
  		FREE(space, M_UFSMNT);
  done:
  	free(copy_fs->fs_csp, M_UFSMNT);
  	bawrite(sbp);
  out:
  	if (saved_nice > 0) {
  		PROC_LOCK(td->td_proc);
  		mtx_lock_spin(&sched_lock);
  		sched_nice(td->td_ksegrp, saved_nice);
  		mtx_unlock_spin(&sched_lock);
  		PROC_UNLOCK(td->td_proc);
  	}
  	if (fs->fs_active != 0) {
  		FREE(fs->fs_active, M_DEVBUF);
  		fs->fs_active = 0;
  	}
  	mp->mnt_flag = flag;
  	if (error)
  		(void) UFS_TRUNCATE(vp, (off_t)0, 0, NOCRED, td);
  	(void) VOP_FSYNC(vp, KERNCRED, MNT_WAIT, td);
  	if (error)
  		vput(vp);
  	else
  		VOP_UNLOCK(vp, 0, td);
  	vn_finished_write(wrtmp);
  	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
  cgaccount(cg, vp, nbp, passno)
  	int cg;
  	struct vnode *vp;
  	struct buf *nbp;
  	int passno;
  {
  	struct buf *bp, *ibp;
  	struct inode *ip;
  	struct cg *cgp;
  	struct fs *fs;
  	ufs2_daddr_t base, numblks;
  	int error, len, loc, indiroff;
  
  	ip = VTOI(vp);
  	fs = ip->i_fs;
  	error = bread(ip->i_devvp, fsbtodb(fs, cgtod(fs, cg)),
  		(int)fs->fs_cgsize, KERNCRED, &bp);
  	if (error) {
  		brelse(bp);
  		return (error);
  	}
  	cgp = (struct cg *)bp->b_data;
  	if (!cg_chkmagic(cgp)) {
  		brelse(bp);
  		return (EIO);
  	}
  	atomic_set_int(&ACTIVECGNUM(fs, cg), ACTIVECGOFF(cg));
  	bcopy(bp->b_data, nbp->b_data, fs->fs_cgsize);
  	if (fs->fs_cgsize < fs->fs_bsize)
  		bzero(&nbp->b_data[fs->fs_cgsize],
  		    fs->fs_bsize - fs->fs_cgsize);
  	if (passno == 2)
  		nbp->b_flags |= B_VALIDSUSPWRT;
  	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 < NDADDR) {
  		for ( ; loc < NDADDR; loc++) {
  			if (ffs_isblock(fs, cg_blksfree(cgp), loc))
  				DIP(ip, i_db[loc]) = BLK_NOCOPY;
  			else if (passno == 2 && DIP(ip, i_db[loc])== BLK_NOCOPY)
  				DIP(ip, i_db[loc]) = 0;
  			else if (passno == 1 && DIP(ip, i_db[loc])== BLK_NOCOPY)
  				panic("ffs_snapshot: lost direct block");
  		}
  	}
  	error = UFS_BALLOC(vp, lblktosize(fs, (off_t)(base + loc)),
  	    fs->fs_bsize, KERNCRED, BA_METAONLY, &ibp);
  	if (error) {
  		brelse(bp);
  		return (error);
  	}
  	indiroff = (base + loc - NDADDR) % NINDIR(fs);
  	for ( ; loc < len; loc++, indiroff++) {
  		if (indiroff >= NINDIR(fs)) {
  			if (passno == 2)
  				ibp->b_flags |= B_VALIDSUSPWRT;
  			bawrite(ibp);
  			error = UFS_BALLOC(vp,
  			    lblktosize(fs, (off_t)(base + loc)),
  			    fs->fs_bsize, KERNCRED, BA_METAONLY, &ibp);
  			if (error) {
  				brelse(bp);
  				return (error);
  			}
  			indiroff = 0;
  		}
  		if (ip->i_ump->um_fstype == UFS1) {
  			if (ffs_isblock(fs, cg_blksfree(cgp), loc))
  				((ufs1_daddr_t *)(ibp->b_data))[indiroff] =
  				    BLK_NOCOPY;
  			else if (passno == 2 && ((ufs1_daddr_t *)(ibp->b_data))
  			    [indiroff] == BLK_NOCOPY)
  				((ufs1_daddr_t *)(ibp->b_data))[indiroff] = 0;
  			else if (passno == 1 && ((ufs1_daddr_t *)(ibp->b_data))
  			    [indiroff] == BLK_NOCOPY)
  				panic("ffs_snapshot: lost indirect block");
  			continue;
  		}
  		if (ffs_isblock(fs, cg_blksfree(cgp), loc))
  			((ufs2_daddr_t *)(ibp->b_data))[indiroff] = BLK_NOCOPY;
  		else if (passno == 2 &&
  		    ((ufs2_daddr_t *)(ibp->b_data)) [indiroff] == BLK_NOCOPY)
  			((ufs2_daddr_t *)(ibp->b_data))[indiroff] = 0;
  		else if (passno == 1 &&
  		    ((ufs2_daddr_t *)(ibp->b_data)) [indiroff] == BLK_NOCOPY)
  			panic("ffs_snapshot: lost indirect block");
  	}
  	bqrelse(bp);
  	if (passno == 2)
  		ibp->b_flags |= B_VALIDSUSPWRT;
  	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. This code
   * is reproduced once each for UFS1 and UFS2.
   */
  static int
  expunge_ufs1(snapvp, cancelip, fs, acctfunc, expungetype)
  	struct vnode *snapvp;
  	struct inode *cancelip;
  	struct fs *fs;
  	int (*acctfunc)(struct vnode *, ufs1_daddr_t *, ufs1_daddr_t *,
  	    struct fs *, ufs_lbn_t, int);
  	int expungetype;
  {
  	int i, error, indiroff;
  	ufs_lbn_t lbn, rlbn;
  	ufs2_daddr_t len, blkno, numblks, blksperindir;
  	struct ufs1_dinode *dip;
  	struct thread *td = curthread;
  	struct buf *bp;
  
  	/*
  	 * Prepare to expunge the inode. If its inode block has not
  	 * yet been copied, then allocate and fill the copy.
  	 */
  	lbn = fragstoblks(fs, ino_to_fsba(fs, cancelip->i_number));
  	blkno = 0;
  	if (lbn < NDADDR) {
  		blkno = VTOI(snapvp)->i_din1->di_db[lbn];
  	} else {
  		td->td_pflags |= TDP_COWINPROGRESS;
  		error = UFS_BALLOC(snapvp, lblktosize(fs, (off_t)lbn),
  		   fs->fs_bsize, KERNCRED, BA_METAONLY, &bp);
  		td->td_pflags &= ~TDP_COWINPROGRESS;
  		if (error)
  			return (error);
  		indiroff = (lbn - NDADDR) % NINDIR(fs);
  		blkno = ((ufs1_daddr_t *)(bp->b_data))[indiroff];
  		bqrelse(bp);
  	}
  	if (blkno != 0) {
  		if ((error = bread(snapvp, lbn, fs->fs_bsize, KERNCRED, &bp)))
  			return (error);
  	} else {
  		error = UFS_BALLOC(snapvp, lblktosize(fs, (off_t)lbn),
  		    fs->fs_bsize, KERNCRED, 0, &bp);
  		if (error)
  			return (error);
  		if ((error = readblock(bp, lbn)) != 0)
  			return (error);
  	}
  	/*
  	 * Set a snapshot inode to be a zero length file, regular files
  	 * to be completely unallocated.
  	 */
  	dip = (struct ufs1_dinode *)bp->b_data +
  	    ino_to_fsbo(fs, cancelip->i_number);
  	if (expungetype == BLK_NOCOPY)
  		dip->di_mode = 0;
  	dip->di_size = 0;
  	dip->di_blocks = 0;
  	dip->di_flags &= ~SF_SNAPSHOT;
  	bzero(&dip->di_db[0], (NDADDR + NIADDR) * sizeof(ufs1_daddr_t));
  	bdwrite(bp);
  	/*
  	 * Now go through and expunge all the blocks in the file
  	 * using the function requested.
  	 */
  	numblks = howmany(cancelip->i_size, fs->fs_bsize);
  	if ((error = (*acctfunc)(snapvp, &cancelip->i_din1->di_db[0],
  	    &cancelip->i_din1->di_db[NDADDR], fs, 0, expungetype)))
  		return (error);
  	if ((error = (*acctfunc)(snapvp, &cancelip->i_din1->di_ib[0],
  	    &cancelip->i_din1->di_ib[NIADDR], fs, -1, expungetype)))
  		return (error);
  	blksperindir = 1;
  	lbn = -NDADDR;
  	len = numblks - NDADDR;
  	rlbn = NDADDR;
  	for (i = 0; len > 0 && i < NIADDR; i++) {
  		error = indiracct_ufs1(snapvp, ITOV(cancelip), i,
  		    cancelip->i_din1->di_ib[i], lbn, rlbn, len,
  		    blksperindir, fs, acctfunc, expungetype);
  		if (error)
  			return (error);
  		blksperindir *= 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_ufs1(snapvp, cancelvp, level, blkno, lbn, rlbn, remblks,
  	    blksperindir, fs, acctfunc, expungetype)
  	struct vnode *snapvp;
  	struct vnode *cancelvp;
  	int level;
  	ufs1_daddr_t blkno;
  	ufs_lbn_t lbn;
  	ufs_lbn_t rlbn;
  	ufs_lbn_t remblks;
  	ufs_lbn_t blksperindir;
  	struct fs *fs;
  	int (*acctfunc)(struct vnode *, ufs1_daddr_t *, ufs1_daddr_t *,
  	    struct fs *, ufs_lbn_t, int);
  	int expungetype;
  {
  	int error, num, i;
  	ufs_lbn_t subblksperindir;
  	struct indir indirs[NIADDR + 2];
  	ufs1_daddr_t last, *bap;
  	struct buf *bp;
  
  	if (blkno == 0) {
  		if (expungetype == BLK_NOCOPY)
  			return (0);
  		panic("indiracct_ufs1: 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_ufs1: 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.
  	 */
  	bp = getblk(cancelvp, lbn, fs->fs_bsize, 0, 0, 0);
  	bp->b_blkno = fsbtodb(fs, blkno);
  	if ((bp->b_flags & (B_DONE | B_DELWRI)) == 0 &&
  	    (error = readblock(bp, fragstoblks(fs, blkno)))) {
  		brelse(bp);
  		return (error);
  	}
  	/*
  	 * Account for the block pointers in this indirect block.
  	 */
  	last = howmany(remblks, blksperindir);
  	if (last > NINDIR(fs))
  		last = NINDIR(fs);
  	MALLOC(bap, ufs1_daddr_t *, fs->fs_bsize, M_DEVBUF, M_WAITOK);
  	bcopy(bp->b_data, (caddr_t)bap, fs->fs_bsize);
  	bqrelse(bp);
  	error = (*acctfunc)(snapvp, &bap[0], &bap[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 / NINDIR(fs);
  	for (lbn++, level--, i = 0; i < last; i++) {
  		error = indiracct_ufs1(snapvp, cancelvp, level, 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_ufs1(vp, oldblkp, lastblkp, fs, lblkno, exptype)
  	struct vnode *vp;
  	ufs1_daddr_t *oldblkp, *lastblkp;
  	struct fs *fs;
  	ufs_lbn_t lblkno;
  	int exptype;	/* BLK_SNAP or BLK_NOCOPY */
  {
  	int error;
  
  	if ((error = snapacct_ufs1(vp, oldblkp, lastblkp, fs, lblkno, exptype)))
  		return (error);
  	return (mapacct_ufs1(vp, oldblkp, lastblkp, fs, lblkno, exptype));
  }
  
  /*
   * Identify a set of blocks allocated in a snapshot inode.
   */
  static int
  snapacct_ufs1(vp, oldblkp, lastblkp, fs, lblkno, expungetype)
  	struct vnode *vp;
  	ufs1_daddr_t *oldblkp, *lastblkp;
  	struct fs *fs;
  	ufs_lbn_t lblkno;
  	int expungetype;	/* BLK_SNAP or BLK_NOCOPY */
  {
  	struct inode *ip = VTOI(vp);
  	ufs1_daddr_t blkno, *blkp;
  	ufs_lbn_t lbn;
  	struct buf *ibp;
  	int error;
  
  	for ( ; oldblkp < lastblkp; oldblkp++) {
  		blkno = *oldblkp;
  		if (blkno == 0 || blkno == BLK_NOCOPY || blkno == BLK_SNAP)
  			continue;
  		lbn = fragstoblks(fs, blkno);
  		if (lbn < NDADDR) {
  			blkp = &ip->i_din1->di_db[lbn];
  			ip->i_flag |= IN_CHANGE | IN_UPDATE;
  		} else {
  			error = UFS_BALLOC(vp, lblktosize(fs, (off_t)lbn),
  			    fs->fs_bsize, KERNCRED, BA_METAONLY, &ibp);
  			if (error)
  				return (error);
  			blkp = &((ufs1_daddr_t *)(ibp->b_data))
  			    [(lbn - NDADDR) % 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 && *blkp == BLK_NOCOPY) {
  			if (lbn >= NDADDR)
  				brelse(ibp);
  		} else {
  			if (*blkp != 0)
  				panic("snapacct_ufs1: bad block");
  			*blkp = expungetype;
  			if (lbn >= NDADDR)
  				bdwrite(ibp);
  		}
  	}
  	return (0);
  }
  
  /*
   * Account for a set of blocks allocated in a snapshot inode.
   */
  static int
  mapacct_ufs1(vp, oldblkp, lastblkp, fs, lblkno, expungetype)
  	struct vnode *vp;
  	ufs1_daddr_t *oldblkp, *lastblkp;
  	struct fs *fs;
  	ufs_lbn_t lblkno;
  	int expungetype;
  {
  	ufs1_daddr_t blkno;
  	struct inode *ip;
  	ino_t inum;
  	int acctit;
  
  	ip = VTOI(vp);
  	inum = ip->i_number;
  	if (lblkno == -1)
  		acctit = 0;
  	else
  		acctit = 1;
  	for ( ; oldblkp < lastblkp; oldblkp++, lblkno++) {
  		blkno = *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 = blkstofrags(fs, lblkno);
  		ffs_blkfree(fs, vp, blkno, fs->fs_bsize, inum);
  	}
  	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. This code
   * is reproduced once each for UFS1 and UFS2.
   */
  static int
  expunge_ufs2(snapvp, cancelip, fs, acctfunc, expungetype)
  	struct vnode *snapvp;
  	struct inode *cancelip;
  	struct fs *fs;
  	int (*acctfunc)(struct vnode *, ufs2_daddr_t *, ufs2_daddr_t *,
  	    struct fs *, ufs_lbn_t, int);
  	int expungetype;
  {
  	int i, error, indiroff;
  	ufs_lbn_t lbn, rlbn;
  	ufs2_daddr_t len, blkno, numblks, blksperindir;
  	struct ufs2_dinode *dip;
  	struct thread *td = curthread;
  	struct buf *bp;
  
  	/*
  	 * Prepare to expunge the inode. If its inode block has not
  	 * yet been copied, then allocate and fill the copy.
  	 */
  	lbn = fragstoblks(fs, ino_to_fsba(fs, cancelip->i_number));
  	blkno = 0;
  	if (lbn < NDADDR) {
  		blkno = VTOI(snapvp)->i_din2->di_db[lbn];
  	} else {
  		td->td_pflags |= TDP_COWINPROGRESS;
  		error = UFS_BALLOC(snapvp, lblktosize(fs, (off_t)lbn),
  		   fs->fs_bsize, KERNCRED, BA_METAONLY, &bp);
  		td->td_pflags &= ~TDP_COWINPROGRESS;
  		if (error)
  			return (error);
  		indiroff = (lbn - NDADDR) % NINDIR(fs);
  		blkno = ((ufs2_daddr_t *)(bp->b_data))[indiroff];
  		bqrelse(bp);
  	}
  	if (blkno != 0) {
  		if ((error = bread(snapvp, lbn, fs->fs_bsize, KERNCRED, &bp)))
  			return (error);
  	} else {
  		error = UFS_BALLOC(snapvp, lblktosize(fs, (off_t)lbn),
  		    fs->fs_bsize, KERNCRED, 0, &bp);
  		if (error)
  			return (error);
  		if ((error = readblock(bp, lbn)) != 0)
  			return (error);
  	}
  	/*
  	 * Set a snapshot inode to be a zero length file, regular files
  	 * to be completely unallocated.
  	 */
  	dip = (struct ufs2_dinode *)bp->b_data +
  	    ino_to_fsbo(fs, cancelip->i_number);
  	if (expungetype == BLK_NOCOPY)
  		dip->di_mode = 0;
  	dip->di_size = 0;
  	dip->di_blocks = 0;
  	dip->di_flags &= ~SF_SNAPSHOT;
  	bzero(&dip->di_db[0], (NDADDR + NIADDR) * sizeof(ufs2_daddr_t));
  	bdwrite(bp);
  	/*
  	 * Now go through and expunge all the blocks in the file
  	 * using the function requested.
  	 */
  	numblks = howmany(cancelip->i_size, fs->fs_bsize);
  	if ((error = (*acctfunc)(snapvp, &cancelip->i_din2->di_db[0],
  	    &cancelip->i_din2->di_db[NDADDR], fs, 0, expungetype)))
  		return (error);
  	if ((error = (*acctfunc)(snapvp, &cancelip->i_din2->di_ib[0],
  	    &cancelip->i_din2->di_ib[NIADDR], fs, -1, expungetype)))
  		return (error);
  	blksperindir = 1;
  	lbn = -NDADDR;
  	len = numblks - NDADDR;
  	rlbn = NDADDR;
  	for (i = 0; len > 0 && i < NIADDR; i++) {
  		error = indiracct_ufs2(snapvp, ITOV(cancelip), i,
  		    cancelip->i_din2->di_ib[i], lbn, rlbn, len,
  		    blksperindir, fs, acctfunc, expungetype);
  		if (error)
  			return (error);
  		blksperindir *= 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_ufs2(snapvp, cancelvp, level, blkno, lbn, rlbn, remblks,
  	    blksperindir, fs, acctfunc, expungetype)
  	struct vnode *snapvp;
  	struct vnode *cancelvp;
  	int level;
  	ufs2_daddr_t blkno;
  	ufs_lbn_t lbn;
  	ufs_lbn_t rlbn;
  	ufs_lbn_t remblks;
  	ufs_lbn_t blksperindir;
  	struct fs *fs;
  	int (*acctfunc)(struct vnode *, ufs2_daddr_t *, ufs2_daddr_t *,
  	    struct fs *, ufs_lbn_t, int);
  	int expungetype;
  {
  	int error, num, i;
  	ufs_lbn_t subblksperindir;
  	struct indir indirs[NIADDR + 2];
  	ufs2_daddr_t last, *bap;
  	struct buf *bp;
  
  	if (blkno == 0) {
  		if (expungetype == BLK_NOCOPY)
  			return (0);
  		panic("indiracct_ufs2: 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_ufs2: 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.
  	 */
  	bp = getblk(cancelvp, lbn, fs->fs_bsize, 0, 0, 0);
  	bp->b_blkno = fsbtodb(fs, blkno);
  	if ((bp->b_flags & (B_DONE | B_DELWRI)) == 0 &&
  	    (error = readblock(bp, fragstoblks(fs, blkno)))) {
  		brelse(bp);
  		return (error);
  	}
  	/*
  	 * Account for the block pointers in this indirect block.
  	 */
  	last = howmany(remblks, blksperindir);
  	if (last > NINDIR(fs))
  		last = NINDIR(fs);
  	MALLOC(bap, ufs2_daddr_t *, fs->fs_bsize, M_DEVBUF, M_WAITOK);
  	bcopy(bp->b_data, (caddr_t)bap, fs->fs_bsize);
  	bqrelse(bp);
  	error = (*acctfunc)(snapvp, &bap[0], &bap[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 / NINDIR(fs);
  	for (lbn++, level--, i = 0; i < last; i++) {
  		error = indiracct_ufs2(snapvp, cancelvp, level, 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_ufs2(vp, oldblkp, lastblkp, fs, lblkno, exptype)
  	struct vnode *vp;
  	ufs2_daddr_t *oldblkp, *lastblkp;
  	struct fs *fs;
  	ufs_lbn_t lblkno;
  	int exptype;	/* BLK_SNAP or BLK_NOCOPY */
  {
  	int error;
  
  	if ((error = snapacct_ufs2(vp, oldblkp, lastblkp, fs, lblkno, exptype)))
  		return (error);
  	return (mapacct_ufs2(vp, oldblkp, lastblkp, fs, lblkno, exptype));
  }
  
  /*
   * Identify a set of blocks allocated in a snapshot inode.
   */
  static int
  snapacct_ufs2(vp, oldblkp, lastblkp, fs, lblkno, expungetype)
  	struct vnode *vp;
  	ufs2_daddr_t *oldblkp, *lastblkp;
  	struct fs *fs;
  	ufs_lbn_t lblkno;
  	int expungetype;	/* BLK_SNAP or BLK_NOCOPY */
  {
  	struct inode *ip = VTOI(vp);
  	ufs2_daddr_t blkno, *blkp;
  	ufs_lbn_t lbn;
  	struct buf *ibp;
  	int error;
  
  	for ( ; oldblkp < lastblkp; oldblkp++) {
  		blkno = *oldblkp;
  		if (blkno == 0 || blkno == BLK_NOCOPY || blkno == BLK_SNAP)
  			continue;
  		lbn = fragstoblks(fs, blkno);
  		if (lbn < NDADDR) {
  			blkp = &ip->i_din2->di_db[lbn];
  			ip->i_flag |= IN_CHANGE | IN_UPDATE;
  		} else {
  			error = UFS_BALLOC(vp, lblktosize(fs, (off_t)lbn),
  			    fs->fs_bsize, KERNCRED, BA_METAONLY, &ibp);
  			if (error)
  				return (error);
  			blkp = &((ufs2_daddr_t *)(ibp->b_data))
  			    [(lbn - NDADDR) % 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 && *blkp == BLK_NOCOPY) {
  			if (lbn >= NDADDR)
  				brelse(ibp);
  		} else {
  			if (*blkp != 0)
  				panic("snapacct_ufs2: bad block");
  			*blkp = expungetype;
  			if (lbn >= NDADDR)
  				bdwrite(ibp);
  		}
  	}
  	return (0);
  }
  
  /*
   * Account for a set of blocks allocated in a snapshot inode.
   */
  static int
  mapacct_ufs2(vp, oldblkp, lastblkp, fs, lblkno, expungetype)
  	struct vnode *vp;
  	ufs2_daddr_t *oldblkp, *lastblkp;
  	struct fs *fs;
  	ufs_lbn_t lblkno;
  	int expungetype;
  {
  	ufs2_daddr_t blkno;
  	struct inode *ip;
  	ino_t inum;
  	int acctit;
  
  	ip = VTOI(vp);
  	inum = ip->i_number;
  	if (lblkno == -1)
  		acctit = 0;
  	else
  		acctit = 1;
  	for ( ; oldblkp < lastblkp; oldblkp++, lblkno++) {
  		blkno = *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 = blkstofrags(fs, lblkno);
  		ffs_blkfree(fs, vp, blkno, fs->fs_bsize, inum);
  	}
  	return (0);
  }
  
  /*
   * 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(ip)
  	struct inode *ip;
  {
  	struct inode *xp;
  	struct fs *fs;
  	int snaploc;
  
  	/*
  	 * Find snapshot in incore list.
  	 */
  	TAILQ_FOREACH(xp, &ip->i_devvp->v_rdev->si_snapshots, i_nextsnap)
  		if (xp == ip)
  			break;
  	if (xp != NULL)
  		vrele(ITOV(ip));
  	else if (snapdebug)
  		printf("ffs_snapgone: lost snapshot vnode %d\n",
  		    ip->i_number);
  	/*
  	 * Delete snapshot inode from superblock. Keep list dense.
  	 */
  	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;
  	}
  }
  
  /*
   * Prepare a snapshot file for being removed.
   */
  void
  ffs_snapremove(vp)
  	struct vnode *vp;
  {
  	struct inode *ip;
  	struct vnode *devvp;
  	struct lock *lkp;
  	struct buf *ibp;
  	struct fs *fs;
  	struct thread *td = curthread;
  	ufs2_daddr_t numblks, blkno, dblk, *snapblklist;
  	int error, loc, last;
  
  	ip = VTOI(vp);
  	fs = ip->i_fs;
  	devvp = ip->i_devvp;
  	/*
  	 * 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.
  	 */
  	if (ip->i_nextsnap.tqe_prev != 0) {
  		VI_LOCK(devvp);
  		lockmgr(&vp->v_lock, LK_INTERLOCK | LK_EXCLUSIVE,
  		    VI_MTX(devvp), td);
  		VI_LOCK(devvp);
  		TAILQ_REMOVE(&devvp->v_rdev->si_snapshots, ip, i_nextsnap);
  		ip->i_nextsnap.tqe_prev = 0;
  		lkp = vp->v_vnlock;
  		vp->v_vnlock = &vp->v_lock;
  		lockmgr(lkp, LK_RELEASE, NULL, td);
  		if (TAILQ_FIRST(&devvp->v_rdev->si_snapshots) != 0) {
  			VI_UNLOCK(devvp);
  		} else {
  			snapblklist = devvp->v_rdev->si_snapblklist;
  			devvp->v_rdev->si_snapblklist = 0;
  			devvp->v_rdev->si_snaplistsize = 0;
  			devvp->v_rdev->si_copyonwrite = 0;
  			devvp->v_vflag &= ~VV_COPYONWRITE;
  			lockmgr(lkp, LK_DRAIN|LK_INTERLOCK, VI_MTX(devvp), td);
  			lockmgr(lkp, LK_RELEASE, NULL, td);
  			lockdestroy(lkp);
  			FREE(lkp, M_UFSMNT);
  			FREE(snapblklist, M_UFSMNT);
  		}
  	}
  	/*
  	 * Clear all BLK_NOCOPY fields. Pass any block claims to other
  	 * snapshots that want them (see ffs_snapblkfree below).
  	 */
  	for (blkno = 1; blkno < NDADDR; blkno++) {
  		dblk = DIP(ip, i_db[blkno]);
  		if (dblk == BLK_NOCOPY || dblk == BLK_SNAP)
  			DIP(ip, i_db[blkno]) = 0;
  		else if ((dblk == blkstofrags(fs, blkno) &&
  		     ffs_snapblkfree(fs, ip->i_devvp, dblk, fs->fs_bsize,
  		     ip->i_number))) {
  			DIP(ip, i_blocks) -= btodb(fs->fs_bsize);
  			DIP(ip, i_db[blkno]) = 0;
  		}
  	}
  	numblks = howmany(ip->i_size, fs->fs_bsize);
  	for (blkno = NDADDR; blkno < numblks; blkno += NINDIR(fs)) {
  		error = UFS_BALLOC(vp, lblktosize(fs, (off_t)blkno),
  		    fs->fs_bsize, KERNCRED, BA_METAONLY, &ibp);
  		if (error)
  			continue;
  		if (fs->fs_size - blkno > NINDIR(fs))
  			last = NINDIR(fs);
  		else
  			last = fs->fs_size - blkno;
  		for (loc = 0; loc < last; loc++) {
  			if (ip->i_ump->um_fstype == UFS1) {
  				dblk = ((ufs1_daddr_t *)(ibp->b_data))[loc];
  				if (dblk == BLK_NOCOPY || dblk == BLK_SNAP)
  					((ufs1_daddr_t *)(ibp->b_data))[loc]= 0;
  				else if ((dblk == blkstofrags(fs, blkno) &&
  				     ffs_snapblkfree(fs, ip->i_devvp, dblk,
  				     fs->fs_bsize, ip->i_number))) {
  					ip->i_din1->di_blocks -=
  					    btodb(fs->fs_bsize);
  					((ufs1_daddr_t *)(ibp->b_data))[loc]= 0;
  				}
  				continue;
  			}
  			dblk = ((ufs2_daddr_t *)(ibp->b_data))[loc];
  			if (dblk == BLK_NOCOPY || dblk == BLK_SNAP)
  				((ufs2_daddr_t *)(ibp->b_data))[loc] = 0;
  			else if ((dblk == blkstofrags(fs, blkno) &&
  			     ffs_snapblkfree(fs, ip->i_devvp, dblk,
  			     fs->fs_bsize, ip->i_number))) {
  				ip->i_din2->di_blocks -= btodb(fs->fs_bsize);
  				((ufs2_daddr_t *)(ibp->b_data))[loc] = 0;
  			}
  		}
  		bawrite(ibp);
  	}
  	/*
  	 * Clear snapshot flag and drop reference.
  	 */
  	ip->i_flags &= ~SF_SNAPSHOT;
  	DIP(ip, i_flags) = ip->i_flags;
  	ip->i_flag |= IN_CHANGE | IN_UPDATE;
  }
  
  /*
   * 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(fs, devvp, bno, size, inum)
  	struct fs *fs;
  	struct vnode *devvp;
  	ufs2_daddr_t bno;
  	long size;
  	ino_t inum;
  {
  	struct buf *ibp, *cbp, *savedcbp = 0;
  	struct thread *td = curthread;
  	struct inode *ip;
  	struct vnode *vp = NULL;
  	ufs_lbn_t lbn;
  	ufs2_daddr_t blkno;
  	int indiroff = 0, snapshot_locked = 0, error = 0, claimedblk = 0;
  	struct snaphead *snaphead;
  
  	lbn = fragstoblks(fs, bno);
  retry:
  	VI_LOCK(devvp);
  	snaphead = &devvp->v_rdev->si_snapshots;
  	TAILQ_FOREACH(ip, snaphead, i_nextsnap) {
  		vp = ITOV(ip);
  		/*
  		 * Lookup block being written.
  		 */
  		if (lbn < NDADDR) {
  			blkno = DIP(ip, i_db[lbn]);
  		} else {
  			if (snapshot_locked == 0 &&
  			    lockmgr(vp->v_vnlock,
  			      LK_INTERLOCK | LK_EXCLUSIVE | LK_SLEEPFAIL,
  			      VI_MTX(devvp), td) != 0)
  				goto retry;
  			snapshot_locked = 1;
  			td->td_pflags |= TDP_COWINPROGRESS;
  			error = UFS_BALLOC(vp, lblktosize(fs, (off_t)lbn),
  			    fs->fs_bsize, KERNCRED, BA_METAONLY, &ibp);
  			td->td_pflags &= ~TDP_COWINPROGRESS;
  			if (error)
  				break;
  			indiroff = (lbn - NDADDR) % NINDIR(fs);
  			if (ip->i_ump->um_fstype == UFS1)
  				blkno=((ufs1_daddr_t *)(ibp->b_data))[indiroff];
  			else
  				blkno=((ufs2_daddr_t *)(ibp->b_data))[indiroff];
  		}
  		/*
  		 * 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 (snapshot_locked == 0 &&
  			    lockmgr(vp->v_vnlock,
  			      LK_INTERLOCK | LK_EXCLUSIVE | LK_NOWAIT,
  			      VI_MTX(devvp), td) != 0) {
  				if (lbn >= NDADDR)
  					bqrelse(ibp);
  				vn_lock(vp, LK_EXCLUSIVE | LK_SLEEPFAIL, td);
  				goto retry;
  			}
  			snapshot_locked = 1;
  			if (lbn < NDADDR) {
  				DIP(ip, i_db[lbn]) = BLK_NOCOPY;
  				ip->i_flag |= IN_CHANGE | IN_UPDATE;
  			} else if (ip->i_ump->um_fstype == UFS1) {
  				((ufs1_daddr_t *)(ibp->b_data))[indiroff] =
  				    BLK_NOCOPY;
  				bdwrite(ibp);
  			} else {
  				((ufs2_daddr_t *)(ibp->b_data))[indiroff] =
  				    BLK_NOCOPY;
  				bdwrite(ibp);
  			}
  			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 >= NDADDR)
  				bqrelse(ibp);
  			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 (snapshot_locked == 0 &&
  		    lockmgr(vp->v_vnlock,
  		      LK_INTERLOCK | LK_EXCLUSIVE | LK_NOWAIT,
  		      VI_MTX(devvp), td) != 0) {
  			if (lbn >= NDADDR)
  				bqrelse(ibp);
  			vn_lock(vp, LK_EXCLUSIVE | LK_SLEEPFAIL, td);
  			goto retry;
  		}
  		snapshot_locked = 1;
  		if (size == fs->fs_bsize) {
  #ifdef DEBUG
  			if (snapdebug)
  				printf("%s %d lbn %jd from inum %d\n",
  				    "Grabonremove: snapino", ip->i_number,
  				    (intmax_t)lbn, inum);
  #endif
  			if (lbn < NDADDR) {
  				DIP(ip, i_db[lbn]) = bno;
  			} else if (ip->i_ump->um_fstype == UFS1) {
  				((ufs1_daddr_t *)(ibp->b_data))[indiroff] = bno;
  				bdwrite(ibp);
  			} else {
  				((ufs2_daddr_t *)(ibp->b_data))[indiroff] = bno;
  				bdwrite(ibp);
  			}
  			DIP(ip, i_blocks) += btodb(size);
  			ip->i_flag |= IN_CHANGE | IN_UPDATE;
  			VOP_UNLOCK(vp, 0, td);
  			return (1);
  		}
  		if (lbn >= NDADDR)
  			bqrelse(ibp);
  		/*
  		 * Allocate the block into which to do the copy. Note that this
  		 * allocation will never require any additional allocations for
  		 * the snapshot inode.
  		 */
  		td->td_pflags |= TDP_COWINPROGRESS;
  		error = UFS_BALLOC(vp, lblktosize(fs, (off_t)lbn),
  		    fs->fs_bsize, KERNCRED, 0, &cbp);
  		td->td_pflags &= ~TDP_COWINPROGRESS;
  		if (error)
  			break;
  #ifdef DEBUG
  		if (snapdebug)
  			printf("%s%d lbn %jd %s %d size %ld to blkno %jd\n",
  			    "Copyonremove: snapino ", ip->i_number,
  			    (intmax_t)lbn, "for inum", inum, size,
  			    (intmax_t)cbp->b_blkno);
  #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.
  		 */
  		if (savedcbp != 0) {
  			bcopy(savedcbp->b_data, cbp->b_data, fs->fs_bsize);
  			bawrite(cbp);
  			if (dopersistence && ip->i_effnlink > 0)
  				(void) VOP_FSYNC(vp, KERNCRED, MNT_WAIT, td);
  			continue;
  		}
  		/*
  		 * Otherwise, read the old block contents into the buffer.
  		 */
  		if ((error = readblock(cbp, lbn)) != 0) {
  			bzero(cbp->b_data, fs->fs_bsize);
  			bawrite(cbp);
  			if (dopersistence && ip->i_effnlink > 0)
  				(void) VOP_FSYNC(vp, KERNCRED, MNT_WAIT, td);
  			break;
  		}
  		savedcbp = cbp;
  	}
  	/*
  	 * 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 (savedcbp) {
  		vp = savedcbp->b_vp;
  		bawrite(savedcbp);
  		if (dopersistence && VTOI(vp)->i_effnlink > 0)
  			(void) VOP_FSYNC(vp, KERNCRED, MNT_WAIT, td);
  	}
  	/*
  	 * 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.
  	 */
  	if (snapshot_locked)
  		VOP_UNLOCK(vp, 0, td);
  	else
  		VI_UNLOCK(devvp);
  	return (error);
  }
  
  /*
   * Associate snapshot files when mounting.
   */
  void
  ffs_snapshot_mount(mp)
  	struct mount *mp;
  {
  	struct ufsmount *ump = VFSTOUFS(mp);
  	struct vnode *devvp = ump->um_devvp;
  	struct fs *fs = ump->um_fs;
  	struct thread *td = curthread;
  	struct snaphead *snaphead;
  	struct vnode *vp;
  	struct inode *ip, *xp;
  	struct uio auio;
  	struct iovec aiov;
  	void *snapblklist;
  	char *reason;
  	daddr_t snaplistsize;
  	int error, snaploc, loc;
  
  	/*
  	 * XXX The following needs to be set before UFS_TRUNCATE or
  	 * VOP_READ can be called.
  	 */
  	mp->mnt_stat.f_iosize = fs->fs_bsize;
  	/*
  	 * Process each snapshot listed in the superblock.
  	 */
  	vp = NULL;
  	snaphead = &devvp->v_rdev->si_snapshots;
  	for (snaploc = 0; snaploc < FSMAXSNAP; snaploc++) {
  		if (fs->fs_snapinum[snaploc] == 0)
  			break;
  		if ((error = VFS_VGET(mp, fs->fs_snapinum[snaploc],
  		    LK_EXCLUSIVE, &vp)) != 0){
  			printf("ffs_snapshot_mount: vget failed %d\n", error);
  			continue;
  		}
  		ip = VTOI(vp);
  		if ((ip->i_flags & SF_SNAPSHOT) == 0 || ip->i_size ==
  		    lblktosize(fs, howmany(fs->fs_size, fs->fs_frag))) {
  			if ((ip->i_flags & SF_SNAPSHOT) == 0) {
  				reason = "non-snapshot";
  			} else {
  				reason = "old format snapshot";
  				(void)UFS_TRUNCATE(vp, (off_t)0, 0, NOCRED, td);
  				(void)VOP_FSYNC(vp, KERNCRED, MNT_WAIT, td);
  			}
  			printf("ffs_snapshot_mount: %s inode %d\n",
  			    reason, 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;
  		}
  		/*
  		 * If there already exist snapshots on this filesystem, grab a
  		 * reference to their shared lock. If this is the first snapshot
  		 * on this filesystem, we need to allocate a lock for the
  		 * snapshots to share. In either case, acquire the snapshot
  		 * lock and give up our original private lock.
  		 */
  		VI_LOCK(devvp);
  		if ((xp = TAILQ_FIRST(snaphead)) != NULL) {
  			VI_LOCK(vp);
  			vp->v_vnlock = ITOV(xp)->v_vnlock;
  			VI_UNLOCK(devvp);
  		} else {
  			struct lock *lkp;
  
  			VI_UNLOCK(devvp);
  			MALLOC(lkp, struct lock *, sizeof(struct lock),
  			    M_UFSMNT, M_WAITOK);
  			lockinit(lkp, PVFS, "snaplk", VLKTIMEOUT,
  			    LK_CANRECURSE | LK_NOPAUSE);
  			VI_LOCK(vp);
  			vp->v_vnlock = lkp;
  		}
  		vn_lock(vp, LK_INTERLOCK | LK_EXCLUSIVE | LK_RETRY, td);
  		transferlockers(&vp->v_lock, vp->v_vnlock);
  		lockmgr(&vp->v_lock, LK_RELEASE, NULL, td);
  		/*
  		 * Link it onto the active snapshot list.
  		 */
  		VI_LOCK(devvp);
  		if (ip->i_nextsnap.tqe_prev != 0)
  			panic("ffs_snapshot_mount: %d already on list",
  			    ip->i_number);
  		else
  			TAILQ_INSERT_TAIL(snaphead, ip, i_nextsnap);
  		vp->v_vflag |= VV_SYSTEM;
  		VI_UNLOCK(devvp);
  		VOP_UNLOCK(vp, 0, td);
  	}
  	/*
  	 * No usable snapshots found.
  	 */
  	if (vp == NULL)
  		return;
  	/*
  	 * Allocate the space for the block hints list. We always want to
  	 * use the list from the newest snapshot.
  	 */
  	auio.uio_iov = &aiov;
  	auio.uio_iovcnt = 1;
  	aiov.iov_base = (void *)&snaplistsize;
  	aiov.iov_len = sizeof(snaplistsize);
  	auio.uio_resid = aiov.iov_len;
  	auio.uio_offset =
  	    lblktosize(fs, howmany(fs->fs_size, fs->fs_frag));
  	auio.uio_segflg = UIO_SYSSPACE;
  	auio.uio_rw = UIO_READ;
  	auio.uio_td = td;
  	vn_lock(vp, LK_EXCLUSIVE | LK_RETRY, td);
  	if ((error = VOP_READ(vp, &auio, IO_UNIT, td->td_ucred)) != 0) {
  		printf("ffs_snapshot_mount: read_1 failed %d\n", error);
  		VOP_UNLOCK(vp, 0, td);
  		return;
  	}
  	MALLOC(snapblklist, void *, snaplistsize * sizeof(daddr_t),
  	    M_UFSMNT, M_WAITOK);
  	auio.uio_iovcnt = 1;
  	aiov.iov_base = snapblklist;
  	aiov.iov_len = snaplistsize * sizeof (daddr_t);
  	auio.uio_resid = aiov.iov_len;
  	auio.uio_offset -= sizeof(snaplistsize);
  	if ((error = VOP_READ(vp, &auio, IO_UNIT, td->td_ucred)) != 0) {
  		printf("ffs_snapshot_mount: read_2 failed %d\n", error);
  		VOP_UNLOCK(vp, 0, td);
  		FREE(snapblklist, M_UFSMNT);
  		return;
  	}
  	VOP_UNLOCK(vp, 0, td);
  	VI_LOCK(devvp);
  	ASSERT_VOP_LOCKED(devvp, "ffs_snapshot_mount");
  	devvp->v_rdev->si_snaplistsize = snaplistsize;
  	devvp->v_rdev->si_snapblklist = (daddr_t *)snapblklist;
  	devvp->v_rdev->si_copyonwrite = ffs_copyonwrite;
  	devvp->v_vflag |= VV_COPYONWRITE;
  	VI_UNLOCK(devvp);
  }
  
  /*
   * Disassociate snapshot files when unmounting.
   */
  void
  ffs_snapshot_unmount(mp)
  	struct mount *mp;
  {
  	struct vnode *devvp = VFSTOUFS(mp)->um_devvp;
  	struct snaphead *snaphead = &devvp->v_rdev->si_snapshots;
  	struct lock *lkp = NULL;
  	struct inode *xp;
  	struct vnode *vp;
  
  	VI_LOCK(devvp);
  	while ((xp = TAILQ_FIRST(snaphead)) != 0) {
  		vp = ITOV(xp);
  		lkp = vp->v_vnlock;
  		vp->v_vnlock = &vp->v_lock;
  		TAILQ_REMOVE(snaphead, xp, i_nextsnap);
  		xp->i_nextsnap.tqe_prev = 0;
  		if (xp->i_effnlink > 0) {
  			VI_UNLOCK(devvp);
  			vrele(vp);
  			VI_LOCK(devvp);
  		}
  	}
  	if (devvp->v_rdev->si_snapblklist != NULL) {
  		FREE(devvp->v_rdev->si_snapblklist, M_UFSMNT);
  		devvp->v_rdev->si_snapblklist = NULL;
  		devvp->v_rdev->si_snaplistsize = 0;
  	}
  	if (lkp != NULL) {
  		lockdestroy(lkp);
  		FREE(lkp, M_UFSMNT);
  	}
  	ASSERT_VOP_LOCKED(devvp, "ffs_snapshot_unmount");
  	devvp->v_rdev->si_copyonwrite = 0;
  	devvp->v_vflag &= ~VV_COPYONWRITE;
  	VI_UNLOCK(devvp);
  }
  
  /*
   * Check for need to copy block that is about to be written,
   * copying the block if necessary.
   */
  static int
  ffs_copyonwrite(devvp, bp)
  	struct vnode *devvp;
  	struct buf *bp;
  {
  	struct snaphead *snaphead;
  	struct buf *ibp, *cbp, *savedcbp = 0;
  	struct thread *td = curthread;
  	struct fs *fs;
  	struct inode *ip;
  	struct vnode *vp = 0;
  	ufs2_daddr_t lbn, blkno, *snapblklist;
  	int lower, upper, mid, indiroff, snapshot_locked = 0, error = 0;
  
  	if (td->td_pflags & TDP_COWINPROGRESS)
  		panic("ffs_copyonwrite: recursive call");
  	/*
  	 * First check to see if it is in the preallocated list.
  	 * By doing this check we avoid several potential deadlocks.
  	 */
  	VI_LOCK(devvp);
  	snaphead = &devvp->v_rdev->si_snapshots;
  	ip = TAILQ_FIRST(snaphead);
  	fs = ip->i_fs;
  	lbn = fragstoblks(fs, dbtofsb(fs, bp->b_blkno));
  	snapblklist = devvp->v_rdev->si_snapblklist;
  	upper = devvp->v_rdev->si_snaplistsize - 1;
  	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) {
  		VI_UNLOCK(devvp);
  		return (0);
  	}
  	/*
  	 * Not in the precomputed list, so check the snapshots.
  	 */
  retry:
  	TAILQ_FOREACH(ip, snaphead, 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 UFS_BALLOC.
  		 */
  		if (bp->b_vp == vp)
  			continue;
  		/*
  		 * Check to see if block needs to be copied. We do not have
  		 * to hold the snapshot lock while doing this lookup as it
  		 * will never require any additional allocations for the
  		 * snapshot inode.
  		 */
  		if (lbn < NDADDR) {
  			blkno = DIP(ip, i_db[lbn]);
  		} else {
  			if (snapshot_locked == 0 &&
  			    lockmgr(vp->v_vnlock,
  			      LK_INTERLOCK | LK_EXCLUSIVE | LK_SLEEPFAIL,
  			      VI_MTX(devvp), td) != 0) {
  				VI_LOCK(devvp);
  				goto retry;
  			}
  			snapshot_locked = 1;
  			td->td_pflags |= TDP_COWINPROGRESS;
  			error = UFS_BALLOC(vp, lblktosize(fs, (off_t)lbn),
  			   fs->fs_bsize, KERNCRED, BA_METAONLY, &ibp);
  			td->td_pflags &= ~TDP_COWINPROGRESS;
  			if (error)
  				break;
  			indiroff = (lbn - NDADDR) % NINDIR(fs);
  			if (ip->i_ump->um_fstype == UFS1)
  				blkno=((ufs1_daddr_t *)(ibp->b_data))[indiroff];
  			else
  				blkno=((ufs2_daddr_t *)(ibp->b_data))[indiroff];
  			bqrelse(ibp);
  		}
  #ifdef DIAGNOSTIC
  		if (blkno == BLK_SNAP && bp->b_lblkno >= 0)
  			panic("ffs_copyonwrite: bad copy block");
  #endif
  		if (blkno != 0)
  			continue;
  		/*
  		 * 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.
  		 */
  		if (snapshot_locked == 0 &&
  		    lockmgr(vp->v_vnlock,
  		      LK_INTERLOCK | LK_EXCLUSIVE | LK_SLEEPFAIL,
  		      VI_MTX(devvp), td) != 0) {
  			VI_LOCK(devvp);
  			goto retry;
  		}
  		snapshot_locked = 1;
  		td->td_pflags |= TDP_COWINPROGRESS;
  		error = UFS_BALLOC(vp, lblktosize(fs, (off_t)lbn),
  		    fs->fs_bsize, KERNCRED, 0, &cbp);
  		td->td_pflags &= ~TDP_COWINPROGRESS;
  		if (error)
  			break;
  #ifdef DEBUG
  		if (snapdebug) {
  			printf("Copyonwrite: snapino %d lbn %jd for ",
  			    ip->i_number, (intmax_t)lbn);
  			if (bp->b_vp == devvp)
  				printf("fs metadata");
  			else
  				printf("inum %d", VTOI(bp->b_vp)->i_number);
  			printf(" lblkno %jd to blkno %jd\n",
  			    (intmax_t)bp->b_lblkno, (intmax_t)cbp->b_blkno);
  		}
  #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.
  		 */
  		if (savedcbp != 0) {
  			bcopy(savedcbp->b_data, cbp->b_data, fs->fs_bsize);
  			bawrite(cbp);
  			if (dopersistence && ip->i_effnlink > 0)
  				(void) VOP_FSYNC(vp, KERNCRED, MNT_WAIT, td);
  			continue;
  		}
  		/*
  		 * Otherwise, read the old block contents into the buffer.
  		 */
  		if ((error = readblock(cbp, lbn)) != 0) {
  			bzero(cbp->b_data, fs->fs_bsize);
  			bawrite(cbp);
  			if (dopersistence && ip->i_effnlink > 0)
  				(void) VOP_FSYNC(vp, KERNCRED, MNT_WAIT, td);
  			break;
  		}
  		savedcbp = cbp;
  	}
  	/*
  	 * 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 (savedcbp) {
  		vp = savedcbp->b_vp;
  		bawrite(savedcbp);
  		if (dopersistence && VTOI(vp)->i_effnlink > 0)
  			(void) VOP_FSYNC(vp, KERNCRED, MNT_WAIT, td);
  	}
  	if (snapshot_locked)
  		VOP_UNLOCK(vp, 0, td);
  	else
  		VI_UNLOCK(devvp);
  	return (error);
  }
  
  /*
   * Read the specified block into the given buffer.
   * Much of this boiler-plate comes from bwrite().
   */
  static int
  readblock(bp, lbn)
  	struct buf *bp;
  	ufs2_daddr_t lbn;
  {
  	struct uio auio;
  	struct iovec aiov;
  	struct thread *td = curthread;
  	struct inode *ip = VTOI(bp->b_vp);
  
  	aiov.iov_base = bp->b_data;
  	aiov.iov_len = bp->b_bcount;
  	auio.uio_iov = &aiov;
  	auio.uio_iovcnt = 1;
  	auio.uio_offset = dbtob(fsbtodb(ip->i_fs, blkstofrags(ip->i_fs, lbn)));
  	auio.uio_resid = bp->b_bcount;
  	auio.uio_rw = UIO_READ;
  	auio.uio_segflg = UIO_SYSSPACE;
  	auio.uio_td = td;
  	return (physio(ip->i_devvp->v_rdev, &auio, 0));
  }
  