freebsd/sys/kern/vfs_cluster.c

/*-
 * Copyright (c) 1993
 *	The Regents of the University of California.  All rights reserved.
 * Modifications/enhancements:
 * 	Copyright (c) 1995 John S. Dyson.  All rights reserved.
 *
 * 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.
 * 3. All advertising materials mentioning features or use of this software
 *    must display the following acknowledgement:
 *	This product includes software developed by the University of
 *	California, Berkeley and its contributors.
 * 4. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 *
 *	@(#)vfs_cluster.c	8.7 (Berkeley) 2/13/94
 * $Id: vfs_cluster.c,v 1.36 1996/06/03 04:40:35 dyson Exp $
 */

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/proc.h>
#include <sys/buf.h>
#include <sys/vnode.h>
#include <sys/mount.h>
#include <sys/malloc.h>
#include <sys/resourcevar.h>
#include <sys/vmmeter.h>
#include <miscfs/specfs/specdev.h>
#include <vm/vm.h>
#include <vm/vm_param.h>
#include <vm/vm_prot.h>
#include <vm/vm_object.h>
#include <vm/vm_page.h>

#ifdef notyet_block_reallocation_enabled
#ifdef DEBUG
#include <sys/sysctl.h>
#include <sys/kernel.h>

static int	doreallocblks = 0;
SYSCTL_INT(_debug, 13, doreallocblks, CTLFLAG_RW, &doreallocblks, 0, "");
#else
#define	doreallocblks 0
#endif
#endif /* notyet_block_reallocation_enabled */

#ifdef notyet_block_reallocation_enabled
static struct cluster_save *
	cluster_collectbufs __P((struct vnode *vp, struct buf *last_bp));
#endif
static struct buf *
	cluster_rbuild __P((struct vnode *vp, u_quad_t filesize, daddr_t lbn,
			    daddr_t blkno, long size, int run));

static int	totreads;
static int	totreadblocks;
extern vm_page_t	bogus_page;

#ifdef DIAGNOSTIC
/*
 * Set to 1 if reads of block zero should cause readahead to be done.
 * Set to 0 treats a read of block zero as a non-sequential read.
 *
 * Setting to one assumes that most reads of block zero of files are due to
 * sequential passes over the files (e.g. cat, sum) where additional blocks
 * will soon be needed.  Setting to zero assumes that the majority are
 * surgical strikes to get particular info (e.g. size, file) where readahead
 * blocks will not be used and, in fact, push out other potentially useful
 * blocks from the cache.  The former seems intuitive, but some quick tests
 * showed that the latter performed better from a system-wide point of view.
 */
	int doclusterraz = 0;

#define ISSEQREAD(vp, blk) \
	(((blk) != 0 || doclusterraz) && \
	 ((blk) == (vp)->v_lastr + 1 || (blk) == (vp)->v_lastr))
#else
#define ISSEQREAD(vp, blk) \
	(/* (blk) != 0 && */ ((blk) == (vp)->v_lastr + 1 || (blk) == (vp)->v_lastr))
#endif

/*
 * allow for three entire read-aheads...  The system will
 * adjust downwards rapidly if needed...
 */
#define RA_MULTIPLE_FAST	2
#define RA_MULTIPLE_SLOW	3
#define RA_SHIFTDOWN	1	/* approx lg2(RA_MULTIPLE) */
/*
 * This replaces bread.  If this is a bread at the beginning of a file and
 * lastr is 0, we assume this is the first read and we'll read up to two
 * blocks if they are sequential.  After that, we'll do regular read ahead
 * in clustered chunks.
 * 	bp is the block requested.
 *	rbp is the read-ahead block.
 *	If either is NULL, then you don't have to do the I/O.
 */
int
cluster_read(vp, filesize, lblkno, size, cred, bpp)
	struct vnode *vp;
	u_quad_t filesize;
	daddr_t lblkno;
	long size;
	struct ucred *cred;
	struct buf **bpp;
{
	struct buf *bp, *rbp;
	daddr_t blkno, rablkno, origlblkno;
	int error, num_ra, alreadyincore;
	int i;
	int seq;

	error = 0;
	/*
	 * get the requested block
	 */
	origlblkno = lblkno;
	*bpp = bp = getblk(vp, lblkno, size, 0, 0);

	seq = ISSEQREAD(vp, lblkno);
	/*
	 * if it is in the cache, then check to see if the reads have been
	 * sequential.  If they have, then try some read-ahead, otherwise
	 * back-off on prospective read-aheads.
	 */
	if (bp->b_flags & B_CACHE) {
		if (!seq) {
			vp->v_maxra = bp->b_lblkno + bp->b_bcount / size;
			vp->v_ralen >>= RA_SHIFTDOWN;
			return 0;
		} else if( vp->v_maxra > lblkno) {
			if ((vp->v_ralen + 1) < RA_MULTIPLE_FAST * (MAXPHYS / size))
				++vp->v_ralen;
			if ( vp->v_maxra > lblkno + vp->v_ralen ) {
				return 0;
			}
			lblkno = vp->v_maxra;
		} else {
			lblkno += 1;
		}
		bp = NULL;
	} else {
		/*
		 * if it isn't in the cache, then get a chunk from disk if
		 * sequential, otherwise just get the block.
		 */
		bp->b_flags |= B_READ;
		lblkno += 1;
		curproc->p_stats->p_ru.ru_inblock++;	/* XXX */
		vp->v_ralen = 0;
	}
	/*
	 * assume no read-ahead
	 */
	alreadyincore = 1;
	rablkno = lblkno;

	/*
	 * if we have been doing sequential I/O, then do some read-ahead
	 */
	if (seq) {
		alreadyincore = 0;

	/*
	 * bump ralen a bit...
	 */
		if ((vp->v_ralen + 1) < RA_MULTIPLE_SLOW*(MAXPHYS / size))
			++vp->v_ralen;
		/*
		 * this code makes sure that the stuff that we have read-ahead
		 * is still in the cache.  If it isn't, we have been reading
		 * ahead too much, and we need to back-off, otherwise we might
		 * try to read more.
		 */
		for (i = 0; i < vp->v_maxra - lblkno; i++) {
			rablkno = lblkno + i;
			alreadyincore = (int) incore(vp, rablkno);
			if (!alreadyincore) {
				vp->v_maxra = rablkno;
				vp->v_ralen >>= RA_SHIFTDOWN;
				alreadyincore = 1;
			}
		}
	}
	/*
	 * we now build the read-ahead buffer if it is desirable.
	 */
	rbp = NULL;
	if (!alreadyincore &&
	    ((u_quad_t)(rablkno + 1) * size) <= filesize &&
	    !(error = VOP_BMAP(vp, rablkno, NULL, &blkno, &num_ra, NULL)) &&
	    blkno != -1) {
		if (num_ra > vp->v_ralen)
			num_ra = vp->v_ralen;

		if (num_ra) {
			rbp = cluster_rbuild(vp, filesize, rablkno, blkno, size,
				num_ra + 1);
		} else {
			rbp = getblk(vp, rablkno, size, 0, 0);
			rbp->b_flags |= B_READ | B_ASYNC;
			rbp->b_blkno = blkno;
		}
	}

	/*
	 * handle the synchronous read
	 */
	if (bp) {
		if (bp->b_flags & (B_DONE | B_DELWRI))
			panic("cluster_read: DONE bp");
		else {
			vfs_busy_pages(bp, 0);
			error = VOP_STRATEGY(bp);
			vp->v_maxra = bp->b_lblkno + bp->b_bcount / size;
			totreads++;
			totreadblocks += bp->b_bcount / size;
			curproc->p_stats->p_ru.ru_inblock++;
		}
	}
	/*
	 * and if we have read-aheads, do them too
	 */
	if (rbp) {
		vp->v_maxra = rbp->b_lblkno + rbp->b_bcount / size;
		if (error) {
			rbp->b_flags &= ~(B_ASYNC | B_READ);
			brelse(rbp);
		} else if (rbp->b_flags & B_CACHE) {
			rbp->b_flags &= ~(B_ASYNC | B_READ);
			bqrelse(rbp);
		} else {
			if ((rbp->b_flags & B_CLUSTER) == 0)
				vfs_busy_pages(rbp, 0);
			(void) VOP_STRATEGY(rbp);
			totreads++;
			totreadblocks += rbp->b_bcount / size;
			curproc->p_stats->p_ru.ru_inblock++;
		}
	}
	if (bp && ((bp->b_flags & B_ASYNC) == 0))
		return (biowait(bp));
	return (error);
}

/*
 * If blocks are contiguous on disk, use this to provide clustered
 * read ahead.  We will read as many blocks as possible sequentially
 * and then parcel them up into logical blocks in the buffer hash table.
 */
static struct buf *
cluster_rbuild(vp, filesize, lbn, blkno, size, run)
	struct vnode *vp;
	u_quad_t filesize;
	daddr_t lbn;
	daddr_t blkno;
	long size;
	int run;
{
	struct buf *bp, *tbp;
	daddr_t bn;
	int i, inc, j;

#ifdef DIAGNOSTIC
	if (size != vp->v_mount->mnt_stat.f_iosize)
		panic("cluster_rbuild: size %d != filesize %d\n",
		    size, vp->v_mount->mnt_stat.f_iosize);
#endif
	/*
	 * avoid a division
	 */
	while ((u_quad_t) size * (lbn + run) > filesize) {
		--run;
	}

	tbp = getblk(vp, lbn, size, 0, 0);
	if (tbp->b_flags & B_CACHE)
		return tbp;

	tbp->b_blkno = blkno;
	tbp->b_flags |= B_ASYNC | B_READ; 
	if( (tbp->b_flags & B_MALLOC) ||
		((tbp->b_flags & B_VMIO) == 0) || (run <= 1) )
		return tbp;

	bp = trypbuf();
	if (bp == 0)
		return tbp;

	(vm_offset_t) bp->b_data |= ((vm_offset_t) tbp->b_data) & PAGE_MASK;
	bp->b_flags = B_ASYNC | B_READ | B_CALL | B_BUSY | B_CLUSTER | B_VMIO;
	bp->b_iodone = cluster_callback;
	bp->b_blkno = blkno;
	bp->b_lblkno = lbn;
	pbgetvp(vp, bp);

	TAILQ_INIT(&bp->b_cluster.cluster_head);

	bp->b_bcount = 0;
	bp->b_bufsize = 0;
	bp->b_npages = 0;

	inc = btodb(size);
	for (bn = blkno, i = 0; i < run; ++i, bn += inc) {
		if (i != 0) {
			if ((bp->b_npages * PAGE_SIZE) +
				round_page(size) > MAXPHYS)
				break;

			if (incore(vp, lbn + i))
				break;

			tbp = getblk(vp, lbn + i, size, 0, 0);

			if ((tbp->b_flags & B_CACHE) ||
				(tbp->b_flags & B_VMIO) == 0) {
				bqrelse(tbp);
				break;
			}

			for (j=0;j<tbp->b_npages;j++) {
				if (tbp->b_pages[j]->valid) {
					break;
				}
			}

			if (j != tbp->b_npages) {
				/*
				 * force buffer to be re-constituted later
				 */
				tbp->b_flags |= B_RELBUF;
				brelse(tbp);
				break;
			}

			tbp->b_flags |= B_READ | B_ASYNC;
			if (tbp->b_blkno == tbp->b_lblkno) {
				tbp->b_blkno = bn;
			} else if (tbp->b_blkno != bn) {
				brelse(tbp);
				break;
			}
		}
		TAILQ_INSERT_TAIL(&bp->b_cluster.cluster_head,
			tbp, b_cluster.cluster_entry);
		for (j = 0; j < tbp->b_npages; j += 1) {
			vm_page_t m;
			m = tbp->b_pages[j];
			++m->busy;
			++m->object->paging_in_progress;
			if ((m->valid & VM_PAGE_BITS_ALL) == VM_PAGE_BITS_ALL) {
				m = bogus_page;
			}
			if ((bp->b_npages == 0) ||
				(bp->b_pages[bp->b_npages-1] != m)) {
				bp->b_pages[bp->b_npages] = m;
				bp->b_npages++;
			}
		}
		bp->b_bcount += tbp->b_bcount;
		bp->b_bufsize += tbp->b_bufsize;
	}
	pmap_qenter(trunc_page((vm_offset_t) bp->b_data),
		(vm_page_t *)bp->b_pages, bp->b_npages);
	return (bp);
}

/*
 * Cleanup after a clustered read or write.
 * This is complicated by the fact that any of the buffers might have
 * extra memory (if there were no empty buffer headers at allocbuf time)
 * that we will need to shift around.
 */
void
cluster_callback(bp)
	struct buf *bp;
{
	struct buf *nbp, *tbp;
	int error = 0;

	/*
	 * Must propogate errors to all the components.
	 */
	if (bp->b_flags & B_ERROR)
		error = bp->b_error;

	pmap_qremove(trunc_page((vm_offset_t) bp->b_data), bp->b_npages);
	/*
	 * Move memory from the large cluster buffer into the component
	 * buffers and mark IO as done on these.
	 */
	for (tbp = bp->b_cluster.cluster_head.tqh_first;
		tbp; tbp = nbp) {
		nbp = tbp->b_cluster.cluster_entry.tqe_next;
		if (error) {
			tbp->b_flags |= B_ERROR;
			tbp->b_error = error;
		}
		biodone(tbp);
	}
	relpbuf(bp);
}

/*
 * Do clustered write for FFS.
 *
 * Three cases:
 *	1. Write is not sequential (write asynchronously)
 *	Write is sequential:
 *	2.	beginning of cluster - begin cluster
 *	3.	middle of a cluster - add to cluster
 *	4.	end of a cluster - asynchronously write cluster
 */
void
cluster_write(bp, filesize)
	struct buf *bp;
	u_quad_t filesize;
{
	struct vnode *vp;
	daddr_t lbn;
	int maxclen, cursize;
	int lblocksize;
	int async;

	vp = bp->b_vp;
	async = (vp->v_mount && (vp->v_mount->mnt_flag & MNT_ASYNC));
	lblocksize = vp->v_mount->mnt_stat.f_iosize;
	lbn = bp->b_lblkno;

	/* Initialize vnode to beginning of file. */
	if (lbn == 0)
		vp->v_lasta = vp->v_clen = vp->v_cstart = vp->v_lastw = 0;

	if (vp->v_clen == 0 || lbn != vp->v_lastw + 1 ||
	    (bp->b_blkno != vp->v_lasta + btodb(lblocksize))) {
		maxclen = MAXPHYS / lblocksize - 1;
		if (vp->v_clen != 0) {
			/*
			 * Next block is not sequential.
			 *
			 * If we are not writing at end of file, the process
			 * seeked to another point in the file since its last
			 * write, or we have reached our maximum cluster size,
			 * then push the previous cluster. Otherwise try
			 * reallocating to make it sequential.
			 */
			cursize = vp->v_lastw - vp->v_cstart + 1;
#ifndef notyet_block_reallocation_enabled
			if (((u_quad_t)(lbn + 1) * lblocksize) != filesize ||
				lbn != vp->v_lastw + 1 ||
				vp->v_clen <= cursize) {
				if (!async)
					cluster_wbuild(vp, lblocksize,
						vp->v_cstart, cursize);
			}
#else
			if (!doreallocblks ||
			    (lbn + 1) * lblocksize != filesize ||
			    lbn != vp->v_lastw + 1 || vp->v_clen <= cursize) {
				if (!async)
					cluster_wbuild(vp, lblocksize,
						vp->v_cstart, cursize);
			} else {
				struct buf **bpp, **endbp;
				struct cluster_save *buflist;

				buflist = cluster_collectbufs(vp, bp);
				endbp = &buflist->bs_children
				    [buflist->bs_nchildren - 1];
				if (VOP_REALLOCBLKS(vp, buflist)) {
					/*
					 * Failed, push the previous cluster.
					 */
					for (bpp = buflist->bs_children;
					     bpp < endbp; bpp++)
						brelse(*bpp);
					free(buflist, M_SEGMENT);
					cluster_wbuild(vp, lblocksize,
					    vp->v_cstart, cursize);
				} else {
					/*
					 * Succeeded, keep building cluster.
					 */
					for (bpp = buflist->bs_children;
					     bpp <= endbp; bpp++)
						bdwrite(*bpp);
					free(buflist, M_SEGMENT);
					vp->v_lastw = lbn;
					vp->v_lasta = bp->b_blkno;
					return;
				}
			}
#endif /* notyet_block_reallocation_enabled */
		}
		/*
		 * Consider beginning a cluster. If at end of file, make
		 * cluster as large as possible, otherwise find size of
		 * existing cluster.
		 */
		if (((u_quad_t) (lbn + 1) * lblocksize) != filesize &&
		    (bp->b_blkno == bp->b_lblkno) &&
		    (VOP_BMAP(vp, lbn, NULL, &bp->b_blkno, &maxclen, NULL) ||
		     bp->b_blkno == -1)) {
			bawrite(bp);
			vp->v_clen = 0;
			vp->v_lasta = bp->b_blkno;
			vp->v_cstart = lbn + 1;
			vp->v_lastw = lbn;
			return;
		}
		vp->v_clen = maxclen;
		if (!async && maxclen == 0) {	/* I/O not contiguous */
			vp->v_cstart = lbn + 1;
			bawrite(bp);
		} else {	/* Wait for rest of cluster */
			vp->v_cstart = lbn;
			bdwrite(bp);
		}
	} else if (lbn == vp->v_cstart + vp->v_clen) {
		/*
		 * At end of cluster, write it out.
		 */
		bdwrite(bp);
		cluster_wbuild(vp, lblocksize, vp->v_cstart, vp->v_clen + 1);
		vp->v_clen = 0;
		vp->v_cstart = lbn + 1;
	} else
		/*
		 * In the middle of a cluster, so just delay the I/O for now.
		 */
		bdwrite(bp);
	vp->v_lastw = lbn;
	vp->v_lasta = bp->b_blkno;
}


/*
 * This is an awful lot like cluster_rbuild...wish they could be combined.
 * The last lbn argument is the current block on which I/O is being
 * performed.  Check to see that it doesn't fall in the middle of
 * the current block (if last_bp == NULL).
 */
int
cluster_wbuild(vp, size, start_lbn, len)
	struct vnode *vp;
	long size;
	daddr_t start_lbn;
	int len;
{
	struct buf *bp, *tbp;
	int i, j, s;
	int totalwritten = 0;
	int dbsize = btodb(size);
	while (len > 0) {
		s = splbio();
		if ( ((tbp = gbincore(vp, start_lbn)) == NULL) ||
			((tbp->b_flags & (B_INVAL|B_BUSY|B_DELWRI)) != B_DELWRI)) {
			++start_lbn;
			--len;
			splx(s);
			continue;
		}
		bremfree(tbp);
		tbp->b_flags |= B_BUSY;
		tbp->b_flags &= ~B_DONE;
		splx(s);

	/*
	 * Extra memory in the buffer, punt on this buffer. XXX we could
	 * handle this in most cases, but we would have to push the extra
	 * memory down to after our max possible cluster size and then
	 * potentially pull it back up if the cluster was terminated
	 * prematurely--too much hassle.
	 */
		if (((tbp->b_flags & (B_CLUSTEROK|B_MALLOC)) != B_CLUSTEROK) ||
			(tbp->b_bcount != tbp->b_bufsize) ||
			(tbp->b_bcount != size) ||
			len == 1) {
			totalwritten += tbp->b_bufsize;
			bawrite(tbp);
			++start_lbn;
			--len;
			continue;
		}

		bp = trypbuf();
		if (bp == NULL) {
			totalwritten += tbp->b_bufsize;
			bawrite(tbp);
			++start_lbn;
			--len;
			continue;
		}

		TAILQ_INIT(&bp->b_cluster.cluster_head);
		bp->b_bcount = 0;
		bp->b_bufsize = 0;
		bp->b_npages = 0;
		if (tbp->b_wcred != NOCRED) {
		    bp->b_wcred = tbp->b_wcred;
		    crhold(bp->b_wcred);
		}

		bp->b_blkno = tbp->b_blkno;
		bp->b_lblkno = tbp->b_lblkno;
		(vm_offset_t) bp->b_data |= ((vm_offset_t) tbp->b_data) & PAGE_MASK;
		bp->b_flags |= B_CALL | B_BUSY | B_CLUSTER | (tbp->b_flags & (B_VMIO|B_NEEDCOMMIT));
		bp->b_iodone = cluster_callback;
		pbgetvp(vp, bp);

		for (i = 0; i < len; ++i, ++start_lbn) {
			if (i != 0) {
				s = splbio();
				if ((tbp = gbincore(vp, start_lbn)) == NULL) {
					splx(s);
					break;
				}

				if ((tbp->b_flags & (B_VMIO|B_CLUSTEROK|B_INVAL|B_BUSY|B_DELWRI|B_NEEDCOMMIT)) != (B_DELWRI|B_CLUSTEROK|(bp->b_flags & (B_VMIO|B_NEEDCOMMIT)))) {
					splx(s);
					break;
				}

				if (tbp->b_wcred != bp->b_wcred) {
					splx(s);
					break;
				}

				if ((tbp->b_bcount != size) ||
					((bp->b_blkno + dbsize * i) != tbp->b_blkno) ||
					((tbp->b_npages + bp->b_npages) > (MAXPHYS / PAGE_SIZE))) {
					splx(s);
					break;
				}
				bremfree(tbp);
				tbp->b_flags |= B_BUSY;
				tbp->b_flags &= ~B_DONE;
				splx(s);
			}
			if (tbp->b_flags & B_VMIO) {
				for (j = 0; j < tbp->b_npages; j += 1) {
					vm_page_t m;
					m = tbp->b_pages[j];
					++m->busy;
					++m->object->paging_in_progress;
					if ((bp->b_npages == 0) ||
						(bp->b_pages[bp->b_npages - 1] != m)) {
						bp->b_pages[bp->b_npages] = m;
						bp->b_npages++;
					}
				}
			}
			bp->b_bcount += size;
			bp->b_bufsize += size;

			tbp->b_flags &= ~(B_READ | B_DONE | B_ERROR | B_DELWRI);
			tbp->b_flags |= B_ASYNC;
			s = splbio();
			reassignbuf(tbp, tbp->b_vp);	/* put on clean list */
			++tbp->b_vp->v_numoutput;
			splx(s);
			TAILQ_INSERT_TAIL(&bp->b_cluster.cluster_head,
				tbp, b_cluster.cluster_entry);
		}
		pmap_qenter(trunc_page((vm_offset_t) bp->b_data),
			(vm_page_t *) bp->b_pages, bp->b_npages);
		totalwritten += bp->b_bufsize;
		bp->b_dirtyoff = 0;
		bp->b_dirtyend = bp->b_bufsize;
		bawrite(bp);

		len -= i;
	}
	return totalwritten;
}

#ifdef notyet_block_reallocation_enabled
/*
 * Collect together all the buffers in a cluster.
 * Plus add one additional buffer.
 */
static struct cluster_save *
cluster_collectbufs(vp, last_bp)
	struct vnode *vp;
	struct buf *last_bp;
{
	struct cluster_save *buflist;
	daddr_t lbn;
	int i, len;

	len = vp->v_lastw - vp->v_cstart + 1;
	buflist = malloc(sizeof(struct buf *) * (len + 1) + sizeof(*buflist),
	    M_SEGMENT, M_WAITOK);
	buflist->bs_nchildren = 0;
	buflist->bs_children = (struct buf **) (buflist + 1);
	for (lbn = vp->v_cstart, i = 0; i < len; lbn++, i++)
		(void) bread(vp, lbn, last_bp->b_bcount, NOCRED,
		    &buflist->bs_children[i]);
	buflist->bs_children[i] = last_bp;
	buflist->bs_nchildren = i + 1;
	return (buflist);
}
#endif /* notyet_block_reallocation_enabled */