fs/xfs/xfs_iomap.c

/*
 * Copyright (c) 2000-2006 Silicon Graphics, Inc.
 * All Rights Reserved.
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License as
 * published by the Free Software Foundation.
 *
 * This program is distributed in the hope that it would be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write the Free Software Foundation,
 * Inc.,  51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
 */
#include "xfs.h"
#include "xfs_fs.h"
#include "xfs_shared.h"
#include "xfs_format.h"
#include "xfs_log_format.h"
#include "xfs_trans_resv.h"
#include "xfs_mount.h"
#include "xfs_inode.h"
#include "xfs_btree.h"
#include "xfs_bmap_btree.h"
#include "xfs_bmap.h"
#include "xfs_bmap_util.h"
#include "xfs_error.h"
#include "xfs_trans.h"
#include "xfs_trans_space.h"
#include "xfs_iomap.h"
#include "xfs_trace.h"
#include "xfs_icache.h"
#include "xfs_quota.h"
#include "xfs_dquot_item.h"
#include "xfs_dquot.h"


#define XFS_WRITEIO_ALIGN(mp,off)	(((off) >> mp->m_writeio_log) \
						<< mp->m_writeio_log)
#define XFS_WRITE_IMAPS		XFS_BMAP_MAX_NMAP

STATIC int
xfs_iomap_eof_align_last_fsb(
	xfs_mount_t	*mp,
	xfs_inode_t	*ip,
	xfs_extlen_t	extsize,
	xfs_fileoff_t	*last_fsb)
{
	xfs_extlen_t	align = 0;
	int		eof, error;

	if (!XFS_IS_REALTIME_INODE(ip)) {
		/*
		 * Round up the allocation request to a stripe unit
		 * (m_dalign) boundary if the file size is >= stripe unit
		 * size, and we are allocating past the allocation eof.
		 *
		 * If mounted with the "-o swalloc" option the alignment is
		 * increased from the strip unit size to the stripe width.
		 */
		if (mp->m_swidth && (mp->m_flags & XFS_MOUNT_SWALLOC))
			align = mp->m_swidth;
		else if (mp->m_dalign)
			align = mp->m_dalign;

		if (align && XFS_ISIZE(ip) < XFS_FSB_TO_B(mp, align))
			align = 0;
	}

	/*
	 * Always round up the allocation request to an extent boundary
	 * (when file on a real-time subvolume or has di_extsize hint).
	 */
	if (extsize) {
		if (align)
			align = roundup_64(align, extsize);
		else
			align = extsize;
	}

	if (align) {
		xfs_fileoff_t	new_last_fsb = roundup_64(*last_fsb, align);
		error = xfs_bmap_eof(ip, new_last_fsb, XFS_DATA_FORK, &eof);
		if (error)
			return error;
		if (eof)
			*last_fsb = new_last_fsb;
	}
	return 0;
}

STATIC int
xfs_alert_fsblock_zero(
	xfs_inode_t	*ip,
	xfs_bmbt_irec_t	*imap)
{
	xfs_alert_tag(ip->i_mount, XFS_PTAG_FSBLOCK_ZERO,
			"Access to block zero in inode %llu "
			"start_block: %llx start_off: %llx "
			"blkcnt: %llx extent-state: %x",
		(unsigned long long)ip->i_ino,
		(unsigned long long)imap->br_startblock,
		(unsigned long long)imap->br_startoff,
		(unsigned long long)imap->br_blockcount,
		imap->br_state);
	return -EFSCORRUPTED;
}

int
xfs_iomap_write_direct(
	xfs_inode_t	*ip,
	xfs_off_t	offset,
	size_t		count,
	xfs_bmbt_irec_t *imap,
	int		nmaps)
{
	xfs_mount_t	*mp = ip->i_mount;
	xfs_fileoff_t	offset_fsb;
	xfs_fileoff_t	last_fsb;
	xfs_filblks_t	count_fsb, resaligned;
	xfs_fsblock_t	firstfsb;
	xfs_extlen_t	extsz, temp;
	int		nimaps;
	int		quota_flag;
	int		rt;
	xfs_trans_t	*tp;
	xfs_bmap_free_t free_list;
	uint		qblocks, resblks, resrtextents;
	int		committed;
	int		error;
	int		lockmode;
	int		bmapi_flags = XFS_BMAPI_PREALLOC;

	rt = XFS_IS_REALTIME_INODE(ip);
	extsz = xfs_get_extsz_hint(ip);
	lockmode = XFS_ILOCK_SHARED;	/* locked by caller */

	ASSERT(xfs_isilocked(ip, lockmode));

	offset_fsb = XFS_B_TO_FSBT(mp, offset);
	last_fsb = XFS_B_TO_FSB(mp, ((xfs_ufsize_t)(offset + count)));
	if ((offset + count) > XFS_ISIZE(ip)) {
		/*
		 * Assert that the in-core extent list is present since this can
		 * call xfs_iread_extents() and we only have the ilock shared.
		 * This should be safe because the lock was held around a bmapi
		 * call in the caller and we only need it to access the in-core
		 * list.
		 */
		ASSERT(XFS_IFORK_PTR(ip, XFS_DATA_FORK)->if_flags &
								XFS_IFEXTENTS);
		error = xfs_iomap_eof_align_last_fsb(mp, ip, extsz, &last_fsb);
		if (error)
			goto out_unlock;
	} else {
		if (nmaps && (imap->br_startblock == HOLESTARTBLOCK))
			last_fsb = MIN(last_fsb, (xfs_fileoff_t)
					imap->br_blockcount +
					imap->br_startoff);
	}
	count_fsb = last_fsb - offset_fsb;
	ASSERT(count_fsb > 0);

	resaligned = count_fsb;
	if (unlikely(extsz)) {
		if ((temp = do_mod(offset_fsb, extsz)))
			resaligned += temp;
		if ((temp = do_mod(resaligned, extsz)))
			resaligned += extsz - temp;
	}

	if (unlikely(rt)) {
		resrtextents = qblocks = resaligned;
		resrtextents /= mp->m_sb.sb_rextsize;
		resblks = XFS_DIOSTRAT_SPACE_RES(mp, 0);
		quota_flag = XFS_QMOPT_RES_RTBLKS;
	} else {
		resrtextents = 0;
		resblks = qblocks = XFS_DIOSTRAT_SPACE_RES(mp, resaligned);
		quota_flag = XFS_QMOPT_RES_REGBLKS;
	}

	/*
	 * Drop the shared lock acquired by the caller, attach the dquot if
	 * necessary and move on to transaction setup.
	 */
	xfs_iunlock(ip, lockmode);
	error = xfs_qm_dqattach(ip, 0);
	if (error)
		return error;

	/*
	 * Allocate and setup the transaction
	 */
	tp = xfs_trans_alloc(mp, XFS_TRANS_DIOSTRAT);

	/*
	 * For DAX, we do not allocate unwritten extents, but instead we zero
	 * the block before we commit the transaction.  Ideally we'd like to do
	 * this outside the transaction context, but if we commit and then crash
	 * we may not have zeroed the blocks and this will be exposed on
	 * recovery of the allocation. Hence we must zero before commit.
	 * Further, if we are mapping unwritten extents here, we need to zero
	 * and convert them to written so that we don't need an unwritten extent
	 * callback for DAX. This also means that we need to be able to dip into
	 * the reserve block pool if there is no space left but we need to do
	 * unwritten extent conversion.
	 */
	if (IS_DAX(VFS_I(ip))) {
		bmapi_flags = XFS_BMAPI_CONVERT | XFS_BMAPI_ZERO;
		tp->t_flags |= XFS_TRANS_RESERVE;
	}
	error = xfs_trans_reserve(tp, &M_RES(mp)->tr_write,
				  resblks, resrtextents);
	/*
	 * Check for running out of space, note: need lock to return
	 */
	if (error) {
		xfs_trans_cancel(tp);
		return error;
	}

	lockmode = XFS_ILOCK_EXCL;
	xfs_ilock(ip, lockmode);

	error = xfs_trans_reserve_quota_nblks(tp, ip, qblocks, 0, quota_flag);
	if (error)
		goto out_trans_cancel;

	xfs_trans_ijoin(tp, ip, 0);

	/*
	 * From this point onwards we overwrite the imap pointer that the
	 * caller gave to us.
	 */
	xfs_bmap_init(&free_list, &firstfsb);
	nimaps = 1;
	error = xfs_bmapi_write(tp, ip, offset_fsb, count_fsb,
				bmapi_flags, &firstfsb, resblks, imap,
				&nimaps, &free_list);
	if (error)
		goto out_bmap_cancel;

	/*
	 * Complete the transaction
	 */
	error = xfs_bmap_finish(&tp, &free_list, &committed);
	if (error)
		goto out_bmap_cancel;

	error = xfs_trans_commit(tp);
	if (error)
		goto out_unlock;

	/*
	 * Copy any maps to caller's array and return any error.
	 */
	if (nimaps == 0) {
		error = -ENOSPC;
		goto out_unlock;
	}

	if (!(imap->br_startblock || XFS_IS_REALTIME_INODE(ip)))
		error = xfs_alert_fsblock_zero(ip, imap);

out_unlock:
	xfs_iunlock(ip, lockmode);
	return error;

out_bmap_cancel:
	xfs_bmap_cancel(&free_list);
	xfs_trans_unreserve_quota_nblks(tp, ip, (long)qblocks, 0, quota_flag);
out_trans_cancel:
	xfs_trans_cancel(tp);
	goto out_unlock;
}

/*
 * If the caller is doing a write at the end of the file, then extend the
 * allocation out to the file system's write iosize.  We clean up any extra
 * space left over when the file is closed in xfs_inactive().
 *
 * If we find we already have delalloc preallocation beyond EOF, don't do more
 * preallocation as it it not needed.
 */
STATIC int
xfs_iomap_eof_want_preallocate(
	xfs_mount_t	*mp,
	xfs_inode_t	*ip,
	xfs_off_t	offset,
	size_t		count,
	xfs_bmbt_irec_t *imap,
	int		nimaps,
	int		*prealloc)
{
	xfs_fileoff_t   start_fsb;
	xfs_filblks_t   count_fsb;
	int		n, error, imaps;
	int		found_delalloc = 0;

	*prealloc = 0;
	if (offset + count <= XFS_ISIZE(ip))
		return 0;

	/*
	 * If the file is smaller than the minimum prealloc and we are using
	 * dynamic preallocation, don't do any preallocation at all as it is
	 * likely this is the only write to the file that is going to be done.
	 */
	if (!(mp->m_flags & XFS_MOUNT_DFLT_IOSIZE) &&
	    XFS_ISIZE(ip) < XFS_FSB_TO_B(mp, mp->m_writeio_blocks))
		return 0;

	/*
	 * If there are any real blocks past eof, then don't
	 * do any speculative allocation.
	 */
	start_fsb = XFS_B_TO_FSBT(mp, ((xfs_ufsize_t)(offset + count - 1)));
	count_fsb = XFS_B_TO_FSB(mp, mp->m_super->s_maxbytes);
	while (count_fsb > 0) {
		imaps = nimaps;
		error = xfs_bmapi_read(ip, start_fsb, count_fsb, imap, &imaps,
				       0);
		if (error)
			return error;
		for (n = 0; n < imaps; n++) {
			if ((imap[n].br_startblock != HOLESTARTBLOCK) &&
			    (imap[n].br_startblock != DELAYSTARTBLOCK))
				return 0;
			start_fsb += imap[n].br_blockcount;
			count_fsb -= imap[n].br_blockcount;

			if (imap[n].br_startblock == DELAYSTARTBLOCK)
				found_delalloc = 1;
		}
	}
	if (!found_delalloc)
		*prealloc = 1;
	return 0;
}

/*
 * Determine the initial size of the preallocation. We are beyond the current
 * EOF here, but we need to take into account whether this is a sparse write or
 * an extending write when determining the preallocation size.  Hence we need to
 * look up the extent that ends at the current write offset and use the result
 * to determine the preallocation size.
 *
 * If the extent is a hole, then preallocation is essentially disabled.
 * Otherwise we take the size of the preceeding data extent as the basis for the
 * preallocation size. If the size of the extent is greater than half the
 * maximum extent length, then use the current offset as the basis. This ensures
 * that for large files the preallocation size always extends to MAXEXTLEN
 * rather than falling short due to things like stripe unit/width alignment of
 * real extents.
 */
STATIC xfs_fsblock_t
xfs_iomap_eof_prealloc_initial_size(
	struct xfs_mount	*mp,
	struct xfs_inode	*ip,
	xfs_off_t		offset,
	xfs_bmbt_irec_t		*imap,
	int			nimaps)
{
	xfs_fileoff_t   start_fsb;
	int		imaps = 1;
	int		error;

	ASSERT(nimaps >= imaps);

	/* if we are using a specific prealloc size, return now */
	if (mp->m_flags & XFS_MOUNT_DFLT_IOSIZE)
		return 0;

	/* If the file is small, then use the minimum prealloc */
	if (XFS_ISIZE(ip) < XFS_FSB_TO_B(mp, mp->m_dalign))
		return 0;

	/*
	 * As we write multiple pages, the offset will always align to the
	 * start of a page and hence point to a hole at EOF. i.e. if the size is
	 * 4096 bytes, we only have one block at FSB 0, but XFS_B_TO_FSB(4096)
	 * will return FSB 1. Hence if there are blocks in the file, we want to
	 * point to the block prior to the EOF block and not the hole that maps
	 * directly at @offset.
	 */
	start_fsb = XFS_B_TO_FSB(mp, offset);
	if (start_fsb)
		start_fsb--;
	error = xfs_bmapi_read(ip, start_fsb, 1, imap, &imaps, XFS_BMAPI_ENTIRE);
	if (error)
		return 0;

	ASSERT(imaps == 1);
	if (imap[0].br_startblock == HOLESTARTBLOCK)
		return 0;
	if (imap[0].br_blockcount <= (MAXEXTLEN >> 1))
		return imap[0].br_blockcount << 1;
	return XFS_B_TO_FSB(mp, offset);
}

STATIC bool
xfs_quota_need_throttle(
	struct xfs_inode *ip,
	int type,
	xfs_fsblock_t alloc_blocks)
{
	struct xfs_dquot *dq = xfs_inode_dquot(ip, type);

	if (!dq || !xfs_this_quota_on(ip->i_mount, type))
		return false;

	/* no hi watermark, no throttle */
	if (!dq->q_prealloc_hi_wmark)
		return false;

	/* under the lo watermark, no throttle */
	if (dq->q_res_bcount + alloc_blocks < dq->q_prealloc_lo_wmark)
		return false;

	return true;
}

STATIC void
xfs_quota_calc_throttle(
	struct xfs_inode *ip,
	int type,
	xfs_fsblock_t *qblocks,
	int *qshift,
	int64_t	*qfreesp)
{
	int64_t freesp;
	int shift = 0;
	struct xfs_dquot *dq = xfs_inode_dquot(ip, type);

	/* no dq, or over hi wmark, squash the prealloc completely */
	if (!dq || dq->q_res_bcount >= dq->q_prealloc_hi_wmark) {
		*qblocks = 0;
		*qfreesp = 0;
		return;
	}

	freesp = dq->q_prealloc_hi_wmark - dq->q_res_bcount;
	if (freesp < dq->q_low_space[XFS_QLOWSP_5_PCNT]) {
		shift = 2;
		if (freesp < dq->q_low_space[XFS_QLOWSP_3_PCNT])
			shift += 2;
		if (freesp < dq->q_low_space[XFS_QLOWSP_1_PCNT])
			shift += 2;
	}

	if (freesp < *qfreesp)
		*qfreesp = freesp;

	/* only overwrite the throttle values if we are more aggressive */
	if ((freesp >> shift) < (*qblocks >> *qshift)) {
		*qblocks = freesp;
		*qshift = shift;
	}
}

/*
 * If we don't have a user specified preallocation size, dynamically increase
 * the preallocation size as the size of the file grows. Cap the maximum size
 * at a single extent or less if the filesystem is near full. The closer the
 * filesystem is to full, the smaller the maximum prealocation.
 */
STATIC xfs_fsblock_t
xfs_iomap_prealloc_size(
	struct xfs_mount	*mp,
	struct xfs_inode	*ip,
	xfs_off_t		offset,
	struct xfs_bmbt_irec	*imap,
	int			nimaps)
{
	xfs_fsblock_t		alloc_blocks = 0;
	int			shift = 0;
	int64_t			freesp;
	xfs_fsblock_t		qblocks;
	int			qshift = 0;

	alloc_blocks = xfs_iomap_eof_prealloc_initial_size(mp, ip, offset,
							   imap, nimaps);
	if (!alloc_blocks)
		goto check_writeio;
	qblocks = alloc_blocks;

	/*
	 * MAXEXTLEN is not a power of two value but we round the prealloc down
	 * to the nearest power of two value after throttling. To prevent the
	 * round down from unconditionally reducing the maximum supported prealloc
	 * size, we round up first, apply appropriate throttling, round down and
	 * cap the value to MAXEXTLEN.
	 */
	alloc_blocks = XFS_FILEOFF_MIN(roundup_pow_of_two(MAXEXTLEN),
				       alloc_blocks);

	freesp = percpu_counter_read_positive(&mp->m_fdblocks);
	if (freesp < mp->m_low_space[XFS_LOWSP_5_PCNT]) {
		shift = 2;
		if (freesp < mp->m_low_space[XFS_LOWSP_4_PCNT])
			shift++;
		if (freesp < mp->m_low_space[XFS_LOWSP_3_PCNT])
			shift++;
		if (freesp < mp->m_low_space[XFS_LOWSP_2_PCNT])
			shift++;
		if (freesp < mp->m_low_space[XFS_LOWSP_1_PCNT])
			shift++;
	}

	/*
	 * Check each quota to cap the prealloc size, provide a shift value to
	 * throttle with and adjust amount of available space.
	 */
	if (xfs_quota_need_throttle(ip, XFS_DQ_USER, alloc_blocks))
		xfs_quota_calc_throttle(ip, XFS_DQ_USER, &qblocks, &qshift,
					&freesp);
	if (xfs_quota_need_throttle(ip, XFS_DQ_GROUP, alloc_blocks))
		xfs_quota_calc_throttle(ip, XFS_DQ_GROUP, &qblocks, &qshift,
					&freesp);
	if (xfs_quota_need_throttle(ip, XFS_DQ_PROJ, alloc_blocks))
		xfs_quota_calc_throttle(ip, XFS_DQ_PROJ, &qblocks, &qshift,
					&freesp);

	/*
	 * The final prealloc size is set to the minimum of free space available
	 * in each of the quotas and the overall filesystem.
	 *
	 * The shift throttle value is set to the maximum value as determined by
	 * the global low free space values and per-quota low free space values.
	 */
	alloc_blocks = MIN(alloc_blocks, qblocks);
	shift = MAX(shift, qshift);

	if (shift)
		alloc_blocks >>= shift;
	/*
	 * rounddown_pow_of_two() returns an undefined result if we pass in
	 * alloc_blocks = 0.
	 */
	if (alloc_blocks)
		alloc_blocks = rounddown_pow_of_two(alloc_blocks);
	if (alloc_blocks > MAXEXTLEN)
		alloc_blocks = MAXEXTLEN;

	/*
	 * If we are still trying to allocate more space than is
	 * available, squash the prealloc hard. This can happen if we
	 * have a large file on a small filesystem and the above
	 * lowspace thresholds are smaller than MAXEXTLEN.
	 */
	while (alloc_blocks && alloc_blocks >= freesp)
		alloc_blocks >>= 4;

check_writeio:
	if (alloc_blocks < mp->m_writeio_blocks)
		alloc_blocks = mp->m_writeio_blocks;

	trace_xfs_iomap_prealloc_size(ip, alloc_blocks, shift,
				      mp->m_writeio_blocks);

	return alloc_blocks;
}

int
xfs_iomap_write_delay(
	xfs_inode_t	*ip,
	xfs_off_t	offset,
	size_t		count,
	xfs_bmbt_irec_t *ret_imap)
{
	xfs_mount_t	*mp = ip->i_mount;
	xfs_fileoff_t	offset_fsb;
	xfs_fileoff_t	last_fsb;
	xfs_off_t	aligned_offset;
	xfs_fileoff_t	ioalign;
	xfs_extlen_t	extsz;
	int		nimaps;
	xfs_bmbt_irec_t imap[XFS_WRITE_IMAPS];
	int		prealloc;
	int		error;

	ASSERT(xfs_isilocked(ip, XFS_ILOCK_EXCL));

	/*
	 * Make sure that the dquots are there. This doesn't hold
	 * the ilock across a disk read.
	 */
	error = xfs_qm_dqattach_locked(ip, 0);
	if (error)
		return error;

	extsz = xfs_get_extsz_hint(ip);
	offset_fsb = XFS_B_TO_FSBT(mp, offset);

	error = xfs_iomap_eof_want_preallocate(mp, ip, offset, count,
				imap, XFS_WRITE_IMAPS, &prealloc);
	if (error)
		return error;

retry:
	if (prealloc) {
		xfs_fsblock_t	alloc_blocks;

		alloc_blocks = xfs_iomap_prealloc_size(mp, ip, offset, imap,
						       XFS_WRITE_IMAPS);

		aligned_offset = XFS_WRITEIO_ALIGN(mp, (offset + count - 1));
		ioalign = XFS_B_TO_FSBT(mp, aligned_offset);
		last_fsb = ioalign + alloc_blocks;
	} else {
		last_fsb = XFS_B_TO_FSB(mp, ((xfs_ufsize_t)(offset + count)));
	}

	if (prealloc || extsz) {
		error = xfs_iomap_eof_align_last_fsb(mp, ip, extsz, &last_fsb);
		if (error)
			return error;
	}

	/*
	 * Make sure preallocation does not create extents beyond the range we
	 * actually support in this filesystem.
	 */
	if (last_fsb > XFS_B_TO_FSB(mp, mp->m_super->s_maxbytes))
		last_fsb = XFS_B_TO_FSB(mp, mp->m_super->s_maxbytes);

	ASSERT(last_fsb > offset_fsb);

	nimaps = XFS_WRITE_IMAPS;
	error = xfs_bmapi_delay(ip, offset_fsb, last_fsb - offset_fsb,
				imap, &nimaps, XFS_BMAPI_ENTIRE);
	switch (error) {
	case 0:
	case -ENOSPC:
	case -EDQUOT:
		break;
	default:
		return error;
	}

	/*
	 * If bmapi returned us nothing, we got either ENOSPC or EDQUOT. Retry
	 * without EOF preallocation.
	 */
	if (nimaps == 0) {
		trace_xfs_delalloc_enospc(ip, offset, count);
		if (prealloc) {
			prealloc = 0;
			error = 0;
			goto retry;
		}
		return error ? error : -ENOSPC;
	}

	if (!(imap[0].br_startblock || XFS_IS_REALTIME_INODE(ip)))
		return xfs_alert_fsblock_zero(ip, &imap[0]);

	/*
	 * Tag the inode as speculatively preallocated so we can reclaim this
	 * space on demand, if necessary.
	 */
	if (prealloc)
		xfs_inode_set_eofblocks_tag(ip);

	*ret_imap = imap[0];
	return 0;
}

/*
 * Pass in a delayed allocate extent, convert it to real extents;
 * return to the caller the extent we create which maps on top of
 * the originating callers request.
 *
 * Called without a lock on the inode.
 *
 * We no longer bother to look at the incoming map - all we have to
 * guarantee is that whatever we allocate fills the required range.
 */
int
xfs_iomap_write_allocate(
	xfs_inode_t	*ip,
	xfs_off_t	offset,
	xfs_bmbt_irec_t *imap)
{
	xfs_mount_t	*mp = ip->i_mount;
	xfs_fileoff_t	offset_fsb, last_block;
	xfs_fileoff_t	end_fsb, map_start_fsb;
	xfs_fsblock_t	first_block;
	xfs_bmap_free_t	free_list;
	xfs_filblks_t	count_fsb;
	xfs_trans_t	*tp;
	int		nimaps, committed;
	int		error = 0;
	int		nres;

	/*
	 * Make sure that the dquots are there.
	 */
	error = xfs_qm_dqattach(ip, 0);
	if (error)
		return error;

	offset_fsb = XFS_B_TO_FSBT(mp, offset);
	count_fsb = imap->br_blockcount;
	map_start_fsb = imap->br_startoff;

	XFS_STATS_ADD(mp, xs_xstrat_bytes, XFS_FSB_TO_B(mp, count_fsb));

	while (count_fsb != 0) {
		/*
		 * Set up a transaction with which to allocate the
		 * backing store for the file.  Do allocations in a
		 * loop until we get some space in the range we are
		 * interested in.  The other space that might be allocated
		 * is in the delayed allocation extent on which we sit
		 * but before our buffer starts.
		 */

		nimaps = 0;
		while (nimaps == 0) {
			tp = xfs_trans_alloc(mp, XFS_TRANS_STRAT_WRITE);
			tp->t_flags |= XFS_TRANS_RESERVE;
			nres = XFS_EXTENTADD_SPACE_RES(mp, XFS_DATA_FORK);
			error = xfs_trans_reserve(tp, &M_RES(mp)->tr_write,
						  nres, 0);
			if (error) {
				xfs_trans_cancel(tp);
				return error;
			}
			xfs_ilock(ip, XFS_ILOCK_EXCL);
			xfs_trans_ijoin(tp, ip, 0);

			xfs_bmap_init(&free_list, &first_block);

			/*
			 * it is possible that the extents have changed since
			 * we did the read call as we dropped the ilock for a
			 * while. We have to be careful about truncates or hole
			 * punchs here - we are not allowed to allocate
			 * non-delalloc blocks here.
			 *
			 * The only protection against truncation is the pages
			 * for the range we are being asked to convert are
			 * locked and hence a truncate will block on them
			 * first.
			 *
			 * As a result, if we go beyond the range we really
			 * need and hit an delalloc extent boundary followed by
			 * a hole while we have excess blocks in the map, we
			 * will fill the hole incorrectly and overrun the
			 * transaction reservation.
			 *
			 * Using a single map prevents this as we are forced to
			 * check each map we look for overlap with the desired
			 * range and abort as soon as we find it. Also, given
			 * that we only return a single map, having one beyond
			 * what we can return is probably a bit silly.
			 *
			 * We also need to check that we don't go beyond EOF;
			 * this is a truncate optimisation as a truncate sets
			 * the new file size before block on the pages we
			 * currently have locked under writeback. Because they
			 * are about to be tossed, we don't need to write them
			 * back....
			 */
			nimaps = 1;
			end_fsb = XFS_B_TO_FSB(mp, XFS_ISIZE(ip));
			error = xfs_bmap_last_offset(ip, &last_block,
							XFS_DATA_FORK);
			if (error)
				goto trans_cancel;

			last_block = XFS_FILEOFF_MAX(last_block, end_fsb);
			if ((map_start_fsb + count_fsb) > last_block) {
				count_fsb = last_block - map_start_fsb;
				if (count_fsb == 0) {
					error = -EAGAIN;
					goto trans_cancel;
				}
			}

			/*
			 * From this point onwards we overwrite the imap
			 * pointer that the caller gave to us.
			 */
			error = xfs_bmapi_write(tp, ip, map_start_fsb,
						count_fsb, 0, &first_block,
						nres, imap, &nimaps,
						&free_list);
			if (error)
				goto trans_cancel;

			error = xfs_bmap_finish(&tp, &free_list, &committed);
			if (error)
				goto trans_cancel;

			error = xfs_trans_commit(tp);
			if (error)
				goto error0;

			xfs_iunlock(ip, XFS_ILOCK_EXCL);
		}

		/*
		 * See if we were able to allocate an extent that
		 * covers at least part of the callers request
		 */
		if (!(imap->br_startblock || XFS_IS_REALTIME_INODE(ip)))
			return xfs_alert_fsblock_zero(ip, imap);

		if ((offset_fsb >= imap->br_startoff) &&
		    (offset_fsb < (imap->br_startoff +
				   imap->br_blockcount))) {
			XFS_STATS_INC(mp, xs_xstrat_quick);
			return 0;
		}

		/*
		 * So far we have not mapped the requested part of the
		 * file, just surrounding data, try again.
		 */
		count_fsb -= imap->br_blockcount;
		map_start_fsb = imap->br_startoff + imap->br_blockcount;
	}

trans_cancel:
	xfs_bmap_cancel(&free_list);
	xfs_trans_cancel(tp);
error0:
	xfs_iunlock(ip, XFS_ILOCK_EXCL);
	return error;
}

int
xfs_iomap_write_unwritten(
	xfs_inode_t	*ip,
	xfs_off_t	offset,
	xfs_off_t	count)
{
	xfs_mount_t	*mp = ip->i_mount;
	xfs_fileoff_t	offset_fsb;
	xfs_filblks_t	count_fsb;
	xfs_filblks_t	numblks_fsb;
	xfs_fsblock_t	firstfsb;
	int		nimaps;
	xfs_trans_t	*tp;
	xfs_bmbt_irec_t imap;
	xfs_bmap_free_t free_list;
	xfs_fsize_t	i_size;
	uint		resblks;
	int		committed;
	int		error;

	trace_xfs_unwritten_convert(ip, offset, count);

	offset_fsb = XFS_B_TO_FSBT(mp, offset);
	count_fsb = XFS_B_TO_FSB(mp, (xfs_ufsize_t)offset + count);
	count_fsb = (xfs_filblks_t)(count_fsb - offset_fsb);

	/*
	 * Reserve enough blocks in this transaction for two complete extent
	 * btree splits.  We may be converting the middle part of an unwritten
	 * extent and in this case we will insert two new extents in the btree
	 * each of which could cause a full split.
	 *
	 * This reservation amount will be used in the first call to
	 * xfs_bmbt_split() to select an AG with enough space to satisfy the
	 * rest of the operation.
	 */
	resblks = XFS_DIOSTRAT_SPACE_RES(mp, 0) << 1;

	do {
		/*
		 * set up a transaction to convert the range of extents
		 * from unwritten to real. Do allocations in a loop until
		 * we have covered the range passed in.
		 *
		 * Note that we open code the transaction allocation here
		 * to pass KM_NOFS--we can't risk to recursing back into
		 * the filesystem here as we might be asked to write out
		 * the same inode that we complete here and might deadlock
		 * on the iolock.
		 */
		sb_start_intwrite(mp->m_super);
		tp = _xfs_trans_alloc(mp, XFS_TRANS_STRAT_WRITE, KM_NOFS);
		tp->t_flags |= XFS_TRANS_RESERVE | XFS_TRANS_FREEZE_PROT;
		error = xfs_trans_reserve(tp, &M_RES(mp)->tr_write,
					  resblks, 0);
		if (error) {
			xfs_trans_cancel(tp);
			return error;
		}

		xfs_ilock(ip, XFS_ILOCK_EXCL);
		xfs_trans_ijoin(tp, ip, 0);

		/*
		 * Modify the unwritten extent state of the buffer.
		 */
		xfs_bmap_init(&free_list, &firstfsb);
		nimaps = 1;
		error = xfs_bmapi_write(tp, ip, offset_fsb, count_fsb,
					XFS_BMAPI_CONVERT, &firstfsb, resblks,
					&imap, &nimaps, &free_list);
		if (error)
			goto error_on_bmapi_transaction;

		/*
		 * Log the updated inode size as we go.  We have to be careful
		 * to only log it up to the actual write offset if it is
		 * halfway into a block.
		 */
		i_size = XFS_FSB_TO_B(mp, offset_fsb + count_fsb);
		if (i_size > offset + count)
			i_size = offset + count;

		i_size = xfs_new_eof(ip, i_size);
		if (i_size) {
			ip->i_d.di_size = i_size;
			xfs_trans_log_inode(tp, ip, XFS_ILOG_CORE);
		}

		error = xfs_bmap_finish(&tp, &free_list, &committed);
		if (error)
			goto error_on_bmapi_transaction;

		error = xfs_trans_commit(tp);
		xfs_iunlock(ip, XFS_ILOCK_EXCL);
		if (error)
			return error;

		if (!(imap.br_startblock || XFS_IS_REALTIME_INODE(ip)))
			return xfs_alert_fsblock_zero(ip, &imap);

		if ((numblks_fsb = imap.br_blockcount) == 0) {
			/*
			 * The numblks_fsb value should always get
			 * smaller, otherwise the loop is stuck.
			 */
			ASSERT(imap.br_blockcount);
			break;
		}
		offset_fsb += numblks_fsb;
		count_fsb -= numblks_fsb;
	} while (count_fsb > 0);

	return 0;

error_on_bmapi_transaction:
	xfs_bmap_cancel(&free_list);
	xfs_trans_cancel(tp);
	xfs_iunlock(ip, XFS_ILOCK_EXCL);
	return error;
}