fs/xfs/xfs_reflink.c

/*
 * Copyright (C) 2016 Oracle.  All Rights Reserved.
 *
 * Author: Darrick J. Wong <darrick.wong@oracle.com>
 *
 * 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; either version 2
 * of the License, or (at your option) any later version.
 *
 * 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_defer.h"
#include "xfs_da_format.h"
#include "xfs_da_btree.h"
#include "xfs_inode.h"
#include "xfs_trans.h"
#include "xfs_inode_item.h"
#include "xfs_bmap.h"
#include "xfs_bmap_util.h"
#include "xfs_error.h"
#include "xfs_dir2.h"
#include "xfs_dir2_priv.h"
#include "xfs_ioctl.h"
#include "xfs_trace.h"
#include "xfs_log.h"
#include "xfs_icache.h"
#include "xfs_pnfs.h"
#include "xfs_btree.h"
#include "xfs_refcount_btree.h"
#include "xfs_refcount.h"
#include "xfs_bmap_btree.h"
#include "xfs_trans_space.h"
#include "xfs_bit.h"
#include "xfs_alloc.h"
#include "xfs_quota_defs.h"
#include "xfs_quota.h"
#include "xfs_btree.h"
#include "xfs_bmap_btree.h"
#include "xfs_reflink.h"
#include "xfs_iomap.h"
#include "xfs_rmap_btree.h"

/*
 * Copy on Write of Shared Blocks
 *
 * XFS must preserve "the usual" file semantics even when two files share
 * the same physical blocks.  This means that a write to one file must not
 * alter the blocks in a different file; the way that we'll do that is
 * through the use of a copy-on-write mechanism.  At a high level, that
 * means that when we want to write to a shared block, we allocate a new
 * block, write the data to the new block, and if that succeeds we map the
 * new block into the file.
 *
 * XFS provides a "delayed allocation" mechanism that defers the allocation
 * of disk blocks to dirty-but-not-yet-mapped file blocks as long as
 * possible.  This reduces fragmentation by enabling the filesystem to ask
 * for bigger chunks less often, which is exactly what we want for CoW.
 *
 * The delalloc mechanism begins when the kernel wants to make a block
 * writable (write_begin or page_mkwrite).  If the offset is not mapped, we
 * create a delalloc mapping, which is a regular in-core extent, but without
 * a real startblock.  (For delalloc mappings, the startblock encodes both
 * a flag that this is a delalloc mapping, and a worst-case estimate of how
 * many blocks might be required to put the mapping into the BMBT.)  delalloc
 * mappings are a reservation against the free space in the filesystem;
 * adjacent mappings can also be combined into fewer larger mappings.
 *
 * When dirty pages are being written out (typically in writepage), the
 * delalloc reservations are converted into real mappings by allocating
 * blocks and replacing the delalloc mapping with real ones.  A delalloc
 * mapping can be replaced by several real ones if the free space is
 * fragmented.
 *
 * We want to adapt the delalloc mechanism for copy-on-write, since the
 * write paths are similar.  The first two steps (creating the reservation
 * and allocating the blocks) are exactly the same as delalloc except that
 * the mappings must be stored in a separate CoW fork because we do not want
 * to disturb the mapping in the data fork until we're sure that the write
 * succeeded.  IO completion in this case is the process of removing the old
 * mapping from the data fork and moving the new mapping from the CoW fork to
 * the data fork.  This will be discussed shortly.
 *
 * For now, unaligned directio writes will be bounced back to the page cache.
 * Block-aligned directio writes will use the same mechanism as buffered
 * writes.
 *
 * CoW remapping must be done after the data block write completes,
 * because we don't want to destroy the old data fork map until we're sure
 * the new block has been written.  Since the new mappings are kept in a
 * separate fork, we can simply iterate these mappings to find the ones
 * that cover the file blocks that we just CoW'd.  For each extent, simply
 * unmap the corresponding range in the data fork, map the new range into
 * the data fork, and remove the extent from the CoW fork.
 *
 * Since the remapping operation can be applied to an arbitrary file
 * range, we record the need for the remap step as a flag in the ioend
 * instead of declaring a new IO type.  This is required for direct io
 * because we only have ioend for the whole dio, and we have to be able to
 * remember the presence of unwritten blocks and CoW blocks with a single
 * ioend structure.  Better yet, the more ground we can cover with one
 * ioend, the better.
 */

/*
 * Given an AG extent, find the lowest-numbered run of shared blocks
 * within that range and return the range in fbno/flen.  If
 * find_end_of_shared is true, return the longest contiguous extent of
 * shared blocks.  If there are no shared extents, fbno and flen will
 * be set to NULLAGBLOCK and 0, respectively.
 */
int
xfs_reflink_find_shared(
	struct xfs_mount	*mp,
	xfs_agnumber_t		agno,
	xfs_agblock_t		agbno,
	xfs_extlen_t		aglen,
	xfs_agblock_t		*fbno,
	xfs_extlen_t		*flen,
	bool			find_end_of_shared)
{
	struct xfs_buf		*agbp;
	struct xfs_btree_cur	*cur;
	int			error;

	error = xfs_alloc_read_agf(mp, NULL, agno, 0, &agbp);
	if (error)
		return error;

	cur = xfs_refcountbt_init_cursor(mp, NULL, agbp, agno, NULL);

	error = xfs_refcount_find_shared(cur, agbno, aglen, fbno, flen,
			find_end_of_shared);

	xfs_btree_del_cursor(cur, error ? XFS_BTREE_ERROR : XFS_BTREE_NOERROR);

	xfs_buf_relse(agbp);
	return error;
}

/*
 * Trim the mapping to the next block where there's a change in the
 * shared/unshared status.  More specifically, this means that we
 * find the lowest-numbered extent of shared blocks that coincides with
 * the given block mapping.  If the shared extent overlaps the start of
 * the mapping, trim the mapping to the end of the shared extent.  If
 * the shared region intersects the mapping, trim the mapping to the
 * start of the shared extent.  If there are no shared regions that
 * overlap, just return the original extent.
 */
int
xfs_reflink_trim_around_shared(
	struct xfs_inode	*ip,
	struct xfs_bmbt_irec	*irec,
	bool			*shared,
	bool			*trimmed)
{
	xfs_agnumber_t		agno;
	xfs_agblock_t		agbno;
	xfs_extlen_t		aglen;
	xfs_agblock_t		fbno;
	xfs_extlen_t		flen;
	int			error = 0;

	/* Holes, unwritten, and delalloc extents cannot be shared */
	if (!xfs_is_reflink_inode(ip) ||
	    ISUNWRITTEN(irec) ||
	    irec->br_startblock == HOLESTARTBLOCK ||
	    irec->br_startblock == DELAYSTARTBLOCK) {
		*shared = false;
		return 0;
	}

	trace_xfs_reflink_trim_around_shared(ip, irec);

	agno = XFS_FSB_TO_AGNO(ip->i_mount, irec->br_startblock);
	agbno = XFS_FSB_TO_AGBNO(ip->i_mount, irec->br_startblock);
	aglen = irec->br_blockcount;

	error = xfs_reflink_find_shared(ip->i_mount, agno, agbno,
			aglen, &fbno, &flen, true);
	if (error)
		return error;

	*shared = *trimmed = false;
	if (fbno == NULLAGBLOCK) {
		/* No shared blocks at all. */
		return 0;
	} else if (fbno == agbno) {
		/*
		 * The start of this extent is shared.  Truncate the
		 * mapping at the end of the shared region so that a
		 * subsequent iteration starts at the start of the
		 * unshared region.
		 */
		irec->br_blockcount = flen;
		*shared = true;
		if (flen != aglen)
			*trimmed = true;
		return 0;
	} else {
		/*
		 * There's a shared extent midway through this extent.
		 * Truncate the mapping at the start of the shared
		 * extent so that a subsequent iteration starts at the
		 * start of the shared region.
		 */
		irec->br_blockcount = fbno - agbno;
		*trimmed = true;
		return 0;
	}
}

/* Create a CoW reservation for a range of blocks within a file. */
static int
__xfs_reflink_reserve_cow(
	struct xfs_inode	*ip,
	xfs_fileoff_t		*offset_fsb,
	xfs_fileoff_t		end_fsb,
	bool			*skipped)
{
	struct xfs_bmbt_irec	got, prev, imap;
	xfs_fileoff_t		orig_end_fsb;
	int			nimaps, eof = 0, error = 0;
	bool			shared = false, trimmed = false;
	xfs_extnum_t		idx;

	/* Already reserved?  Skip the refcount btree access. */
	xfs_bmap_search_extents(ip, *offset_fsb, XFS_COW_FORK, &eof, &idx,
			&got, &prev);
	if (!eof && got.br_startoff <= *offset_fsb) {
		end_fsb = orig_end_fsb = got.br_startoff + got.br_blockcount;
		trace_xfs_reflink_cow_found(ip, &got);
		goto done;
	}

	/* Read extent from the source file. */
	nimaps = 1;
	error = xfs_bmapi_read(ip, *offset_fsb, end_fsb - *offset_fsb,
			&imap, &nimaps, 0);
	if (error)
		goto out_unlock;
	ASSERT(nimaps == 1);

	/* Trim the mapping to the nearest shared extent boundary. */
	error = xfs_reflink_trim_around_shared(ip, &imap, &shared, &trimmed);
	if (error)
		goto out_unlock;

	end_fsb = orig_end_fsb = imap.br_startoff + imap.br_blockcount;

	/* Not shared?  Just report the (potentially capped) extent. */
	if (!shared) {
		*skipped = true;
		goto done;
	}

	/*
	 * Fork all the shared blocks from our write offset until the end of
	 * the extent.
	 */
	error = xfs_qm_dqattach_locked(ip, 0);
	if (error)
		goto out_unlock;

retry:
	error = xfs_bmapi_reserve_delalloc(ip, XFS_COW_FORK, *offset_fsb,
			end_fsb - *offset_fsb, &got,
			&prev, &idx, eof);
	switch (error) {
	case 0:
		break;
	case -ENOSPC:
	case -EDQUOT:
		/* retry without any preallocation */
		trace_xfs_reflink_cow_enospc(ip, &imap);
		if (end_fsb != orig_end_fsb) {
			end_fsb = orig_end_fsb;
			goto retry;
		}
		/*FALLTHRU*/
	default:
		goto out_unlock;
	}

	trace_xfs_reflink_cow_alloc(ip, &got);
done:
	*offset_fsb = end_fsb;
out_unlock:
	return error;
}

/* Create a CoW reservation for part of a file. */
int
xfs_reflink_reserve_cow_range(
	struct xfs_inode	*ip,
	xfs_off_t		offset,
	xfs_off_t		count)
{
	struct xfs_mount	*mp = ip->i_mount;
	xfs_fileoff_t		offset_fsb, end_fsb;
	bool			skipped = false;
	int			error;

	trace_xfs_reflink_reserve_cow_range(ip, offset, count);

	offset_fsb = XFS_B_TO_FSBT(mp, offset);
	end_fsb = XFS_B_TO_FSB(mp, offset + count);

	xfs_ilock(ip, XFS_ILOCK_EXCL);
	while (offset_fsb < end_fsb) {
		error = __xfs_reflink_reserve_cow(ip, &offset_fsb, end_fsb,
				&skipped);
		if (error) {
			trace_xfs_reflink_reserve_cow_range_error(ip, error,
				_RET_IP_);
			break;
		}
	}
	xfs_iunlock(ip, XFS_ILOCK_EXCL);

	return error;
}

/* Allocate all CoW reservations covering a range of blocks in a file. */
static int
__xfs_reflink_allocate_cow(
	struct xfs_inode	*ip,
	xfs_fileoff_t		*offset_fsb,
	xfs_fileoff_t		end_fsb)
{
	struct xfs_mount	*mp = ip->i_mount;
	struct xfs_bmbt_irec	imap;
	struct xfs_defer_ops	dfops;
	struct xfs_trans	*tp;
	xfs_fsblock_t		first_block;
	xfs_fileoff_t		next_fsb;
	int			nimaps = 1, error;
	bool			skipped = false;

	xfs_defer_init(&dfops, &first_block);

	error = xfs_trans_alloc(mp, &M_RES(mp)->tr_write, 0, 0,
			XFS_TRANS_RESERVE, &tp);
	if (error)
		return error;

	xfs_ilock(ip, XFS_ILOCK_EXCL);

	next_fsb = *offset_fsb;
	error = __xfs_reflink_reserve_cow(ip, &next_fsb, end_fsb, &skipped);
	if (error)
		goto out_trans_cancel;

	if (skipped) {
		*offset_fsb = next_fsb;
		goto out_trans_cancel;
	}

	xfs_trans_ijoin(tp, ip, 0);
	error = xfs_bmapi_write(tp, ip, *offset_fsb, next_fsb - *offset_fsb,
			XFS_BMAPI_COWFORK, &first_block,
			XFS_EXTENTADD_SPACE_RES(mp, XFS_DATA_FORK),
			&imap, &nimaps, &dfops);
	if (error)
		goto out_trans_cancel;

	/* We might not have been able to map the whole delalloc extent */
	*offset_fsb = min(*offset_fsb + imap.br_blockcount, next_fsb);

	error = xfs_defer_finish(&tp, &dfops, NULL);
	if (error)
		goto out_trans_cancel;

	error = xfs_trans_commit(tp);

out_unlock:
	xfs_iunlock(ip, XFS_ILOCK_EXCL);
	return error;
out_trans_cancel:
	xfs_defer_cancel(&dfops);
	xfs_trans_cancel(tp);
	goto out_unlock;
}

/* Allocate all CoW reservations covering a part of a file. */
int
xfs_reflink_allocate_cow_range(
	struct xfs_inode	*ip,
	xfs_off_t		offset,
	xfs_off_t		count)
{
	struct xfs_mount	*mp = ip->i_mount;
	xfs_fileoff_t		offset_fsb = XFS_B_TO_FSBT(mp, offset);
	xfs_fileoff_t		end_fsb = XFS_B_TO_FSB(mp, offset + count);
	int			error;

	ASSERT(xfs_is_reflink_inode(ip));

	trace_xfs_reflink_allocate_cow_range(ip, offset, count);

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

	while (offset_fsb < end_fsb) {
		error = __xfs_reflink_allocate_cow(ip, &offset_fsb, end_fsb);
		if (error) {
			trace_xfs_reflink_allocate_cow_range_error(ip, error,
					_RET_IP_);
			break;
		}
	}

	return error;
}

/*
 * Find the CoW reservation (and whether or not it needs block allocation)
 * for a given byte offset of a file.
 */
bool
xfs_reflink_find_cow_mapping(
	struct xfs_inode		*ip,
	xfs_off_t			offset,
	struct xfs_bmbt_irec		*imap,
	bool				*need_alloc)
{
	struct xfs_bmbt_irec		irec;
	struct xfs_ifork		*ifp;
	struct xfs_bmbt_rec_host	*gotp;
	xfs_fileoff_t			bno;
	xfs_extnum_t			idx;

	ASSERT(xfs_isilocked(ip, XFS_ILOCK_EXCL | XFS_ILOCK_SHARED));
	ASSERT(xfs_is_reflink_inode(ip));

	/* Find the extent in the CoW fork. */
	ifp = XFS_IFORK_PTR(ip, XFS_COW_FORK);
	bno = XFS_B_TO_FSBT(ip->i_mount, offset);
	gotp = xfs_iext_bno_to_ext(ifp, bno, &idx);
	if (!gotp)
		return false;

	xfs_bmbt_get_all(gotp, &irec);
	if (bno >= irec.br_startoff + irec.br_blockcount ||
	    bno < irec.br_startoff)
		return false;

	trace_xfs_reflink_find_cow_mapping(ip, offset, 1, XFS_IO_OVERWRITE,
			&irec);

	/* If it's still delalloc, we must allocate later. */
	*imap = irec;
	*need_alloc = !!(isnullstartblock(irec.br_startblock));

	return true;
}

/*
 * Trim an extent to end at the next CoW reservation past offset_fsb.
 */
int
xfs_reflink_trim_irec_to_next_cow(
	struct xfs_inode		*ip,
	xfs_fileoff_t			offset_fsb,
	struct xfs_bmbt_irec		*imap)
{
	struct xfs_bmbt_irec		irec;
	struct xfs_ifork		*ifp;
	struct xfs_bmbt_rec_host	*gotp;
	xfs_extnum_t			idx;

	if (!xfs_is_reflink_inode(ip))
		return 0;

	/* Find the extent in the CoW fork. */
	ifp = XFS_IFORK_PTR(ip, XFS_COW_FORK);
	gotp = xfs_iext_bno_to_ext(ifp, offset_fsb, &idx);
	if (!gotp)
		return 0;
	xfs_bmbt_get_all(gotp, &irec);

	/* This is the extent before; try sliding up one. */
	if (irec.br_startoff < offset_fsb) {
		idx++;
		if (idx >= ifp->if_bytes / sizeof(xfs_bmbt_rec_t))
			return 0;
		gotp = xfs_iext_get_ext(ifp, idx);
		xfs_bmbt_get_all(gotp, &irec);
	}

	if (irec.br_startoff >= imap->br_startoff + imap->br_blockcount)
		return 0;

	imap->br_blockcount = irec.br_startoff - imap->br_startoff;
	trace_xfs_reflink_trim_irec(ip, imap);

	return 0;
}

/*
 * Cancel all pending CoW reservations for some block range of an inode.
 */
int
xfs_reflink_cancel_cow_blocks(
	struct xfs_inode		*ip,
	struct xfs_trans		**tpp,
	xfs_fileoff_t			offset_fsb,
	xfs_fileoff_t			end_fsb)
{
	struct xfs_bmbt_irec		irec;
	xfs_filblks_t			count_fsb;
	xfs_fsblock_t			firstfsb;
	struct xfs_defer_ops		dfops;
	int				error = 0;
	int				nimaps;

	if (!xfs_is_reflink_inode(ip))
		return 0;

	/* Go find the old extent in the CoW fork. */
	while (offset_fsb < end_fsb) {
		nimaps = 1;
		count_fsb = (xfs_filblks_t)(end_fsb - offset_fsb);
		error = xfs_bmapi_read(ip, offset_fsb, count_fsb, &irec,
				&nimaps, XFS_BMAPI_COWFORK);
		if (error)
			break;
		ASSERT(nimaps == 1);

		trace_xfs_reflink_cancel_cow(ip, &irec);

		if (irec.br_startblock == DELAYSTARTBLOCK) {
			/* Free a delayed allocation. */
			xfs_mod_fdblocks(ip->i_mount, irec.br_blockcount,
					false);
			ip->i_delayed_blks -= irec.br_blockcount;

			/* Remove the mapping from the CoW fork. */
			error = xfs_bunmapi_cow(ip, &irec);
			if (error)
				break;
		} else if (irec.br_startblock == HOLESTARTBLOCK) {
			/* empty */
		} else {
			xfs_trans_ijoin(*tpp, ip, 0);
			xfs_defer_init(&dfops, &firstfsb);

			/* Free the CoW orphan record. */
			error = xfs_refcount_free_cow_extent(ip->i_mount,
					&dfops, irec.br_startblock,
					irec.br_blockcount);
			if (error)
				break;

			xfs_bmap_add_free(ip->i_mount, &dfops,
					irec.br_startblock, irec.br_blockcount,
					NULL);

			/* Update quota accounting */
			xfs_trans_mod_dquot_byino(*tpp, ip, XFS_TRANS_DQ_BCOUNT,
					-(long)irec.br_blockcount);

			/* Roll the transaction */
			error = xfs_defer_finish(tpp, &dfops, ip);
			if (error) {
				xfs_defer_cancel(&dfops);
				break;
			}

			/* Remove the mapping from the CoW fork. */
			error = xfs_bunmapi_cow(ip, &irec);
			if (error)
				break;
		}

		/* Roll on... */
		offset_fsb = irec.br_startoff + irec.br_blockcount;
	}

	return error;
}

/*
 * Cancel all pending CoW reservations for some byte range of an inode.
 */
int
xfs_reflink_cancel_cow_range(
	struct xfs_inode	*ip,
	xfs_off_t		offset,
	xfs_off_t		count)
{
	struct xfs_trans	*tp;
	xfs_fileoff_t		offset_fsb;
	xfs_fileoff_t		end_fsb;
	int			error;

	trace_xfs_reflink_cancel_cow_range(ip, offset, count);

	offset_fsb = XFS_B_TO_FSBT(ip->i_mount, offset);
	if (count == NULLFILEOFF)
		end_fsb = NULLFILEOFF;
	else
		end_fsb = XFS_B_TO_FSB(ip->i_mount, offset + count);

	/* Start a rolling transaction to remove the mappings */
	error = xfs_trans_alloc(ip->i_mount, &M_RES(ip->i_mount)->tr_write,
			0, 0, 0, &tp);
	if (error)
		goto out;

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

	/* Scrape out the old CoW reservations */
	error = xfs_reflink_cancel_cow_blocks(ip, &tp, offset_fsb, end_fsb);
	if (error)
		goto out_cancel;

	error = xfs_trans_commit(tp);

	xfs_iunlock(ip, XFS_ILOCK_EXCL);
	return error;

out_cancel:
	xfs_trans_cancel(tp);
	xfs_iunlock(ip, XFS_ILOCK_EXCL);
out:
	trace_xfs_reflink_cancel_cow_range_error(ip, error, _RET_IP_);
	return error;
}

/*
 * Remap parts of a file's data fork after a successful CoW.
 */
int
xfs_reflink_end_cow(
	struct xfs_inode		*ip,
	xfs_off_t			offset,
	xfs_off_t			count)
{
	struct xfs_bmbt_irec		irec;
	struct xfs_bmbt_irec		uirec;
	struct xfs_trans		*tp;
	xfs_fileoff_t			offset_fsb;
	xfs_fileoff_t			end_fsb;
	xfs_filblks_t			count_fsb;
	xfs_fsblock_t			firstfsb;
	struct xfs_defer_ops		dfops;
	int				error;
	unsigned int			resblks;
	xfs_filblks_t			ilen;
	xfs_filblks_t			rlen;
	int				nimaps;

	trace_xfs_reflink_end_cow(ip, offset, count);

	offset_fsb = XFS_B_TO_FSBT(ip->i_mount, offset);
	end_fsb = XFS_B_TO_FSB(ip->i_mount, offset + count);
	count_fsb = (xfs_filblks_t)(end_fsb - offset_fsb);

	/* Start a rolling transaction to switch the mappings */
	resblks = XFS_EXTENTADD_SPACE_RES(ip->i_mount, XFS_DATA_FORK);
	error = xfs_trans_alloc(ip->i_mount, &M_RES(ip->i_mount)->tr_write,
			resblks, 0, 0, &tp);
	if (error)
		goto out;

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

	/* Go find the old extent in the CoW fork. */
	while (offset_fsb < end_fsb) {
		/* Read extent from the source file */
		nimaps = 1;
		count_fsb = (xfs_filblks_t)(end_fsb - offset_fsb);
		error = xfs_bmapi_read(ip, offset_fsb, count_fsb, &irec,
				&nimaps, XFS_BMAPI_COWFORK);
		if (error)
			goto out_cancel;
		ASSERT(nimaps == 1);

		ASSERT(irec.br_startblock != DELAYSTARTBLOCK);
		trace_xfs_reflink_cow_remap(ip, &irec);

		/*
		 * We can have a hole in the CoW fork if part of a directio
		 * write is CoW but part of it isn't.
		 */
		rlen = ilen = irec.br_blockcount;
		if (irec.br_startblock == HOLESTARTBLOCK)
			goto next_extent;

		/* Unmap the old blocks in the data fork. */
		while (rlen) {
			xfs_defer_init(&dfops, &firstfsb);
			error = __xfs_bunmapi(tp, ip, irec.br_startoff,
					&rlen, 0, 1, &firstfsb, &dfops);
			if (error)
				goto out_defer;

			/*
			 * Trim the extent to whatever got unmapped.
			 * Remember, bunmapi works backwards.
			 */
			uirec.br_startblock = irec.br_startblock + rlen;
			uirec.br_startoff = irec.br_startoff + rlen;
			uirec.br_blockcount = irec.br_blockcount - rlen;
			irec.br_blockcount = rlen;
			trace_xfs_reflink_cow_remap_piece(ip, &uirec);

			/* Free the CoW orphan record. */
			error = xfs_refcount_free_cow_extent(tp->t_mountp,
					&dfops, uirec.br_startblock,
					uirec.br_blockcount);
			if (error)
				goto out_defer;

			/* Map the new blocks into the data fork. */
			error = xfs_bmap_map_extent(tp->t_mountp, &dfops,
					ip, &uirec);
			if (error)
				goto out_defer;

			/* Remove the mapping from the CoW fork. */
			error = xfs_bunmapi_cow(ip, &uirec);
			if (error)
				goto out_defer;

			error = xfs_defer_finish(&tp, &dfops, ip);
			if (error)
				goto out_defer;
		}

next_extent:
		/* Roll on... */
		offset_fsb = irec.br_startoff + ilen;
	}

	error = xfs_trans_commit(tp);
	xfs_iunlock(ip, XFS_ILOCK_EXCL);
	if (error)
		goto out;
	return 0;

out_defer:
	xfs_defer_cancel(&dfops);
out_cancel:
	xfs_trans_cancel(tp);
	xfs_iunlock(ip, XFS_ILOCK_EXCL);
out:
	trace_xfs_reflink_end_cow_error(ip, error, _RET_IP_);
	return error;
}

/*
 * Free leftover CoW reservations that didn't get cleaned out.
 */
int
xfs_reflink_recover_cow(
	struct xfs_mount	*mp)
{
	xfs_agnumber_t		agno;
	int			error = 0;

	if (!xfs_sb_version_hasreflink(&mp->m_sb))
		return 0;

	for (agno = 0; agno < mp->m_sb.sb_agcount; agno++) {
		error = xfs_refcount_recover_cow_leftovers(mp, agno);
		if (error)
			break;
	}

	return error;
}

/*
 * Reflinking (Block) Ranges of Two Files Together
 *
 * First, ensure that the reflink flag is set on both inodes.  The flag is an
 * optimization to avoid unnecessary refcount btree lookups in the write path.
 *
 * Now we can iteratively remap the range of extents (and holes) in src to the
 * corresponding ranges in dest.  Let drange and srange denote the ranges of
 * logical blocks in dest and src touched by the reflink operation.
 *
 * While the length of drange is greater than zero,
 *    - Read src's bmbt at the start of srange ("imap")
 *    - If imap doesn't exist, make imap appear to start at the end of srange
 *      with zero length.
 *    - If imap starts before srange, advance imap to start at srange.
 *    - If imap goes beyond srange, truncate imap to end at the end of srange.
 *    - Punch (imap start - srange start + imap len) blocks from dest at
 *      offset (drange start).
 *    - If imap points to a real range of pblks,
 *         > Increase the refcount of the imap's pblks
 *         > Map imap's pblks into dest at the offset
 *           (drange start + imap start - srange start)
 *    - Advance drange and srange by (imap start - srange start + imap len)
 *
 * Finally, if the reflink made dest longer, update both the in-core and
 * on-disk file sizes.
 *
 * ASCII Art Demonstration:
 *
 * Let's say we want to reflink this source file:
 *
 * ----SSSSSSS-SSSSS----SSSSSS (src file)
 *   <-------------------->
 *
 * into this destination file:
 *
 * --DDDDDDDDDDDDDDDDDDD--DDD (dest file)
 *        <-------------------->
 * '-' means a hole, and 'S' and 'D' are written blocks in the src and dest.
 * Observe that the range has different logical offsets in either file.
 *
 * Consider that the first extent in the source file doesn't line up with our
 * reflink range.  Unmapping  and remapping are separate operations, so we can
 * unmap more blocks from the destination file than we remap.
 *
 * ----SSSSSSS-SSSSS----SSSSSS
 *   <------->
 * --DDDDD---------DDDDD--DDD
 *        <------->
 *
 * Now remap the source extent into the destination file:
 *
 * ----SSSSSSS-SSSSS----SSSSSS
 *   <------->
 * --DDDDD--SSSSSSSDDDDD--DDD
 *        <------->
 *
 * Do likewise with the second hole and extent in our range.  Holes in the
 * unmap range don't affect our operation.
 *
 * ----SSSSSSS-SSSSS----SSSSSS
 *            <---->
 * --DDDDD--SSSSSSS-SSSSS-DDD
 *                 <---->
 *
 * Finally, unmap and remap part of the third extent.  This will increase the
 * size of the destination file.
 *
 * ----SSSSSSS-SSSSS----SSSSSS
 *                  <----->
 * --DDDDD--SSSSSSS-SSSSS----SSS
 *                       <----->
 *
 * Once we update the destination file's i_size, we're done.
 */

/*
 * Ensure the reflink bit is set in both inodes.
 */
STATIC int
xfs_reflink_set_inode_flag(
	struct xfs_inode	*src,
	struct xfs_inode	*dest)
{
	struct xfs_mount	*mp = src->i_mount;
	int			error;
	struct xfs_trans	*tp;

	if (xfs_is_reflink_inode(src) && xfs_is_reflink_inode(dest))
		return 0;

	error = xfs_trans_alloc(mp, &M_RES(mp)->tr_ichange, 0, 0, 0, &tp);
	if (error)
		goto out_error;

	/* Lock both files against IO */
	if (src->i_ino == dest->i_ino)
		xfs_ilock(src, XFS_ILOCK_EXCL);
	else
		xfs_lock_two_inodes(src, dest, XFS_ILOCK_EXCL);

	if (!xfs_is_reflink_inode(src)) {
		trace_xfs_reflink_set_inode_flag(src);
		xfs_trans_ijoin(tp, src, XFS_ILOCK_EXCL);
		src->i_d.di_flags2 |= XFS_DIFLAG2_REFLINK;
		xfs_trans_log_inode(tp, src, XFS_ILOG_CORE);
		xfs_ifork_init_cow(src);
	} else
		xfs_iunlock(src, XFS_ILOCK_EXCL);

	if (src->i_ino == dest->i_ino)
		goto commit_flags;

	if (!xfs_is_reflink_inode(dest)) {
		trace_xfs_reflink_set_inode_flag(dest);
		xfs_trans_ijoin(tp, dest, XFS_ILOCK_EXCL);
		dest->i_d.di_flags2 |= XFS_DIFLAG2_REFLINK;
		xfs_trans_log_inode(tp, dest, XFS_ILOG_CORE);
		xfs_ifork_init_cow(dest);
	} else
		xfs_iunlock(dest, XFS_ILOCK_EXCL);

commit_flags:
	error = xfs_trans_commit(tp);
	if (error)
		goto out_error;
	return error;

out_error:
	trace_xfs_reflink_set_inode_flag_error(dest, error, _RET_IP_);
	return error;
}

/*
 * Update destination inode size, if necessary.
 */
STATIC int
xfs_reflink_update_dest(
	struct xfs_inode	*dest,
	xfs_off_t		newlen)
{
	struct xfs_mount	*mp = dest->i_mount;
	struct xfs_trans	*tp;
	int			error;

	if (newlen <= i_size_read(VFS_I(dest)))
		return 0;

	error = xfs_trans_alloc(mp, &M_RES(mp)->tr_ichange, 0, 0, 0, &tp);
	if (error)
		goto out_error;

	xfs_ilock(dest, XFS_ILOCK_EXCL);
	xfs_trans_ijoin(tp, dest, XFS_ILOCK_EXCL);

	trace_xfs_reflink_update_inode_size(dest, newlen);
	i_size_write(VFS_I(dest), newlen);
	dest->i_d.di_size = newlen;
	xfs_trans_log_inode(tp, dest, XFS_ILOG_CORE);

	error = xfs_trans_commit(tp);
	if (error)
		goto out_error;
	return error;

out_error:
	trace_xfs_reflink_update_inode_size_error(dest, error, _RET_IP_);
	return error;
}

/*
 * Unmap a range of blocks from a file, then map other blocks into the hole.
 * The range to unmap is (destoff : destoff + srcioff + irec->br_blockcount).
 * The extent irec is mapped into dest at irec->br_startoff.
 */
STATIC int
xfs_reflink_remap_extent(
	struct xfs_inode	*ip,
	struct xfs_bmbt_irec	*irec,
	xfs_fileoff_t		destoff,
	xfs_off_t		new_isize)
{
	struct xfs_mount	*mp = ip->i_mount;
	struct xfs_trans	*tp;
	xfs_fsblock_t		firstfsb;
	unsigned int		resblks;
	struct xfs_defer_ops	dfops;
	struct xfs_bmbt_irec	uirec;
	bool			real_extent;
	xfs_filblks_t		rlen;
	xfs_filblks_t		unmap_len;
	xfs_off_t		newlen;
	int			error;

	unmap_len = irec->br_startoff + irec->br_blockcount - destoff;
	trace_xfs_reflink_punch_range(ip, destoff, unmap_len);

	/* Only remap normal extents. */
	real_extent =  (irec->br_startblock != HOLESTARTBLOCK &&
			irec->br_startblock != DELAYSTARTBLOCK &&
			!ISUNWRITTEN(irec));

	/* Start a rolling transaction to switch the mappings */
	resblks = XFS_EXTENTADD_SPACE_RES(ip->i_mount, XFS_DATA_FORK);
	error = xfs_trans_alloc(mp, &M_RES(mp)->tr_write, resblks, 0, 0, &tp);
	if (error)
		goto out;

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

	/* If we're not just clearing space, then do we have enough quota? */
	if (real_extent) {
		error = xfs_trans_reserve_quota_nblks(tp, ip,
				irec->br_blockcount, 0, XFS_QMOPT_RES_REGBLKS);
		if (error)
			goto out_cancel;
	}

	trace_xfs_reflink_remap(ip, irec->br_startoff,
				irec->br_blockcount, irec->br_startblock);

	/* Unmap the old blocks in the data fork. */
	rlen = unmap_len;
	while (rlen) {
		xfs_defer_init(&dfops, &firstfsb);
		error = __xfs_bunmapi(tp, ip, destoff, &rlen, 0, 1,
				&firstfsb, &dfops);
		if (error)
			goto out_defer;

		/*
		 * Trim the extent to whatever got unmapped.
		 * Remember, bunmapi works backwards.
		 */
		uirec.br_startblock = irec->br_startblock + rlen;
		uirec.br_startoff = irec->br_startoff + rlen;
		uirec.br_blockcount = unmap_len - rlen;
		unmap_len = rlen;

		/* If this isn't a real mapping, we're done. */
		if (!real_extent || uirec.br_blockcount == 0)
			goto next_extent;

		trace_xfs_reflink_remap(ip, uirec.br_startoff,
				uirec.br_blockcount, uirec.br_startblock);

		/* Update the refcount tree */
		error = xfs_refcount_increase_extent(mp, &dfops, &uirec);
		if (error)
			goto out_defer;

		/* Map the new blocks into the data fork. */
		error = xfs_bmap_map_extent(mp, &dfops, ip, &uirec);
		if (error)
			goto out_defer;

		/* Update quota accounting. */
		xfs_trans_mod_dquot_byino(tp, ip, XFS_TRANS_DQ_BCOUNT,
				uirec.br_blockcount);

		/* Update dest isize if needed. */
		newlen = XFS_FSB_TO_B(mp,
				uirec.br_startoff + uirec.br_blockcount);
		newlen = min_t(xfs_off_t, newlen, new_isize);
		if (newlen > i_size_read(VFS_I(ip))) {
			trace_xfs_reflink_update_inode_size(ip, newlen);
			i_size_write(VFS_I(ip), newlen);
			ip->i_d.di_size = newlen;
			xfs_trans_log_inode(tp, ip, XFS_ILOG_CORE);
		}

next_extent:
		/* Process all the deferred stuff. */
		error = xfs_defer_finish(&tp, &dfops, ip);
		if (error)
			goto out_defer;
	}

	error = xfs_trans_commit(tp);
	xfs_iunlock(ip, XFS_ILOCK_EXCL);
	if (error)
		goto out;
	return 0;

out_defer:
	xfs_defer_cancel(&dfops);
out_cancel:
	xfs_trans_cancel(tp);
	xfs_iunlock(ip, XFS_ILOCK_EXCL);
out:
	trace_xfs_reflink_remap_extent_error(ip, error, _RET_IP_);
	return error;
}

/*
 * Iteratively remap one file's extents (and holes) to another's.
 */
STATIC int
xfs_reflink_remap_blocks(
	struct xfs_inode	*src,
	xfs_fileoff_t		srcoff,
	struct xfs_inode	*dest,
	xfs_fileoff_t		destoff,
	xfs_filblks_t		len,
	xfs_off_t		new_isize)
{
	struct xfs_bmbt_irec	imap;
	int			nimaps;
	int			error = 0;
	xfs_filblks_t		range_len;

	/* drange = (destoff, destoff + len); srange = (srcoff, srcoff + len) */
	while (len) {
		trace_xfs_reflink_remap_blocks_loop(src, srcoff, len,
				dest, destoff);
		/* Read extent from the source file */
		nimaps = 1;
		xfs_ilock(src, XFS_ILOCK_EXCL);
		error = xfs_bmapi_read(src, srcoff, len, &imap, &nimaps, 0);
		xfs_iunlock(src, XFS_ILOCK_EXCL);
		if (error)
			goto err;
		ASSERT(nimaps == 1);

		trace_xfs_reflink_remap_imap(src, srcoff, len, XFS_IO_OVERWRITE,
				&imap);

		/* Translate imap into the destination file. */
		range_len = imap.br_startoff + imap.br_blockcount - srcoff;
		imap.br_startoff += destoff - srcoff;

		/* Clear dest from destoff to the end of imap and map it in. */
		error = xfs_reflink_remap_extent(dest, &imap, destoff,
				new_isize);
		if (error)
			goto err;

		if (fatal_signal_pending(current)) {
			error = -EINTR;
			goto err;
		}

		/* Advance drange/srange */
		srcoff += range_len;
		destoff += range_len;
		len -= range_len;
	}

	return 0;

err:
	trace_xfs_reflink_remap_blocks_error(dest, error, _RET_IP_);
	return error;
}

/*
 * Link a range of blocks from one file to another.
 */
int
xfs_reflink_remap_range(
	struct xfs_inode	*src,
	xfs_off_t		srcoff,
	struct xfs_inode	*dest,
	xfs_off_t		destoff,
	xfs_off_t		len)
{
	struct xfs_mount	*mp = src->i_mount;
	xfs_fileoff_t		sfsbno, dfsbno;
	xfs_filblks_t		fsblen;
	int			error;

	if (!xfs_sb_version_hasreflink(&mp->m_sb))
		return -EOPNOTSUPP;

	if (XFS_FORCED_SHUTDOWN(mp))
		return -EIO;

	/* Don't reflink realtime inodes */
	if (XFS_IS_REALTIME_INODE(src) || XFS_IS_REALTIME_INODE(dest))
		return -EINVAL;

	trace_xfs_reflink_remap_range(src, srcoff, len, dest, destoff);

	/* Lock both files against IO */
	if (src->i_ino == dest->i_ino) {
		xfs_ilock(src, XFS_IOLOCK_EXCL);
		xfs_ilock(src, XFS_MMAPLOCK_EXCL);
	} else {
		xfs_lock_two_inodes(src, dest, XFS_IOLOCK_EXCL);
		xfs_lock_two_inodes(src, dest, XFS_MMAPLOCK_EXCL);
	}

	error = xfs_reflink_set_inode_flag(src, dest);
	if (error)
		goto out_error;

	/*
	 * Invalidate the page cache so that we can clear any CoW mappings
	 * in the destination file.
	 */
	truncate_inode_pages_range(&VFS_I(dest)->i_data, destoff,
				   PAGE_ALIGN(destoff + len) - 1);

	dfsbno = XFS_B_TO_FSBT(mp, destoff);
	sfsbno = XFS_B_TO_FSBT(mp, srcoff);
	fsblen = XFS_B_TO_FSB(mp, len);
	error = xfs_reflink_remap_blocks(src, sfsbno, dest, dfsbno, fsblen,
			destoff + len);
	if (error)
		goto out_error;

	error = xfs_reflink_update_dest(dest, destoff + len);
	if (error)
		goto out_error;

out_error:
	xfs_iunlock(src, XFS_MMAPLOCK_EXCL);
	xfs_iunlock(src, XFS_IOLOCK_EXCL);
	if (src->i_ino != dest->i_ino) {
		xfs_iunlock(dest, XFS_MMAPLOCK_EXCL);
		xfs_iunlock(dest, XFS_IOLOCK_EXCL);
	}
	if (error)
		trace_xfs_reflink_remap_range_error(dest, error, _RET_IP_);
	return error;
}