linux/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_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"

/*
 * 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)
{
	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)
		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;
	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);
		if (error) {
			trace_xfs_reflink_reserve_cow_range_error(ip, error,
				_RET_IP_);
			break;
		}
	}
	xfs_iunlock(ip, XFS_ILOCK_EXCL);

	return error;
}