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Commit d2b3964a authored by Christoph Hellwig's avatar Christoph Hellwig Committed by Darrick J. Wong
Browse files

xfs: fix COW writeback race 



Due to the way how xfs_iomap_write_allocate tries to convert the whole
found extents from delalloc to real space we can run into a race
condition with multiple threads doing writes to this same extent.
For the non-COW case that is harmless as the only thing that can happen
is that we call xfs_bmapi_write on an extent that has already been
converted to a real allocation.  For COW writes where we move the extent
from the COW to the data fork after I/O completion the race is, however,
not quite as harmless.  In the worst case we are now calling
xfs_bmapi_write on a region that contains hole in the COW work, which
will trip up an assert in debug builds or lead to file system corruption
in non-debug builds.  This seems to be reproducible with workloads of
small O_DSYNC write, although so far I've not managed to come up with
a with an isolated reproducer.

The fix for the issue is relatively simple:  tell xfs_bmapi_write
that we are only asked to convert delayed allocations and skip holes
in that case.

Signed-off-by: default avatarChristoph Hellwig <hch@lst.de>
Reviewed-by: default avatarBrian Foster <bfoster@redhat.com>
Reviewed-by: default avatarDarrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: default avatarDarrick J. Wong <darrick.wong@oracle.com>
parent 7a308bb3

fs/xfs/libxfs/xfs_bmap.c

+32 −12
Original line number Diff line number Diff line
@@ -4514,8 +4514,6 @@ xfs_bmapi_write( 
	int			n;		/* current extent index */

	xfs_fileoff_t		obno;		/* old block number (offset) */

	int			whichfork;	/* data or attr fork */

	char			inhole;		/* current location is hole in file */

	char			wasdelay;	/* old extent was delayed */



#ifdef DEBUG

	xfs_fileoff_t		orig_bno;	/* original block number value */
@@ -4603,22 +4601,44 @@ xfs_bmapi_write( 
	bma.firstblock = firstblock;



	while (bno < end && n < *nmap) {

		inhole = eof || bma.got.br_startoff > bno;

		wasdelay = !inhole && isnullstartblock(bma.got.br_startblock);

		bool			need_alloc = false, wasdelay = false;



		/* in hole or beyoned EOF? */

		if (eof || bma.got.br_startoff > bno) {

			if (flags & XFS_BMAPI_DELALLOC) {

				/*

				 * For the COW fork we can reasonably get a

				 * request for converting an extent that races

				 * with other threads already having converted

				 * part of it, as there converting COW to

				 * regular blocks is not protected using the

				 * IOLOCK.

				 */

				ASSERT(flags & XFS_BMAPI_COWFORK);

				if (!(flags & XFS_BMAPI_COWFORK)) {

					error = -EIO;

					goto error0;

				}



				if (eof || bno >= end)

					break;

			} else {

				need_alloc = true;

			}

		} else {

			/*

			 * Make sure we only reflink into a hole.

			 */

		if (flags & XFS_BMAPI_REMAP)

			ASSERT(inhole);

		if (flags & XFS_BMAPI_COWFORK)

			ASSERT(!inhole);

			ASSERT(!(flags & XFS_BMAPI_REMAP));

			if (isnullstartblock(bma.got.br_startblock))

				wasdelay = true;

		}



		/*

		 * First, deal with the hole before the allocated space

		 * that we found, if any.

		 */

		if (inhole || wasdelay) {

		if (need_alloc || wasdelay) {

			bma.eof = eof;

			bma.conv = !!(flags & XFS_BMAPI_CONVERT);

			bma.wasdel = wasdelay;

fs/xfs/libxfs/xfs_bmap.h

+5 −1
Original line number Diff line number Diff line
@@ -110,6 +110,9 @@ struct xfs_extent_free_item 
/* Map something in the CoW fork. */

#define XFS_BMAPI_COWFORK	0x200



/* Only convert delalloc space, don't allocate entirely new extents */

#define XFS_BMAPI_DELALLOC	0x400



#define XFS_BMAPI_FLAGS \

	{ XFS_BMAPI_ENTIRE,	"ENTIRE" }, \

	{ XFS_BMAPI_METADATA,	"METADATA" }, \
@@ -120,7 +123,8 @@ struct xfs_extent_free_item 
	{ XFS_BMAPI_CONVERT,	"CONVERT" }, \

	{ XFS_BMAPI_ZERO,	"ZERO" }, \

	{ XFS_BMAPI_REMAP,	"REMAP" }, \

	{ XFS_BMAPI_COWFORK,	"COWFORK" }

	{ XFS_BMAPI_COWFORK,	"COWFORK" }, \

	{ XFS_BMAPI_DELALLOC,	"DELALLOC" }





static inline int xfs_bmapi_aflag(int w)

fs/xfs/xfs_iomap.c

+1 −1
Original line number Diff line number Diff line
@@ -681,7 +681,7 @@ xfs_iomap_write_allocate( 
	xfs_trans_t	*tp;

	int		nimaps;

	int		error = 0;

	int		flags = 0;

	int		flags = XFS_BMAPI_DELALLOC;

	int		nres;



	if (whichfork == XFS_COW_FORK)