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xfs: fix COW writeback race

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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: Christoph Hellwig <hch@lst.de>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
This commit is contained in:
Christoph Hellwig 2017-01-20 09:31:54 -08:00 committed by Darrick J. Wong
parent 7a308bb301
commit d2b3964a07
3 changed files with 38 additions and 14 deletions
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44
fs/xfs/libxfs/xfs_bmap.c
View File

@ -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;
/*
* Make sure we only reflink into a hole.
*/
if (flags & XFS_BMAPI_REMAP)
ASSERT(inhole);
if (flags & XFS_BMAPI_COWFORK)
ASSERT(!inhole);
/* 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.
*/
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;

6
fs/xfs/libxfs/xfs_bmap.h
View File

@ -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)

2
fs/xfs/xfs_iomap.c
View File

@ -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)