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@xxxxxx>
---
fs/xfs/libxfs/xfs_bmap.c | 41 +++++++++++++++++++++++++++++------------
fs/xfs/libxfs/xfs_bmap.h | 3 +++
fs/xfs/xfs_iomap.c | 2 +-
3 files changed, 33 insertions(+), 13 deletions(-)
diff --git a/fs/xfs/libxfs/xfs_bmap.c b/fs/xfs/libxfs/xfs_bmap.c
index 44773c9..ceae779 100644
--- a/fs/xfs/libxfs/xfs_bmap.c
+++ b/fs/xfs/libxfs/xfs_bmap.c
@@ -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 +4601,41 @@ 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;
+ }
+ } 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;
diff --git a/fs/xfs/libxfs/xfs_bmap.h b/fs/xfs/libxfs/xfs_bmap.h
index cecd094..8f805a7 100644
--- a/fs/xfs/libxfs/xfs_bmap.h
+++ b/fs/xfs/libxfs/xfs_bmap.h
@@ -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" }, \
diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
index ca137b7..7ee8629 100644
--- a/fs/xfs/xfs_iomap.c
+++ b/fs/xfs/xfs_iomap.c
@@ -795,7 +795,7 @@ 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)
--
2.1.4
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