While writing to a file, in inode.c:cow_file_range() (and same applies to
submit_compressed_extents()), after reserving an extent for the file data,
we create a new extent map for the written range and insert it into the
extent map cache. After that, we create an ordered operation, but if it
fails (due to a transient/temporary-ENOMEM), we return without dropping
that extent map, which points to a reserved extent that is freed when we
return. A subsequent incremental fsync (when the btrfs inode doesn't have
the flag BTRFS_INODE_NEEDS_FULL_SYNC) considers this extent map valid and
logs a file extent item based on that extent map, which points to a disk
extent that doesn't contain valid data - it was freed by us earlier, at this
point it might contain any random/garbage data.
Therefore, if we reach an error condition when cowing a file range after
we added the new extent map to the cache, drop it from the cache before
returning.
Some sequence of steps that lead to this:
$ mkfs.btrfs -f /dev/sdd
$ mount -o commit=9999 /dev/sdd /mnt
$ cd /mnt
$ xfs_io -f -c "pwrite -S 0x01 -b 4096 0 4096" -c "fsync" foo
$ xfs_io -c "pwrite -S 0x02 -b 4096 4096 4096"
$ sync
$ od -t x1 foo
0000000 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01
*
0010000 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02
*
0020000
$ xfs_io -c "pwrite -S 0xa1 -b 4096 0 4096" foo
# Now this write + fsync fail with -ENOMEM, which was returned by
# btrfs_add_ordered_extent() in inode.c:cow_file_range().
$ xfs_io -c "pwrite -S 0xff -b 4096 4096 4096" foo
$ xfs_io -c "fsync" foo
fsync: Cannot allocate memory
# Now do a new write + fsync, which will succeed. Our previous
# -ENOMEM was a transient/temporary error.
$ xfs_io -c "pwrite -S 0xee -b 4096 16384 4096" foo
$ xfs_io -c "fsync" foo
# Our file content (in page cache) is now:
$ od -t x1 foo
0000000 a1 a1 a1 a1 a1 a1 a1 a1 a1 a1 a1 a1 a1 a1 a1 a1
*
0010000 ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
*
0020000 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
*
0040000 ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee
*
0050000
# Now reboot the machine, and mount the fs, so that fsync log replay
# takes place.
# The file content is now weird, in particular the first 8Kb, which
# do not match our data before nor after the sync command above.
$ od -t x1 foo
0000000 ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee
*
0010000 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01
*
0020000 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
*
0040000 ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee
*
0050000
# In fact these first 4Kb are a duplicate of the last 4kb block.
# The last write got an extent map/file extent item that points to
# the same disk extent that we got in the write+fsync that failed
# with the -ENOMEM error. btrfs-debug-tree and btrfsck allow us to
# verify that:
$ btrfs-debug-tree /dev/sdd
(...)
item 6 key (257 EXTENT_DATA 0) itemoff 15819 itemsize 53
extent data disk byte 12582912 nr 8192
extent data offset 0 nr 8192 ram 8192
item 7 key (257 EXTENT_DATA 8192) itemoff 15766 itemsize 53
extent data disk byte 0 nr 0
extent data offset 0 nr 8192 ram 8192
item 8 key (257 EXTENT_DATA 16384) itemoff 15713 itemsize 53
extent data disk byte 12582912 nr 4096
extent data offset 0 nr 4096 ram 4096
$ umount /dev/sdd
$ btrfsck /dev/sdd
Checking filesystem on /dev/sdd
UUID: db5e60e1-050d-41e6-8c7f-3d742dea5d8f
checking extents
extent item 12582912 has multiple extent items
ref mismatch on [12582912 4096] extent item 1, found 2
Backref bytes do not match extent backref, bytenr=12582912, ref bytes=4096,
backref bytes=8192
backpointer mismatch on [12582912 4096]
Errors found in extent allocation tree or chunk allocation
checking free space cache
checking fs roots
root 5 inode 257 errors 1000, some csum missing
found 131074 bytes used err is 1
total csum bytes: 4
total tree bytes: 131072
total fs tree bytes: 32768
total extent tree bytes: 16384
btree space waste bytes: 123404
file data blocks allocated: 274432
referenced 274432
Btrfs v3.14.1-96-gcc7fd5a-dirty
Signed-off-by: Filipe Manana <fdman...@suse.com>
---
fs/btrfs/inode.c | 12 +++++++++---
1 file changed, 9 insertions(+), 3 deletions(-)
diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index c678dea..16e8146 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -792,8 +792,12 @@ retry:
ins.offset,
BTRFS_ORDERED_COMPRESSED,
async_extent->compress_type);
- if (ret)
+ if (ret) {
+ btrfs_drop_extent_cache(inode, async_extent->start,
+ async_extent->start +
+ async_extent->ram_size - 1, 0);
goto out_free_reserve;
+ }
/*
* clear dirty, set writeback and unlock the pages.
@@ -985,14 +989,14 @@ static noinline int cow_file_range(struct inode *inode,
ret = btrfs_add_ordered_extent(inode, start, ins.objectid,
ram_size, cur_alloc_size, 0);
if (ret)
- goto out_reserve;
+ goto out_drop_extent_cache;
if (root->root_key.objectid ==
BTRFS_DATA_RELOC_TREE_OBJECTID) {
ret = btrfs_reloc_clone_csums(inode, start,
cur_alloc_size);
if (ret)
- goto out_reserve;
+ goto out_drop_extent_cache;
}
if (disk_num_bytes < cur_alloc_size)
@@ -1020,6 +1024,8 @@ static noinline int cow_file_range(struct inode *inode,
out:
return ret;
+out_drop_extent_cache:
+ btrfs_drop_extent_cache(inode, start, start + ram_size - 1, 0);
out_reserve:
btrfs_free_reserved_extent(root, ins.objectid, ins.offset, 1);
out_unlock:
--
1.9.1
--
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