On Tue, Oct 21, 2014 at 6:12 AM, Filipe Manana <fdman...@suse.com>
wrote:
If right after starting the snapshot creation ioctl we perform a
write against a
file followed by a truncate, with both operations increasing the
file's size, we
can get a snapshot tree that reflects a state of the source
subvolume's tree where
the file truncation happened but the write operation didn't. This
leaves a gap
between 2 file extent items of the inode, which makes btrfs' fsck
complain about it.
For example, if we perform the following file operations:
$ mkfs.btrfs -f /dev/vdd
$ mount /dev/vdd /mnt
$ xfs_io -f \
-c "pwrite -S 0xaa -b 32K 0 32K" \
-c "fsync" \
-c "pwrite -S 0xbb -b 32770 16K 32770" \
-c "truncate 90123" \
/mnt/foobar
and the snapshot creation ioctl was just called before the second
write, we often
can get the following inode items in the snapshot's btree:
item 120 key (257 INODE_ITEM 0) itemoff 7987 itemsize 160
inode generation 146 transid 7 size 90123 block group
0 mode 100600 links 1 uid 0 gid 0 rdev 0 flags 0x0
item 121 key (257 INODE_REF 256) itemoff 7967 itemsize 20
inode ref index 282 namelen 10 name: foobar
item 122 key (257 EXTENT_DATA 0) itemoff 7914 itemsize 53
extent data disk byte 1104855040 nr 32768
extent data offset 0 nr 32768 ram 32768
extent compression 0
item 123 key (257 EXTENT_DATA 53248) itemoff 7861 itemsize 53
extent data disk byte 0 nr 0
extent data offset 0 nr 40960 ram 40960
extent compression 0
There's a file range, corresponding to the interval [32K; ALIGN(16K +
32770, 4096)[
for which there's no file extent item covering it. This is because
the file write
and file truncate operations happened both right after the snapshot
creation ioctl
called btrfs_start_delalloc_inodes(), which means we didn't start and
wait for the
ordered extent that matches the write and, in btrfs_setsize(), we
were able to call
btrfs_cont_expand() before being able to commit the current
transaction in the
snapshot creation ioctl. So this made it possibe to insert the hole
file extent
item in the source subvolume (which represents the region added by
the truncate)
right before the transaction commit from the snapshot creation ioctl.
Btrfs' fsck tool complains about such cases with a message like the
following:
"root 331 inode 257 errors 100, file extent discount"
From a user perspective, the expectation when a snapshot is created
while those
file operations are being performed is that the snapshot will have a
file that
either:
1) is empty
2) only the first write was captured
3) only the 2 writes were captured
4) both writes and the truncation were captured
But never capture a state where only the first write and the
truncation were
captured (since the second write was performed before the truncation).
A test case for xfstests follows.
Signed-off-by: Filipe Manana <fdman...@suse.com>
---
fs/btrfs/inode.c | 3 +++
1 file changed, 3 insertions(+)
diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 0d41741..c28b78f 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -4622,6 +4622,9 @@ static int btrfs_setsize(struct inode *inode,
struct iattr *attr)
}
if (newsize > oldsize) {
+ ret = btrfs_wait_ordered_range(inode, 0, (u64)-1);
+ if (ret)
+ return ret;
Expanding truncates aren't my favorite operation, but we don't want
them to imply fsync. I'm holding off on this one while I work out the
rest of the vacation backlog ;)
-chris
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