The file hole detection logic during a file fsync wasn't correct,
because it didn't look back (in a previous leaf) for the last file
extent item that can be in a leaf to the left of our leaf and that
has a generation lower than the current transaction id. This made it
assume that a hole exists when it really doesn't exist in the file.
Such false positive hole detection happens in the following scenario:
* We have a file that has many file extent items, covering 3 or more
btree leafs (the first leaf must contain non file extent items too).
* Two ranges of the file are modified, with their extent items being
located at 2 different leafs and those leafs aren't consecutive.
* When processing the second modified leaf, we weren't checking if
some file extent item exists that is located in some leaf that is
between our 2 modified leafs, and therefore assumed the range defined
between the last file extent item in the first leaf and the first file
extent item in the second leaf matched a hole.
Fortunately this didn't result in overriding the log with wrong data,
instead it made the last loop in copy_items() attempt to insert a
duplicated key (for a hole file extent item), which makes the file
fsync code return with -EEXIST to file.c:btrfs_sync_file() which in
turn ends up doing a full transaction commit, which is much more
expensive then writing only to the log tree and wait for it to be
durably persisted (as well as the file's modified extents/pages).
Therefore fix the hole detection logic, so that we don't pay the
cost of doing full transaction commits.
I could trigger this issue with the following test for xfstests (which
never fails, either without or with this patch). The last fsync call
results in a full transaction commit, due to the -EEXIST error mentioned
above. I could also observe this behaviour happening frequently when
running xfstests/generic/075 in a loop.
Test:
_cleanup()
{
_cleanup_flakey
rm -fr $tmp
}
# get standard environment, filters and checks
. ./common/rc
. ./common/filter
. ./common/dmflakey
# real QA test starts here
_supported_fs btrfs
_supported_os Linux
_require_scratch
_require_dm_flakey
_need_to_be_root
rm -f $seqres.full
# Create a file with many file extent items, each representing a 4Kb extent.
# These items span 3 btree leaves, of 16Kb each (default mkfs.btrfs leaf size
# as of btrfs-progs 3.12).
_scratch_mkfs -l 16384 >/dev/null 2>&1
_init_flakey
SAVE_MOUNT_OPTIONS="$MOUNT_OPTIONS"
MOUNT_OPTIONS="$MOUNT_OPTIONS -o commit=999"
_mount_flakey
# First fsync, inode has BTRFS_INODE_NEEDS_FULL_SYNC flag set.
$XFS_IO_PROG -f -c "pwrite -S 0x01 -b 4096 0 4096" -c "fsync" \
$SCRATCH_MNT/foo | _filter_xfs_io
# For any of the following fsync calls, inode doesn't have the flag
# BTRFS_INODE_NEEDS_FULL_SYNC set.
for ((i = 1; i <= 500; i++)); do
OFFSET=$((4096 * i))
LEN=4096
$XFS_IO_PROG -c "pwrite -S 0x01 $OFFSET $LEN" -c "fsync" \
$SCRATCH_MNT/foo | _filter_xfs_io
done
# Commit transaction and bump next transaction's id (to 7).
sync
# Truncate will set the BTRFS_INODE_NEEDS_FULL_SYNC flag in the btrfs's
# inode runtime flags.
$XFS_IO_PROG -c "truncate 2048000" $SCRATCH_MNT/foo
# Commit transaction and bump next transaction's id (to 8).
sync
# Touch 1 extent item from the first leaf and 1 from the last leaf. The leaf
# in the middle, containing only file extent items, isn't touched. So the
# next fsync, when calling btrfs_search_forward(), won't visit that middle
# leaf. First and 3rd leaf have now a generation with value 8, while the
# middle leaf remains with a generation with value 6.
$XFS_IO_PROG \
-c "pwrite -S 0xee -b 4096 0 4096" \
-c "pwrite -S 0xff -b 4096 2043904 4096" \
-c "fsync" \
$SCRATCH_MNT/foo | _filter_xfs_io
_load_flakey_table $FLAKEY_DROP_WRITES
md5sum $SCRATCH_MNT/foo | _filter_scratch
_unmount_flakey
_load_flakey_table $FLAKEY_ALLOW_WRITES
# During mount, we'll replay the log created by the fsync above, and the file's
# md5 digest should be the same we got before the unmount.
_mount_flakey
md5sum $SCRATCH_MNT/foo | _filter_scratch
_unmount_flakey
MOUNT_OPTIONS="$SAVE_MOUNT_OPTIONS"
status=0
exit
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Chris Mason <clm@fb.com>
74121f7cbb