Re: [PATCH] Btrfs: fix fsync data loss after append write

Liu Bo
2015-06-18T10:20:54Z

On Wed, Jun 17, 2015 at 12:49:23PM +0100, fdmanana@kernel.org wrote:
> From: Filipe Manana <fdmanana@suse.com>
>
> If we do an append write to a file (which increases its inode's i_size)
> that does not have the flag BTRFS_INODE_NEEDS_FULL_SYNC set in its inode,
> and the previous transaction added a new hard link to the file, which sets
> the flag BTRFS_INODE_COPY_EVERYTHING in the file's inode, and then fsync
> the file, the inode's new i_size isn't logged. This has the consequence
> that after the fsync log is replayed, the file size remains what it was
> before the append write operation, which means users/applications will
> not be able to read the data that was successsfully fsync'ed before.
>
> This happens because neither the inode item nor the delayed inode get
> their i_size updated when the append write is made - doing so would
> require starting a transaction in the buffered write path, something that
> we do not do intentionally for performance reasons.
>
> Fix this by making sure that when the flag BTRFS_INODE_COPY_EVERYTHING is
> set the inode is logged with its current i_size (log the in-memory inode
> into the log tree).

Looks good to me.

Reviewed-by: Liu Bo <bo.li.liu@oracle.com>

Thanks,

-liubo
>
> This issue is not a recent regression and is easy to reproduce with the
> following test case for fstests:
>
> seq=`basename $0`
> seqres=$RESULT_DIR/$seq
> echo "QA output created by $seq"
>
> here=`pwd`
> tmp=/tmp/$$
> status=1 # failure is the default!
>
> _cleanup()
> {
> _cleanup_flakey
> rm -f $tmp.*
> }
> trap "_cleanup; exit \$status" 0 1 2 3 15
>
> # get standard environment, filters and checks
> . ./common/rc
> . ./common/filter
> . ./common/dmflakey
>
> # real QA test starts here
> _supported_fs generic
> _supported_os Linux
> _need_to_be_root
> _require_scratch
> _require_dm_flakey
> _require_metadata_journaling $SCRATCH_DEV
>
> _crash_and_mount()
> {
> # Simulate a crash/power loss.
> _load_flakey_table $FLAKEY_DROP_WRITES
> _unmount_flakey
> # Allow writes again and mount. This makes the fs replay its fsync log.
> _load_flakey_table $FLAKEY_ALLOW_WRITES
> _mount_flakey
> }
>
> rm -f $seqres.full
>
> _scratch_mkfs >> $seqres.full 2>&1
> _init_flakey
> _mount_flakey
>
> # Create the test file with some initial data and then fsync it.
> # The fsync here is only needed to trigger the issue in btrfs, as it causes the
> # the flag BTRFS_INODE_NEEDS_FULL_SYNC to be removed from the btrfs inode.
> $XFS_IO_PROG -f -c "pwrite -S 0xaa 0 32k" \
> -c "fsync" \
> $SCRATCH_MNT/foo | _filter_xfs_io
> sync
>
> # Add a hard link to our file.
> # On btrfs this sets the flag BTRFS_INODE_COPY_EVERYTHING on the btrfs inode,
> # which is a necessary condition to trigger the issue.
> ln $SCRATCH_MNT/foo $SCRATCH_MNT/bar
>
> # Sync the filesystem to force a commit of the current btrfs transaction, this
> # is a necessary condition to trigger the bug on btrfs.
> sync
>
> # Now append more data to our file, increasing its size, and fsync the file.
> # In btrfs because the inode flag BTRFS_INODE_COPY_EVERYTHING was set and the
> # write path did not update the inode item in the btree nor the delayed inode
> # item (in memory struture) in the current transaction (created by the fsync
> # handler), the fsync did not record the inode's new i_size in the fsync
> # log/journal. This made the data unavailable after the fsync log/journal is
> # replayed.
> $XFS_IO_PROG -c "pwrite -S 0xbb 32K 32K" \
> -c "fsync" \
> $SCRATCH_MNT/foo | _filter_xfs_io
>
> echo "File content after fsync and before crash:"
> od -t x1 $SCRATCH_MNT/foo
>
> _crash_and_mount
>
> echo "File content after crash and log replay:"
> od -t x1 $SCRATCH_MNT/foo
>
> status=0
> exit
>
> The expected file output before and after the crash/power failure expects the
> appended data to be available, which is:
>
> 0000000 aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa
> *
> 0100000 bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb
> *
> 0200000
>
> Cc: stable@vger.kernel.org
> Signed-off-by: Filipe Manana <fdmanana@suse.com>
> ---
> fs/btrfs/tree-log.c | 14 +++++++++-----
> 1 file changed, 9 insertions(+), 5 deletions(-)
>
> diff --git a/fs/btrfs/tree-log.c b/fs/btrfs/tree-log.c
> index d049683..4920fce 100644
> --- a/fs/btrfs/tree-log.c
> +++ b/fs/btrfs/tree-log.c
> @@ -4161,6 +4161,7 @@ static int btrfs_log_inode(struct btrfs_trans_handle *trans,
> u64 ino = btrfs_ino(inode);
> struct extent_map_tree *em_tree = &BTRFS_I(inode)->extent_tree;
> u64 logged_isize = 0;
> + bool need_log_inode_item = true;
>
> path = btrfs_alloc_path();
> if (!path)
> @@ -4269,11 +4270,6 @@ static int btrfs_log_inode(struct btrfs_trans_handle *trans,
> } else {
> if (inode_only == LOG_INODE_ALL)
> fast_search = true;
> - ret = log_inode_item(trans, log, dst_path, inode);
> - if (ret) {
> - err = ret;
> - goto out_unlock;
> - }
> goto log_extents;
> }
>
> @@ -4296,6 +4292,9 @@ again:
> if (min_key.type > max_key.type)
> break;
>
> + if (min_key.type == BTRFS_INODE_ITEM_KEY)
> + need_log_inode_item = false;
> +
> src = path->nodes[0];
> if (ins_nr && ins_start_slot + ins_nr == path->slots[0]) {
> ins_nr++;
> @@ -4366,6 +4365,11 @@ next_slot:
> log_extents:
> btrfs_release_path(path);
> btrfs_release_path(dst_path);
> + if (need_log_inode_item) {
> + err = log_inode_item(trans, log, dst_path, inode);
> + if (err)
> + goto out_unlock;
> + }
> if (fast_search) {
> /*
> * Some ordered extents started by fsync might have completed
> --
> 2.1.3
>
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