On Sun, Mar 01, 2015 at 09:08:38AM +0000, Filipe Manana wrote:
> When using the fast file fsync code path we can miss the fact that new
> writes happened since the last file fsync and therefore return without
> waiting for the IO to finish and write the new extents to the fsync log.
>
> Here's an example scenario where the fsync will miss the fact that new
> file data exists that wasn't yet durably persisted:
>
> 1. fs_info->last_trans_committed == N - 1 and current transaction is
> transaction N (fs_info->generation == N);
>
> 2. do a buffered write;
>
> 3. fsync our inode, this clears our inode's full sync flag, starts
> an ordered extent and waits for it to complete - when it completes
> at btrfs_finish_ordered_io(), the inode's last_trans is set to the
> value N (via btrfs_update_inode_fallback -> btrfs_update_inode ->
> btrfs_set_inode_last_trans);
>
> 4. transaction N is committed, so fs_info->last_trans_committed is now
> set to the value N and fs_info->generation remains with the value N;
>
> 5. do another buffered write, when this happens btrfs_file_write_iter
> sets our inode's last_trans to the value N + 1 (that is
> fs_info->generation + 1 == N + 1);
>
> 6. transaction N + 1 is started and fs_info->generation now has the
> value N + 1;
>
> 7. transaction N + 1 is committed, so fs_info->last_trans_committed
> is set to the value N + 1;
>
> 8. fsync our inode - because it doesn't have the full sync flag set,
> we only start the ordered extent, we don't wait for it to complete
> (only in a later phase) therefore its last_trans field has the
> value N + 1 set previously by btrfs_file_write_iter(), and so we
> have:
>
> inode->last_trans <= fs_info->last_trans_committed
> (N + 1) (N + 1)
>
> Which made us not log the last buffered write and exit the fsync
> handler immediately, returning success (0) to user space and resulting
> in data loss after a crash.
>
> This can actually be triggered deterministically and the following excerpt
> from a testcase I made for xfstests triggers the issue. It moves a dummy
> file across directories and then fsyncs the old parent directory - this
> is just to trigger a transaction commit, so moving files around isn't
> directly related to the issue but it was chosen because running 'sync' for
> example does more than just committing the current transaction, as it
> flushes/waits for all file data to be persisted. The issue can also happen
> at random periods, since the transaction kthread periodicaly commits the
> current transaction (about every 30 seconds by default).
> The body of the test is:
>
> _scratch_mkfs >> $seqres.full 2>&1
> _init_flakey
> _mount_flakey
>
> # Create our main test file 'foo', the one we check for data loss.
> # By doing an fsync against our file, it makes btrfs clear the 'needs_full_sync'
> # bit from its flags (btrfs inode specific flags).
> $XFS_IO_PROG -f -c "pwrite -S 0xaa 0 8K" \
> -c "fsync" $SCRATCH_MNT/foo | _filter_xfs_io
>
> # Now create one other file and 2 directories. We will move this second file
> # from one directory to the other later because it forces btrfs to commit its
> # currently open transaction if we fsync the old parent directory. This is
> # necessary to trigger the data loss bug that affected btrfs.
> mkdir $SCRATCH_MNT/testdir_1
> touch $SCRATCH_MNT/testdir_1/bar
> mkdir $SCRATCH_MNT/testdir_2
>
> # Make sure everything is durably persisted.
> sync
>
> # Write more 8Kb of data to our file.
> $XFS_IO_PROG -c "pwrite -S 0xbb 8K 8K" $SCRATCH_MNT/foo | _filter_xfs_io
>
> # Move our 'bar' file into a new directory.
> mv $SCRATCH_MNT/testdir_1/bar $SCRATCH_MNT/testdir_2/bar
>
> # Fsync our first directory. Because it had a file moved into some other
> # directory, this made btrfs commit the currently open transaction. This is
> # a condition necessary to trigger the data loss bug.
> $XFS_IO_PROG -c "fsync" $SCRATCH_MNT/testdir_1
>
> # Now fsync our main test file. If the fsync succeeds, we expect the 8Kb of
> # data we wrote previously to be persisted and available if a crash happens.
> # This did not happen with btrfs, because of the transaction commit that
> # happened when we fsynced the parent directory.
> $XFS_IO_PROG -c "fsync" $SCRATCH_MNT/foo
>
> # Simulate a crash/power loss.
> _load_flakey_table $FLAKEY_DROP_WRITES
> _unmount_flakey
>
> _load_flakey_table $FLAKEY_ALLOW_WRITES
> _mount_flakey
>
> # Now check that all data we wrote before are available.
> echo "File content after log replay:"
> od -t x1 $SCRATCH_MNT/foo
>
> status=0
> exit
>
> The expected golden output for the test, which is what we get with this
> fix applied (or when running against ext3/4 and xfs), is:
>
> wrote 8192/8192 bytes at offset 0
> XXX Bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
> wrote 8192/8192 bytes at offset 8192
> XXX Bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
> File content after log replay:
> 0000000 aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa
> *
> 0020000 bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb
> *
> 0040000
>
> Without this fix applied, the output shows the test file does not have
> the second 8Kb extent that we successfully fsynced:
>
> wrote 8192/8192 bytes at offset 0
> XXX Bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
> wrote 8192/8192 bytes at offset 8192
> XXX Bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
> File content after log replay:
> 0000000 aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa
> *
> 0020000
>
> So fix this by skipping the fsync only if we're doing a full sync and
> if the inode's last_trans is <= fs_info->last_trans_committed, or if
> the inode is already in the log. Also remove setting the inode's
> last_trans in btrfs_file_write_iter since it's useless/unreliable.
>
> A test case for xfstests will be sent soon.
>
> CC: <stable@xxxxxxxxxxxxxxx>
> Reviewed-by: Liu Bo <bo.li.liu@xxxxxxxxxx>
> Signed-off-by: Filipe Manana <fdmanana@xxxxxxxx>
> ---
>
> V2: Removed dead assignment of inode->last_trans in btrfs_file_write_iter
> (and the respective comment) since it's useless now. Added stable to
> cc because it's a data loss fix.
>
> fs/btrfs/file.c | 45 ++++++++++++++++++++++++++++-----------------
> 1 file changed, 28 insertions(+), 17 deletions(-)
>
> diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
> index 2bd72cd..b7334c9 100644
> --- a/fs/btrfs/file.c
> +++ b/fs/btrfs/file.c
> @@ -1811,22 +1811,10 @@ static ssize_t btrfs_file_write_iter(struct kiocb *iocb,
> mutex_unlock(&inode->i_mutex);
>
> /*
> - * we want to make sure fsync finds this change
> - * but we haven't joined a transaction running right now.
> - *
> - * Later on, someone is sure to update the inode and get the
> - * real transid recorded.
> - *
> - * We set last_trans now to the fs_info generation + 1,
> - * this will either be one more than the running transaction
> - * or the generation used for the next transaction if there isn't
> - * one running right now.
> - *
> * We also have to set last_sub_trans to the current log transid,
> * otherwise subsequent syncs to a file that's been synced in this
> * transaction will appear to have already occured.
> */
> - BTRFS_I(inode)->last_trans = root->fs_info->generation + 1;
By thinking twice about it, how about setting ->last_trans with (-1ULL)?
So the benefit is that if new writes have already finished its endio where
calling btrfs_set_inode_last_trans() to set ->last_trans with a transid
at that age, we may get a win for skipping log part if someone else has
updated ->last_trans_committed.
By limiting it to 'full_sync' case we lose the above opportunity.
Thanks,
-liubo
> BTRFS_I(inode)->last_sub_trans = root->log_transid;
> if (num_written > 0) {
> err = generic_write_sync(file, pos, num_written);
> @@ -1971,14 +1959,37 @@ int btrfs_sync_file(struct file *file, loff_t start, loff_t end, int datasync)
> }
>
> /*
> - * if the last transaction that changed this file was before
> - * the current transaction, we can bail out now without any
> - * syncing
> + * If the last transaction that changed this file was before the current
> + * transaction and we have the full sync flag set in our inode, we can
> + * bail out now without any syncing.
> + *
> + * Note that we can't bail out if the full sync flag isn't set. This is
> + * because when the full sync flag is set we start all ordered extents
> + * and wait for them to fully complete - when they complete they update
> + * the inode's last_trans field through:
> + *
> + * btrfs_finish_ordered_io() ->
> + * btrfs_update_inode_fallback() ->
> + * btrfs_update_inode() ->
> + * btrfs_set_inode_last_trans()
> + *
> + * So we are sure that last_trans is up to date and can do this check to
> + * bail out safely. For the fast path, when the full sync flag is not
> + * set in our inode, we can not do it because we start only our ordered
> + * extents and don't wait for them to complete (that is when
> + * btrfs_finish_ordered_io runs), so here at this point their last_trans
> + * value might be less than or equals to fs_info->last_trans_committed,
> + * and setting a speculative last_trans for an inode when a buffered
> + * write is made (such as fs_info->generation + 1 for example) would not
> + * be reliable since after setting the value and before fsync is called
> + * any number of transactions can start and commit (transaction kthread
> + * commits the current transaction periodically), and a transaction
> + * commit does not start nor waits for ordered extents to complete.
> */
> smp_mb();
> if (btrfs_inode_in_log(inode, root->fs_info->generation) ||
> - BTRFS_I(inode)->last_trans <=
> - root->fs_info->last_trans_committed) {
> + (full_sync && BTRFS_I(inode)->last_trans <=
> + root->fs_info->last_trans_committed)) {
> BTRFS_I(inode)->last_trans = 0;
>
> /*
> --
> 2.1.3
>
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