Re: [PATCH v2] Btrfs: fix data loss in the fast fsync path

[Liu Bo <bo.li.liu@oracle.com>]

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@vger.kernel.org>
> Reviewed-by: Liu Bo <bo.li.liu@oracle.com>
> Signed-off-by: Filipe Manana <fdmanana@suse.com>
> ---
> 
> 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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