From: Josef Bacik jbacik@fb.com

Commit b5e6c3e170b77025b5f6174258c7ad71eed2d4de upstream.

There's a priority inversion that exists currently with btrfs fsync. In some cases we will collect outstanding ordered extents onto a list and only wait on them at the very last second. However this "very last second" falls inside of a transaction handle, so if we are in a lower priority cgroup we can end up holding the transaction open for longer than needed, so if a high priority cgroup is also trying to fsync() it'll see latency.

Signed-off-by: Josef Bacik jbacik@fb.com Reviewed-by: Filipe Manana fdmanana@suse.com Signed-off-by: David Sterba dsterba@suse.com Signed-off-by: Anand Jain anand.jain@oracle.com --- fs/btrfs/file.c | 56 ++++--------------------------------------------- 1 file changed, 4 insertions(+), 52 deletions(-)

diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c index dd2504322a87..2f386d8dbd0e 100644 --- a/fs/btrfs/file.c +++ b/fs/btrfs/file.c @@ -2102,53 +2102,12 @@ int btrfs_sync_file(struct file *file, loff_t start, loff_t end, int datasync) atomic_inc(&root->log_batch); full_sync = test_bit(BTRFS_INODE_NEEDS_FULL_SYNC, &BTRFS_I(inode)->runtime_flags); + /* - * We might have have had more pages made dirty after calling - * start_ordered_ops and before acquiring the inode's i_mutex. + * We have to do this here to avoid the priority inversion of waiting on + * IO of a lower priority task while holding a transaciton open. */ - if (full_sync) { - /* - * For a full sync, we need to make sure any ordered operations - * start and finish before we start logging the inode, so that - * all extents are persisted and the respective file extent - * items are in the fs/subvol btree. - */ - ret = btrfs_wait_ordered_range(inode, start, len); - } else { - /* - * Start any new ordered operations before starting to log the - * inode. We will wait for them to finish in btrfs_sync_log(). - * - * Right before acquiring the inode's mutex, we might have new - * writes dirtying pages, which won't immediately start the - * respective ordered operations - that is done through the - * fill_delalloc callbacks invoked from the writepage and - * writepages address space operations. So make sure we start - * all ordered operations before starting to log our inode. Not - * doing this means that while logging the inode, writeback - * could start and invoke writepage/writepages, which would call - * the fill_delalloc callbacks (cow_file_range, - * submit_compressed_extents). These callbacks add first an - * extent map to the modified list of extents and then create - * the respective ordered operation, which means in - * tree-log.c:btrfs_log_inode() we might capture all existing - * ordered operations (with btrfs_get_logged_extents()) before - * the fill_delalloc callback adds its ordered operation, and by - * the time we visit the modified list of extent maps (with - * btrfs_log_changed_extents()), we see and process the extent - * map they created. We then use the extent map to construct a - * file extent item for logging without waiting for the - * respective ordered operation to finish - this file extent - * item points to a disk location that might not have yet been - * written to, containing random data - so after a crash a log - * replay will make our inode have file extent items that point - * to disk locations containing invalid data, as we returned - * success to userspace without waiting for the respective - * ordered operation to finish, because it wasn't captured by - * btrfs_get_logged_extents(). - */ - ret = start_ordered_ops(inode, start, end); - } + ret = btrfs_wait_ordered_range(inode, start, len); if (ret) { up_write(&BTRFS_I(inode)->dio_sem); inode_unlock(inode); @@ -2283,13 +2242,6 @@ int btrfs_sync_file(struct file *file, loff_t start, loff_t end, int datasync) goto out; } } - if (!full_sync) { - ret = btrfs_wait_ordered_range(inode, start, len); - if (ret) { - btrfs_end_transaction(trans); - goto out; - } - } ret = btrfs_commit_transaction(trans); } else { ret = btrfs_end_transaction(trans);