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btrfs: always wait on ordered extents at fsync time

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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>
This commit is contained in:
Josef Bacik 2018-05-23 11:58:33 -04:00 committed by David Sterba
parent 16d1c062c7
commit b5e6c3e170
1 changed files with 4 additions and 52 deletions
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56
fs/btrfs/file.c
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@ -2068,53 +2068,12 @@ 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) {
inode_unlock(inode);
goto out;
@ -2239,13 +2198,6 @@ 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);