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btrfs: always wait on ordered extents at fsync time
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>
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1 changed files with 4 additions and 52 deletions
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@ -2068,53 +2068,12 @@ int btrfs_sync_file(struct file *file, loff_t start, loff_t end, int datasync)
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atomic_inc(&root->log_batch);
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full_sync = test_bit(BTRFS_INODE_NEEDS_FULL_SYNC,
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&BTRFS_I(inode)->runtime_flags);
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/*
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* We might have have had more pages made dirty after calling
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* start_ordered_ops and before acquiring the inode's i_mutex.
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* We have to do this here to avoid the priority inversion of waiting on
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* IO of a lower priority task while holding a transaciton open.
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*/
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if (full_sync) {
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/*
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* For a full sync, we need to make sure any ordered operations
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* start and finish before we start logging the inode, so that
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* all extents are persisted and the respective file extent
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* items are in the fs/subvol btree.
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*/
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ret = btrfs_wait_ordered_range(inode, start, len);
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} else {
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/*
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* Start any new ordered operations before starting to log the
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* inode. We will wait for them to finish in btrfs_sync_log().
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*
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* Right before acquiring the inode's mutex, we might have new
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* writes dirtying pages, which won't immediately start the
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* respective ordered operations - that is done through the
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* fill_delalloc callbacks invoked from the writepage and
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* writepages address space operations. So make sure we start
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* all ordered operations before starting to log our inode. Not
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* doing this means that while logging the inode, writeback
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* could start and invoke writepage/writepages, which would call
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* the fill_delalloc callbacks (cow_file_range,
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* submit_compressed_extents). These callbacks add first an
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* extent map to the modified list of extents and then create
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* the respective ordered operation, which means in
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* tree-log.c:btrfs_log_inode() we might capture all existing
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* ordered operations (with btrfs_get_logged_extents()) before
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* the fill_delalloc callback adds its ordered operation, and by
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* the time we visit the modified list of extent maps (with
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* btrfs_log_changed_extents()), we see and process the extent
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* map they created. We then use the extent map to construct a
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* file extent item for logging without waiting for the
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* respective ordered operation to finish - this file extent
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* item points to a disk location that might not have yet been
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* written to, containing random data - so after a crash a log
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* replay will make our inode have file extent items that point
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* to disk locations containing invalid data, as we returned
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* success to userspace without waiting for the respective
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* ordered operation to finish, because it wasn't captured by
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* btrfs_get_logged_extents().
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*/
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ret = start_ordered_ops(inode, start, end);
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}
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ret = btrfs_wait_ordered_range(inode, start, len);
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if (ret) {
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inode_unlock(inode);
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goto out;
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@ -2239,13 +2198,6 @@ int btrfs_sync_file(struct file *file, loff_t start, loff_t end, int datasync)
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goto out;
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}
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}
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if (!full_sync) {
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ret = btrfs_wait_ordered_range(inode, start, len);
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if (ret) {
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btrfs_end_transaction(trans);
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goto out;
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}
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}
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ret = btrfs_commit_transaction(trans);
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} else {
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ret = btrfs_end_transaction(trans);
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