btrfs: zoned: do not use async metadata checksum on zoned filesystems

On zoned filesystems, btrfs uses per-fs zoned_meta_io_lock to serialize
the metadata write IOs.

Even with this serialization, write bios sent from btree_write_cache_pages
can be reordered by async checksum workers as these workers are per CPU
and not per zone.

To preserve write bio ordering, we disable async metadata checksum on a
zoned filesystem. This does not result in lower performance with HDDs as
a single CPU core is fast enough to do checksum for a single zone write
stream with the maximum possible bandwidth of the device. If multiple
zones are being written simultaneously, HDD seek overhead lowers the
achievable maximum bandwidth, resulting again in a per zone checksum
serialization not affecting the performance.
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/disk-io.c

@@ -871,6 +871,8 @@ static blk_status_t btree_submit_bio_start(struct inode *inode, struct bio *bio,
 static int check_async_write(struct btrfs_fs_info *fs_info,
 			     struct btrfs_inode *bi)
 {
+	if (btrfs_is_zoned(fs_info))
+		return 0;
 	if (atomic_read(&bi->sync_writers))
 		return 0;
 	if (test_bit(BTRFS_FS_CSUM_IMPL_FAST, &fs_info->flags))