In 'commit 752f66a75aba ("bcache: use REQ_PRIO to indicate bio for metadata")' REQ_META is replaced by REQ_PRIO to indicate metadata bio. This assumption is not always correct, e.g. XFS uses REQ_META to mark metadata bio other than REQ_PRIO. This is why Nix noticed that bcache does not cache metadata for XFS after the above commit.

Thanks to Dave Chinner, he explains the difference between REQ_META and REQ_PRIO from view of file system developer. Here I quote part of his explanation from mailing list, REQ_META is used for metadata. REQ_PRIO is used to communicate to the lower layers that the submitter considers this IO to be more important that non REQ_PRIO IO and so dispatch should be expedited.

IOWs, if the filesystem considers metadata IO to be more important that user data IO, then it will use REQ_PRIO | REQ_META rather than just REQ_META.

Then it seems bios with REQ_META or REQ_PRIO should both be cached for performance optimation, because they are all probably low I/O latency demand by upper layer (e.g. file system).

So in this patch, when we want to decide whether to bypass the cache, REQ_META and REQ_PRIO are both checked. Then both metadata and high priority I/O requests will be handled properly.

Reported-by: Nix nix@esperi.org.uk Signed-off-by: Coly Li colyli@suse.de Reviewed-by: Andre Noll maan@tuebingen.mpg.de Cc: stable@vger.kernel.org Cc: Dave Chinner david@fromorbit.com Cc: Christoph Hellwig hch@lst.de --- drivers/md/bcache/request.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/md/bcache/request.c b/drivers/md/bcache/request.c index 3bf35914bb57..62bda90a38dc 100644 --- a/drivers/md/bcache/request.c +++ b/drivers/md/bcache/request.c @@ -395,7 +395,7 @@ static bool check_should_bypass(struct cached_dev *dc, struct bio *bio) * unless the read-ahead request is for metadata (eg, for gfs2 or xfs). */ if (bio->bi_opf & (REQ_RAHEAD|REQ_BACKGROUND) && - !(bio->bi_opf & REQ_PRIO)) + !(bio->bi_opf & (REQ_META|REQ_PRIO))) goto skip;

if (bio->bi_iter.bi_sector & (c->sb.block_size - 1) || @@ -877,7 +877,7 @@ static int cached_dev_cache_miss(struct btree *b, struct search *s, }

if (!(bio->bi_opf & REQ_RAHEAD) && - !(bio->bi_opf & REQ_PRIO) && + !(bio->bi_opf & (REQ_META|REQ_PRIO)) && s->iop.c->gc_stats.in_use < CUTOFF_CACHE_READA) reada = min_t(sector_t, dc->readahead >> 9, get_capacity(bio->bi_disk) - bio_end_sector(bio));