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Subject[PATCH 5.0 196/238] bcache: use (REQ_META|REQ_PRIO) to indicate bio for metadata
Date
5.0-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Coly Li <colyli@suse.de>

commit dc7292a5bcb4c878b076fca2ac3fc22f81b8f8df upstream.

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>
Tested-by: Nix <nix@esperi.org.uk>
Cc: stable@vger.kernel.org
Cc: Dave Chinner <david@fromorbit.com>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
drivers/md/bcache/request.c | 7 ++++---
1 file changed, 4 insertions(+), 3 deletions(-)

--- a/drivers/md/bcache/request.c
+++ b/drivers/md/bcache/request.c
@@ -392,10 +392,11 @@ static bool check_should_bypass(struct c

/*
* Flag for bypass if the IO is for read-ahead or background,
- * unless the read-ahead request is for metadata (eg, for gfs2).
+ * 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 +878,7 @@ static int cached_dev_cache_miss(struct
}

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));

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