Re: [PATCH] bcache: use REQ_PRIO to indicate bio for metadata

On 9/26/18 12:32 AM, Adam Manzanares wrote:

On 9/25/18 9:12 AM, Coly Li wrote:

quoted

In cached_dev_cache_miss() and check_should_bypass(), REQ_META is used
to check whether a bio is for metadata request. REQ_META is used for
blktrace, the correct REQ_ flag should be REQ_PRIO. This flag means the
bio should be prior to other bio, and frequently be used to indicate
metadata io in file system code.

This patch replaces REQ_META with correct flag REQ_PRIO.

CC Adam Manzanares because he explains to me what REQ_PRIO is for.

Sorry for the confusion, I was talking about using the bi_ioprio field
to pass ioprio information down to devices that support prioritized
commands. I haven't used the REQ_PRIO flag in the work that I have done.

Hi Adam,

Aha, I misunderstood you. After 'git blame' I realize REQ_PRIO is 
invented by Christoph Hellwig,

     Add a new REQ_PRIO to let requests preempt others in the cfq I/O 
schedule,
     and lave REQ_META purely for marking requests as metadata in blktrace.

 From log of commit 65299a3b788b ("block: separate priority boosting 
from REQ_META") it seems this patch still uses REQ_PRIO in a correct 
way. And, thank you for clarifying this :-)

Coly Li

quoted

Signed-off-by: Coly Li <redacted>
Cc: Adam Manzanares <redacted>
---
   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 51be355a3309..13d3355a90c0 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).
   	 */
   	if (bio->bi_opf & (REQ_RAHEAD|REQ_BACKGROUND) &&
-	    !(bio->bi_opf & REQ_META))
+	    !(bio->bi_opf & 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_META) &&
+	    !(bio->bi_opf & 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));