block: consider discard merge last

If the next discard range is contiguous with the current range being
considered, it's cheaper to expand the current range than to append an
additional bio.

Signed-off-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/block/blk-merge.c b/block/blk-merge.c
index d3115d7469df07f7b36b3cc9b0a7e494ce2e6150..db08bc90609165d08d337451a85a3bacd1e954e3 100644 (file)
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -712,10 +712,10 @@ static void blk_account_io_merge_request(struct request *req)
 static enum elv_merge blk_try_req_merge(struct request *req,
                                        struct request *next)
 {
-       if (blk_discard_mergable(req))
-               return ELEVATOR_DISCARD_MERGE;
-       else if (blk_rq_pos(req) + blk_rq_sectors(req) == blk_rq_pos(next))
+       if (blk_rq_pos(req) + blk_rq_sectors(req) == blk_rq_pos(next))
                return ELEVATOR_BACK_MERGE;
+       else if (blk_discard_mergable(req))
+               return ELEVATOR_DISCARD_MERGE;
 
        return ELEVATOR_NO_MERGE;
 }
@@ -903,12 +903,12 @@ bool blk_rq_merge_ok(struct request *rq, struct bio *bio)
 
 enum elv_merge blk_try_merge(struct request *rq, struct bio *bio)
 {
-       if (blk_discard_mergable(rq))
-               return ELEVATOR_DISCARD_MERGE;
-       else if (blk_rq_pos(rq) + blk_rq_sectors(rq) == bio->bi_iter.bi_sector)
+       if (blk_rq_pos(rq) + blk_rq_sectors(rq) == bio->bi_iter.bi_sector)
                return ELEVATOR_BACK_MERGE;
        else if (blk_rq_pos(rq) - bio_sectors(bio) == bio->bi_iter.bi_sector)
                return ELEVATOR_FRONT_MERGE;
+       else if (blk_discard_mergable(rq))
+               return ELEVATOR_DISCARD_MERGE;
        return ELEVATOR_NO_MERGE;
 }