block: optimize __blk_segment_map_sg() for single-page bvec

Introduce a fast path for single-page bvec IO, then blk_bvec_map_sg()
can be avoided.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/block/blk-merge.c b/block/blk-merge.c
index c7e8a82..c1ad8ab 100644 (file)
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -447,7 +447,7 @@ static int blk_phys_contig_segment(struct request_queue *q, struct bio *bio,
        return biovec_phys_mergeable(q, &end_bv, &nxt_bv);
 }
 
-static struct scatterlist *blk_next_sg(struct scatterlist **sg,
+static inline struct scatterlist *blk_next_sg(struct scatterlist **sg,
                struct scatterlist *sglist)
 {
        if (!*sg)
@@ -512,7 +512,12 @@ __blk_segment_map_sg(struct request_queue *q, struct bio_vec *bvec,
                (*sg)->length += nbytes;
        } else {
 new_segment:
-               (*nsegs) += blk_bvec_map_sg(q, bvec, sglist, sg);
+               if (bvec->bv_offset + bvec->bv_len <= PAGE_SIZE) {
+                       *sg = blk_next_sg(sg, sglist);
+                       sg_set_page(*sg, bvec->bv_page, nbytes, bvec->bv_offset);
+                       (*nsegs) += 1;
+               } else
+                       (*nsegs) += blk_bvec_map_sg(q, bvec, sglist, sg);
        }
        *bvprv = *bvec;
 }