It is more efficient to use bio_for_each_segment_mp()
for mapping sg, meantime we have to consider splitting
multipage bvec as done in blk_bio_segment_split().

Signed-off-by: Ming Lei <ming....@redhat.com>
---
 block/blk-merge.c | 72 +++++++++++++++++++++++++++++++++++++++----------------
 1 file changed, 52 insertions(+), 20 deletions(-)

diff --git a/block/blk-merge.c b/block/blk-merge.c
index c9b300f91fba..33353ed8c32e 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -439,6 +439,56 @@ static int blk_phys_contig_segment(struct request_queue 
*q, struct bio *bio,
        return 0;
 }
 
+static inline struct scatterlist *blk_next_sg(struct scatterlist **sg,
+               struct scatterlist *sglist)
+{
+       if (!*sg)
+               return sglist;
+       else {
+               /*
+                * If the driver previously mapped a shorter
+                * list, we could see a termination bit
+                * prematurely unless it fully inits the sg
+                * table on each mapping. We KNOW that there
+                * must be more entries here or the driver
+                * would be buggy, so force clear the
+                * termination bit to avoid doing a full
+                * sg_init_table() in drivers for each command.
+                */
+               sg_unmark_end(*sg);
+               return sg_next(*sg);
+       }
+}
+
+static inline unsigned
+blk_bvec_map_sg(struct request_queue *q, struct bio_vec *bvec,
+               struct scatterlist *sglist, struct scatterlist **sg)
+{
+       unsigned nbytes = bvec->bv_len;
+       unsigned nsegs = 0, total = 0;
+
+       while (nbytes > 0) {
+               unsigned seg_size;
+               struct page *pg;
+               unsigned offset, idx;
+
+               *sg = blk_next_sg(sg, sglist);
+
+               seg_size = min(nbytes, queue_max_segment_size(q));
+               offset = (total + bvec->bv_offset) % PAGE_SIZE;
+               idx = (total + bvec->bv_offset) / PAGE_SIZE;
+               pg = nth_page(bvec->bv_page, idx);
+
+               sg_set_page(*sg, pg, seg_size, offset);
+
+               total += seg_size;
+               nbytes -= seg_size;
+               nsegs++;
+       }
+
+       return nsegs;
+}
+
 static inline void
 __blk_segment_map_sg(struct request_queue *q, struct bio_vec *bvec,
                     struct scatterlist *sglist, struct bio_vec *bvprv,
@@ -472,25 +522,7 @@ __blk_segment_map_sg(struct request_queue *q, struct 
bio_vec *bvec,
                (*sg)->length += nbytes;
        } else {
 new_segment:
-               if (!*sg)
-                       *sg = sglist;
-               else {
-                       /*
-                        * If the driver previously mapped a shorter
-                        * list, we could see a termination bit
-                        * prematurely unless it fully inits the sg
-                        * table on each mapping. We KNOW that there
-                        * must be more entries here or the driver
-                        * would be buggy, so force clear the
-                        * termination bit to avoid doing a full
-                        * sg_init_table() in drivers for each command.
-                        */
-                       sg_unmark_end(*sg);
-                       *sg = sg_next(*sg);
-               }
-
-               sg_set_page(*sg, bvec->bv_page, nbytes, bvec->bv_offset);
-               (*nsegs)++;
+               (*nsegs) += blk_bvec_map_sg(q, bvec, sglist, sg);
 
                /* for making iterator happy */
                bvec->bv_offset -= advance;
@@ -516,7 +548,7 @@ static int __blk_bios_map_sg(struct request_queue *q, 
struct bio *bio,
        int cluster = blk_queue_cluster(q), nsegs = 0;
 
        for_each_bio(bio)
-               bio_for_each_segment(bvec, bio, iter)
+               bio_for_each_segment_mp(bvec, bio, iter)
                        __blk_segment_map_sg(q, &bvec, sglist, &bvprv, sg,
                                             &nsegs, &cluster);
 
-- 
2.9.4

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