block: introduce bvec_nth_page()

Single-page bvec can often be seen in small BS workloads, so
introduce bvec_nth_page() for avoiding to call nth_page() unnecessarily,
which looks not cheap.

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

2 files changed, 9 insertions, 4 deletions

diff --git a/block/blk-merge.c b/block/blk-merge.c
index 066b66430523..c7e8a8273460 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -483,7 +483,7 @@ static unsigned blk_bvec_map_sg(struct request_queue *q,
 
                 offset = (total + bvec->bv_offset) % PAGE_SIZE;
                 idx = (total + bvec->bv_offset) / PAGE_SIZE;
-                pg = nth_page(bvec->bv_page, idx);
+                pg = bvec_nth_page(bvec->bv_page, idx);
 
                 sg_set_page(*sg, pg, seg_size, offset);
 
diff --git a/include/linux/bvec.h b/include/linux/bvec.h
index 30a57b68d017..4376f683c08a 100644
--- a/include/linux/bvec.h
+++ b/include/linux/bvec.h
@@ -51,6 +51,11 @@ struct bvec_iter_all {
         unsigned        done;
 };
 
+static inline struct page *bvec_nth_page(struct page *page, int idx)
+{
+        return idx == 0 ? page : nth_page(page, idx);
+}
+
 /*
  * various member access, note that bio_data should of course not be used
  * on highmem page vectors
@@ -87,8 +92,8 @@ struct bvec_iter_all {
               PAGE_SIZE - bvec_iter_offset((bvec), (iter)))
 
 #define bvec_iter_page(bvec, iter)                              \
-        nth_page(mp_bvec_iter_page((bvec), (iter)),             \
-                 mp_bvec_iter_page_idx((bvec), (iter)))
+        bvec_nth_page(mp_bvec_iter_page((bvec), (iter)),                \
+                      mp_bvec_iter_page_idx((bvec), (iter)))
 
 #define bvec_iter_bvec(bvec, iter)                              \
 ((struct bio_vec) {                                             \
@@ -171,7 +176,7 @@ static inline void mp_bvec_last_segment(const struct bio_vec *bvec,
         unsigned total = bvec->bv_offset + bvec->bv_len;
         unsigned last_page = (total - 1) / PAGE_SIZE;
 
-        seg->bv_page = nth_page(bvec->bv_page, last_page);
+        seg->bv_page = bvec_nth_page(bvec->bv_page, last_page);
 
         /* the whole segment is inside the last page */
         if (bvec->bv_offset >= last_page * PAGE_SIZE) {