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>

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);
 

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) {