[PATCH 5.11] block: optimise for_each_bvec() advance

[Pavel Begunkov]

Because of how for_each_bvec() works it never advances across multiple
entries at a time, so bvec_iter_advance() is an overkill. Add
specialised bvec_iter_advance_single() that is faster. It also handles
zero-len bvecs, so can kill bvec_iter_skip_zero_bvec().

   text    data     bss     dec     hex filename
before:
  23977     805       0   24782    60ce lib/iov_iter.o
before, bvec_iter_advance() w/o WARN_ONCE()
  22886     600       0   23486    5bbe ./lib/iov_iter.o
after:
  21862     600       0   22462    57be lib/iov_iter.o

Signed-off-by: Pavel Begunkov <asml.silence@xxxxxxxxx>
---
 include/linux/bvec.h | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/include/linux/bvec.h b/include/linux/bvec.h
index 2efec10bf792..4a304dfafa18 100644
--- a/include/linux/bvec.h
+++ b/include/linux/bvec.h
@@ -121,18 +121,24 @@ static inline bool bvec_iter_advance(const struct bio_vec *bv,
 	return true;
 }
 
-static inline void bvec_iter_skip_zero_bvec(struct bvec_iter *iter)
+static inline void bvec_iter_advance_single(const struct bio_vec *bv,
+				struct bvec_iter *iter, unsigned int bytes)
 {
-	iter->bi_bvec_done = 0;
-	iter->bi_idx++;
+	unsigned int done = iter->bi_bvec_done + bytes;
+
+	if (done == bv[iter->bi_idx].bv_len) {
+		done = 0;
+		iter->bi_idx++;
+	}
+	iter->bi_bvec_done = done;
+	iter->bi_size -= bytes;
 }
 
 #define for_each_bvec(bvl, bio_vec, iter, start)			\
 	for (iter = (start);						\
 	     (iter).bi_size &&						\
 		((bvl = bvec_iter_bvec((bio_vec), (iter))), 1);	\
-	     (bvl).bv_len ? (void)bvec_iter_advance((bio_vec), &(iter),	\
-		     (bvl).bv_len) : bvec_iter_skip_zero_bvec(&(iter)))
+	     bvec_iter_advance_single((bio_vec), &(iter), (bvl).bv_len))
 
 /* for iterating one bio from start to end */
 #define BVEC_ITER_ALL_INIT (struct bvec_iter)				\
-- 
2.24.0