block: optimise for_each_bvec() advance

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@gmail.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/include/linux/bvec.h b/include/linux/bvec.h
index 2efec10..ff832e6 100644 (file)
--- a/include/linux/bvec.h
+++ b/include/linux/bvec.h
@@ -121,18 +121,28 @@ 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)
+/*
+ * A simpler version of bvec_iter_advance(), @bytes should not span
+ * across multiple bvec entries, i.e. bytes <= bv[i->bi_idx].bv_len
+ */
+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)                          \