block: optimize bvec iteration in bvec_iter_advance

There is no need to only iterate in chunks of PAGE_SIZE or less in
bvec_iter_advance, given that the callers pass in the chunk length that
they are operating on - either that already is less than PAGE_SIZE
because they do classic page-based iteration, or it is larger because
the caller operates on multi-page bvecs.

This should help shaving off a few cycles of the I/O hot path.

Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 4 insertions, 3 deletions

diff --git a/include/linux/bvec.h b/include/linux/bvec.h
index 87e82e503a52..f6275c4da13a 100644
--- a/include/linux/bvec.h
+++ b/include/linux/bvec.h
@@ -112,14 +112,15 @@ static inline bool bvec_iter_advance(const struct bio_vec *bv,
         }
 
         while (bytes) {
-                unsigned iter_len = bvec_iter_len(bv, *iter);
-                unsigned len = min(bytes, iter_len);
+                const struct bio_vec *cur = bv + iter->bi_idx;
+                unsigned len = min3(bytes, iter->bi_size,
+                                    cur->bv_len - iter->bi_bvec_done);
 
                 bytes -= len;
                 iter->bi_size -= len;
                 iter->bi_bvec_done += len;
 
-                if (iter->bi_bvec_done == __bvec_iter_bvec(bv, *iter)->bv_len) {
+                if (iter->bi_bvec_done == cur->bv_len) {
                         iter->bi_bvec_done = 0;
                         iter->bi_idx++;
                 }