Re: [PATCH] block: advance by bvec's length for bio_for_each_bvec

[Christoph Hellwig]

On Thu, Feb 28, 2019 at 11:24:21AM +0800, Ming Lei wrote:
> bio_for_each_bvec is used in fast path of bio splitting and sg mapping,
> and what we want to do is to iterate over multi-page bvecs, instead of pages.
> However, bvec_iter_advance() is invisble for this requirement, and
> always advance by page size.
> 
> This way isn't efficient for multipage bvec iterator, also bvec_iter_len()
> isn't as fast as mp_bvec_iter_len().
> 
> So advance by multi-page bvec's length instead of page size for 
> bio_for_each_bvec().
> 
> More than 1% IOPS improvement can be observed in io_uring test on null_blk.

We've been there before, and I still insist that there is not good
reason ever to clamp the iteration to page size in bvec_iter_advance.
Callers that iterate over it already do that in the callers.

So here is a resurretion and rebase of my patch from back then to
just do the right thing:

diff --git a/include/linux/bvec.h b/include/linux/bvec.h
index 2c32e3e151a0..cf06c0647c4f 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++;
                }