Re: [PATCH V11 03/19] block: introduce bio_for_each_bvec()

Christoph Hellwig
2018-11-21T13:32:48Z

> +#define bio_iter_mp_iovec(bio, iter) \
> + segment_iter_bvec((bio)->bi_io_vec, (iter))

Besides the mp naming we'd like to get rid off there also is just
a single user of this macro, please just expand it there.

> +#define segment_iter_bvec(bvec, iter) \
> +((struct bio_vec) { \
> + .bv_page = segment_iter_page((bvec), (iter)), \
> + .bv_len = segment_iter_len((bvec), (iter)), \
> + .bv_offset = segment_iter_offset((bvec), (iter)), \
> +})

And for this one please keep the segment vs bvec versions of these
macros close together in the file please, right now it follow the
bvec_iter_bvec variant closely.

> +static inline void __bio_advance_iter(struct bio *bio, struct bvec_iter *iter,
> + unsigned bytes, unsigned max_seg_len)
> {
> iter->bi_sector += bytes >> 9;
>
> if (bio_no_advance_iter(bio))
> iter->bi_size -= bytes;
> else
> - bvec_iter_advance(bio->bi_io_vec, iter, bytes);
> + __bvec_iter_advance(bio->bi_io_vec, iter, bytes, max_seg_len);
> /* TODO: It is reasonable to complete bio with error here. */
> }
>
> +static inline void bio_advance_iter(struct bio *bio, struct bvec_iter *iter,
> + unsigned bytes)
> +{
> + __bio_advance_iter(bio, iter, bytes, PAGE_SIZE);
> +}

Btw, I think the remaining users of bio_advance_iter() in bio.h
should probably switch to using __bio_advance_iter to make them a little
more clear to read.

> +/* returns one real segment(multi-page bvec) each time */

space before the brace, please.

> +#define BVEC_MAX_LEN ((unsigned int)-1)

> while (bytes) {
> + unsigned segment_len = segment_iter_len(bv, *iter);
>
> - iter->bi_bvec_done += len;
> + if (max_seg_len < BVEC_MAX_LEN)
> + segment_len = min_t(unsigned, segment_len,
> + max_seg_len -
> + bvec_iter_offset(bv, *iter));
> +
> + segment_len = min(bytes, segment_len);

Please stick to passing the magic zero here as can often generate more
efficient code.

Talking about efficent code - I wonder how much code size we'd save
by moving this function out of line..

But while looking over this I wonder why we even need the max_seg_len
here. The only thing __bvec_iter_advance does it to move bi_bvec_done
and bi_idx forward, with corresponding decrements of bi_size. As far
as I can tell the only thing that max_seg_len does is that we need
to more iterations of the while loop to archive the same thing.

And actual bvec used by the caller will be obtained using
bvec_iter_bvec or segment_iter_bvec depending on if they want multi-page
or single-page variants.