Re: [PATCH V10 02/19] block: introduce bio_for_each_bvec()

Ming Lei
2018-11-19T03:31:45Z

On Fri, Nov 16, 2018 at 02:30:28PM +0100, Christoph Hellwig wrote:
> > +static inline void __bio_advance_iter(struct bio *bio, struct bvec_iter *iter,
> > + unsigned bytes, bool mp)
>
> I think these magic 'bool np' arguments and wrappers over wrapper
> don't help anyone to actually understand the code. I'd vote for
> removing as many wrappers as we really don't need, and passing the
> actual segment limit instead of the magic bool flag. Something like
> this untested patch:

I think this way is fine, just a little comment.

>
> diff --git a/include/linux/bio.h b/include/linux/bio.h
> index 277921ad42e7..dcad0b69f57a 100644
> --- a/include/linux/bio.h
> +++ b/include/linux/bio.h
> @@ -138,30 +138,21 @@ static inline bool bio_full(struct bio *bio)
> bvec_for_each_segment(bvl, &((bio)->bi_io_vec[iter_all.idx]), i, iter_all)
>
> static inline void __bio_advance_iter(struct bio *bio, struct bvec_iter *iter,
> - unsigned bytes, bool mp)
> + unsigned bytes, unsigned max_segment)

The new parameter should have been named as 'max_segment_len' or
'max_seg_len'.

> {
> iter->bi_sector += bytes >> 9;
>
> if (bio_no_advance_iter(bio))
> iter->bi_size -= bytes;
> else
> - if (!mp)
> - bvec_iter_advance(bio->bi_io_vec, iter, bytes);
> - else
> - mp_bvec_iter_advance(bio->bi_io_vec, iter, bytes);
> + __bvec_iter_advance(bio->bi_io_vec, iter, bytes, max_segment);
> /* 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, false);
> -}
> -
> -static inline void bio_advance_mp_iter(struct bio *bio, struct bvec_iter *iter,
> - unsigned bytes)
> -{
> - __bio_advance_iter(bio, iter, bytes, true);
> + __bio_advance_iter(bio, iter, bytes, PAGE_SIZE);
> }
>
> #define __bio_for_each_segment(bvl, bio, iter, start) \
> @@ -177,7 +168,7 @@ static inline void bio_advance_mp_iter(struct bio *bio, struct bvec_iter *iter,
> for (iter = (start); \
> (iter).bi_size && \
> ((bvl = bio_iter_mp_iovec((bio), (iter))), 1); \
> - bio_advance_mp_iter((bio), &(iter), (bvl).bv_len))
> + __bio_advance_iter((bio), &(iter), (bvl).bv_len, 0))

Even we might pass '-1' for multi-page segment.

>
> /* returns one real segment(multipage bvec) each time */
> #define bio_for_each_bvec(bvl, bio, iter) \
> diff --git a/include/linux/bvec.h b/include/linux/bvec.h
> index 02f26d2b59ad..5e2ed46c1c88 100644
> --- a/include/linux/bvec.h
> +++ b/include/linux/bvec.h
> @@ -138,8 +138,7 @@ struct bvec_iter_all {
> })
>
> static inline bool __bvec_iter_advance(const struct bio_vec *bv,
> - struct bvec_iter *iter,
> - unsigned bytes, bool mp)
> + struct bvec_iter *iter, unsigned bytes, unsigned max_segment)
> {
> if (WARN_ONCE(bytes > iter->bi_size,
> "Attempted to advance past end of bvec iter\n")) {
> @@ -148,18 +147,18 @@ static inline bool __bvec_iter_advance(const struct bio_vec *bv,
> }
>
> while (bytes) {
> - unsigned len;
> + unsigned segment_len = mp_bvec_iter_len(bv, *iter);
>
> - if (mp)
> - len = mp_bvec_iter_len(bv, *iter);
> - else
> - len = bvec_iter_len(bv, *iter);
> + if (max_segment) {
> + max_segment -= bvec_iter_offset(bv, *iter);
> + segment_len = min(segment_len, max_segment);

Looks 'max_segment' needs to be constant, shouldn't be updated.

If '-1' is passed for multipage case, the above change may become:

segment_len = min_t(segment_len, max_seg_len - bvec_iter_offset(bv, *iter));

This way is more clean, but with extra cost of the above line for multipage
case.

Thanks,
Ming