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

Ming Lei
2018-11-22T10:16:03Z

On Wed, Nov 21, 2018 at 06:12:17PM +0100, Christoph Hellwig wrote:
> On Wed, Nov 21, 2018 at 05:10:25PM +0100, Christoph Hellwig wrote:
> > No - I think we can always use the code without any segment in
> > bvec_iter_advance. Because bvec_iter_advance only operates on the
> > iteractor, the generation of an actual single-page or multi-page
> > bvec is left to the caller using the bvec_iter_bvec or segment_iter_bvec
> > helpers. The only difference is how many bytes you can move the
> > iterator forward in a single loop iteration - so if you pass in
> > PAGE_SIZE as the max_seg_len you just will have to loop more often
> > for a large enough bytes, but not actually do anything different.
>
> FYI, this patch reverts the max_seg_len related changes back to where
> we are in mainline, and as expected everything works fine for me:
>
> diff --git a/include/linux/bio.h b/include/linux/bio.h
> index e5b975fa0558..926550ce2d21 100644
> --- a/include/linux/bio.h
> +++ b/include/linux/bio.h
> @@ -137,24 +137,18 @@ static inline bool bio_full(struct bio *bio)
> for (i = 0, iter_all.idx = 0; iter_all.idx < (bio)->bi_vcnt; iter_all.idx++) \
> 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, unsigned max_seg_len)
> +static inline void bio_advance_iter(struct bio *bio, struct bvec_iter *iter,
> + unsigned bytes)
> {
> 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, max_seg_len);
> + bvec_iter_advance(bio->bi_io_vec, iter, bytes);
> /* 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);
> -}
> -
> #define __bio_for_each_segment(bvl, bio, iter, start) \
> for (iter = (start); \
> (iter).bi_size && \
> @@ -168,7 +162,7 @@ static inline void bio_advance_iter(struct bio *bio, struct bvec_iter *iter,
> for (iter = (start); \
> (iter).bi_size && \
> ((bvl = bio_iter_mp_iovec((bio), (iter))), 1); \
> - __bio_advance_iter((bio), &(iter), (bvl).bv_len, BVEC_MAX_LEN))
> + bio_advance_iter((bio), &(iter), (bvl).bv_len))
>
> /* returns one real segment(multi-page bvec) each time */
> #define bio_for_each_bvec(bvl, bio, iter) \
> diff --git a/include/linux/bvec.h b/include/linux/bvec.h
> index cab36d838ed0..138b4007b8f2 100644
> --- a/include/linux/bvec.h
> +++ b/include/linux/bvec.h
> @@ -25,8 +25,6 @@
> #include <linux/errno.h>
> #include <linux/mm.h>
>
> -#define BVEC_MAX_LEN ((unsigned int)-1)
> -
> /*
> * was unsigned short, but we might as well be ready for > 64kB I/O pages
> */
> @@ -102,8 +100,8 @@ struct bvec_iter_all {
> .bv_offset = segment_iter_offset((bvec), (iter)), \
> })
>
> -static inline bool __bvec_iter_advance(const struct bio_vec *bv,
> - struct bvec_iter *iter, unsigned bytes, unsigned max_seg_len)
> +static inline bool bvec_iter_advance(const struct bio_vec *bv,
> + struct bvec_iter *iter, unsigned bytes)
> {
> if (WARN_ONCE(bytes > iter->bi_size,
> "Attempted to advance past end of bvec iter\n")) {
> @@ -112,18 +110,12 @@ static inline bool __bvec_iter_advance(const struct bio_vec *bv,
> }
>
> while (bytes) {
> - unsigned segment_len = segment_iter_len(bv, *iter);
> -
> - if (max_seg_len < BVEC_MAX_LEN)
> - segment_len = min_t(unsigned, segment_len,
> - max_seg_len -
> - bvec_iter_offset(bv, *iter));
> + unsigned iter_len = bvec_iter_len(bv, *iter);
> + unsigned len = min(bytes, iter_len);

It may not work to always use bvec_iter_len() here, and 'segment_len'
should be max length of the passed 'bv', however we don't know if it is
single-page or mutli-page bvec if no one tells us.

Thanks,
Ming