> + bool new_bio;
>
> if (!bio)
> return 0;
> @@ -377,9 +377,10 @@ static unsigned int __blk_recalc_rq_segments(struct
> request_queue *q,
> fbio = bio;
> seg_size = 0;
> nr_phys_segs = 0;
> + new_bio = false;
I'd just initialize it to false in the declaration line.
> +/* only try to merge bvecs into one sg if they are from two bios */
> +static inline bool
> +__blk_segment_map_sg_merge(struct request_queue *q, struct bio_vec *bvec,
> + struct bio_vec *bvprv, struct scatterlist **sg)
> {
> int nbytes = bvec->bv_len;
>
> + if (!*sg)
> + return false;
>
> + if ((*sg)->length + nbytes > queue_max_segment_size(q))
> + return false;
> +
> + if (!biovec_phys_mergeable(q, bvprv, bvec))
> + return false;
> +
> + (*sg)->length += nbytes;
> +
> + return true;
> +}
> +
> +static inline void
> +__blk_segment_map_sg(struct request_queue *q, struct bio_vec *bvec,
> + struct scatterlist *sglist, struct scatterlist **sg,
> + int *nsegs)
> +{
> + if (bvec->bv_offset + bvec->bv_len <= PAGE_SIZE) {
> + *sg = blk_next_sg(sg, sglist);
> + sg_set_page(*sg, bvec->bv_page, bvec->bv_len, bvec->bv_offset);
> + (*nsegs) += 1;
This branch is basically the same as __blk_bvec_map_sg, I wonder if
we can reuse it.
> + } else
> + (*nsegs) += blk_bvec_map_sg(q, bvec, sglist, sg);
> }
And then maybe just kill off __blk_segment_map_sg entirely by moving
the if else into the caller.
> @@ -535,11 +543,24 @@ static int __blk_bios_map_sg(struct request_queue *q,
> struct bio *bio,
> struct bio_vec bvec, bvprv = { NULL };
> struct bvec_iter iter;
> int nsegs = 0;
> + bool new_bio = false;
>
> + for_each_bio(bio) {
> + bio_for_each_bvec(bvec, bio, iter) {
> + /*
> + * Only try to merge bvecs from two bios given we
> + * have done bio internal merge when adding pages
> + * to bio
> + */
> + if (!new_bio || !__blk_segment_map_sg_merge(q,
> + &bvec, &bvprv, sg))
Somewhat hard to read. Why not:
if (new_bio ||
!__blk_segment_map_sg_merge(q, &bvec, &bvprv, sg))
That being said I'd really like to see some stats on workloads that
actually trigger cross-bio segment merges. If we get a lot of those
we are doing something wrong.
