Re: [PATCH V12 09/20] block: use bio_for_each_bvec() to compute multi-page bvec count

From: Omar Sandoval
Date: Mon Nov 26 2018 - 17:37:17 EST

On Mon, Nov 26, 2018 at 10:17:09AM +0800, Ming Lei wrote:
> First it is more efficient to use bio_for_each_bvec() in both
> blk_bio_segment_split() and __blk_recalc_rq_segments() to compute how
> many multi-page bvecs there are in the bio.
>
> Secondly once bio_for_each_bvec() is used, the bvec may need to be
> splitted because its length can be very longer than max segment size,
> so we have to split the big bvec into several segments.
>
> Thirdly when splitting multi-page bvec into segments, the max segment
> limit may be reached, so the bio split need to be considered under
> this situation too.

Reviewed-by: Omar Sandoval <osandov@xxxxxx>

> Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
> ---
> block/blk-merge.c | 100 +++++++++++++++++++++++++++++++++++++++++++-----------
> 1 file changed, 80 insertions(+), 20 deletions(-)
>
> diff --git a/block/blk-merge.c b/block/blk-merge.c
> index 51ec6ca56a0a..2d8f388d43de 100644
> --- a/block/blk-merge.c
> +++ b/block/blk-merge.c
> @@ -161,6 +161,70 @@ static inline unsigned get_max_io_size(struct request_queue *q,
> return sectors;
> }
>
> +static unsigned get_max_segment_size(struct request_queue *q,
> + unsigned offset)
> +{
> + unsigned long mask = queue_segment_boundary(q);
> +
> + return min_t(unsigned long, mask - (mask & offset) + 1,
> + queue_max_segment_size(q));
> +}
> +
> +/*
> + * Split the bvec @bv into segments, and update all kinds of
> + * variables.
> + */
> +static bool bvec_split_segs(struct request_queue *q, struct bio_vec *bv,
> + unsigned *nsegs, unsigned *last_seg_size,
> + unsigned *front_seg_size, unsigned *sectors)
> +{
> + unsigned len = bv->bv_len;
> + unsigned total_len = 0;
> + unsigned new_nsegs = 0, seg_size = 0;
> +
> + /*
> + * Multipage bvec may be too big to hold in one segment,
> + * so the current bvec has to be splitted as multiple
> + * segments.
> + */
> + while (len && new_nsegs + *nsegs < queue_max_segments(q)) {
> + seg_size = get_max_segment_size(q, bv->bv_offset + total_len);
> + seg_size = min(seg_size, len);
> +
> + new_nsegs++;
> + total_len += seg_size;
> + len -= seg_size;
> +
> + if ((bv->bv_offset + total_len) & queue_virt_boundary(q))
> + break;
> + }
> +
> + if (!new_nsegs)
> + return !!len;
> +
> + /* update front segment size */
> + if (!*nsegs) {
> + unsigned first_seg_size;
> +
> + if (new_nsegs == 1)
> + first_seg_size = get_max_segment_size(q, bv->bv_offset);
> + else
> + first_seg_size = queue_max_segment_size(q);
> +
> + if (*front_seg_size < first_seg_size)
> + *front_seg_size = first_seg_size;
> + }
> +
> + /* update other varibles */
> + *last_seg_size = seg_size;
> + *nsegs += new_nsegs;
> + if (sectors)
> + *sectors += total_len >> 9;
> +
> + /* split in the middle of the bvec if len != 0 */
> + return !!len;
> +}
> +
> static struct bio *blk_bio_segment_split(struct request_queue *q,
> struct bio *bio,
> struct bio_set *bs,
> @@ -174,7 +238,7 @@ static struct bio *blk_bio_segment_split(struct request_queue *q,
> struct bio *new = NULL;
> const unsigned max_sectors = get_max_io_size(q, bio);
>
> - bio_for_each_segment(bv, bio, iter) {
> + bio_for_each_bvec(bv, bio, iter) {
> /*
> * If the queue doesn't support SG gaps and adding this
> * offset would create a gap, disallow it.
> @@ -189,8 +253,12 @@ static struct bio *blk_bio_segment_split(struct request_queue *q,
> */
> if (nsegs < queue_max_segments(q) &&
> sectors < max_sectors) {
> - nsegs++;
> - sectors = max_sectors;
> + /* split in the middle of bvec */
> + bv.bv_len = (max_sectors - sectors) << 9;
> + bvec_split_segs(q, &bv, &nsegs,
> + &seg_size,
> + &front_seg_size,
> + &sectors);
> }
> goto split;
> }
> @@ -212,14 +280,12 @@ static struct bio *blk_bio_segment_split(struct request_queue *q,
> if (nsegs == queue_max_segments(q))
> goto split;
>
> - if (nsegs == 1 && seg_size > front_seg_size)
> - front_seg_size = seg_size;
> -
> - nsegs++;
> bvprv = bv;
> bvprvp = &bvprv;
> - seg_size = bv.bv_len;
> - sectors += bv.bv_len >> 9;
> +
> + if (bvec_split_segs(q, &bv, &nsegs, &seg_size,
> + &front_seg_size, &sectors))
> + goto split;
>
> }
>
> @@ -233,8 +299,6 @@ static struct bio *blk_bio_segment_split(struct request_queue *q,
> bio = new;
> }
>
> - if (nsegs == 1 && seg_size > front_seg_size)
> - front_seg_size = seg_size;
> bio->bi_seg_front_size = front_seg_size;
> if (seg_size > bio->bi_seg_back_size)
> bio->bi_seg_back_size = seg_size;
> @@ -296,6 +360,7 @@ static unsigned int __blk_recalc_rq_segments(struct request_queue *q,
> {
> struct bio_vec bv, bvprv = { NULL };
> unsigned int seg_size, nr_phys_segs;
> + unsigned front_seg_size = bio->bi_seg_front_size;
> struct bio *fbio, *bbio;
> struct bvec_iter iter;
> bool prev = false;
> @@ -316,7 +381,7 @@ static unsigned int __blk_recalc_rq_segments(struct request_queue *q,
> seg_size = 0;
> nr_phys_segs = 0;
> for_each_bio(bio) {
> - bio_for_each_segment(bv, bio, iter) {
> + bio_for_each_bvec(bv, bio, iter) {
> /*
> * If SG merging is disabled, each bio vector is
> * a segment
> @@ -336,20 +401,15 @@ static unsigned int __blk_recalc_rq_segments(struct request_queue *q,
> continue;
> }
> new_segment:
> - if (nr_phys_segs == 1 && seg_size >
> - fbio->bi_seg_front_size)
> - fbio->bi_seg_front_size = seg_size;
> -
> - nr_phys_segs++;
> bvprv = bv;
> prev = true;
> - seg_size = bv.bv_len;
> + bvec_split_segs(q, &bv, &nr_phys_segs, &seg_size,
> + &front_seg_size, NULL);
> }
> bbio = bio;
> }
>
> - if (nr_phys_segs == 1 && seg_size > fbio->bi_seg_front_size)
> - fbio->bi_seg_front_size = seg_size;
> + fbio->bi_seg_front_size = front_seg_size;
> if (seg_size > bbio->bi_seg_back_size)
> bbio->bi_seg_back_size = seg_size;
>
> --
> 2.9.5
>