Re: [PATCH V10 03/19] block: use bio_for_each_bvec() to compute multi-page bvec count

Omar Sandoval
2018-11-15T20:20:34Z

On Thu, Nov 15, 2018 at 04:52:50PM +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.
>
> Cc: Dave Chinner <dchinner@redhat.com>
> Cc: Kent Overstreet <kent.overstreet@gmail.com>
> Cc: Mike Snitzer <snitzer@redhat.com>
> Cc: dm-devel@redhat.com
> Cc: Alexander Viro <viro@zeniv.linux.org.uk>
> Cc: linux-fsdevel@vger.kernel.org
> Cc: Shaohua Li <shli@kernel.org>
> Cc: linux-raid@vger.kernel.org
> Cc: linux-erofs@lists.ozlabs.org
> Cc: David Sterba <dsterba@suse.com>
> Cc: linux-btrfs@vger.kernel.org
> Cc: Darrick J. Wong <darrick.wong@oracle.com>
> Cc: linux-xfs@vger.kernel.org
> Cc: Gao Xiang <gaoxiang25@huawei.com>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Theodore Ts'o <tytso@mit.edu>
> Cc: linux-ext4@vger.kernel.org
> Cc: Coly Li <colyli@suse.de>
> Cc: linux-bcache@vger.kernel.org
> Cc: Boaz Harrosh <ooo@electrozaur.com>
> Cc: Bob Peterson <rpeterso@redhat.com>
> Cc: cluster-devel@redhat.com
> Signed-off-by: Ming Lei <ming.lei@redhat.com>
> ---
> block/blk-merge.c | 90 ++++++++++++++++++++++++++++++++++++++++++++++---------
> 1 file changed, 76 insertions(+), 14 deletions(-)
>
> diff --git a/block/blk-merge.c b/block/blk-merge.c
> index 91b2af332a84..6f7deb94a23f 100644
> --- a/block/blk-merge.c
> +++ b/block/blk-merge.c
> @@ -160,6 +160,62 @@ static inline unsigned get_max_io_size(struct request_queue *q,
> return sectors;
> }
>
> +/*
> + * 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)
> +{
> + bool need_split = false;
> + unsigned len = bv->bv_len;
> + unsigned total_len = 0;
> + unsigned new_nsegs = 0, seg_size = 0;

"unsigned int" here and everywhere else.

> + if ((*nsegs >= queue_max_segments(q)) || !len)
> + return need_split;
> +
> + /*
> + * Multipage bvec may be too big to hold in one segment,
> + * so the current bvec has to be splitted as multiple
> + * segments.
> + */
> + while (new_nsegs + *nsegs < queue_max_segments(q)) {
> + seg_size = min(queue_max_segment_size(q), len);
> +
> + new_nsegs++;
> + total_len += seg_size;
> + len -= seg_size;
> +
> + if ((queue_virt_boundary(q) && ((bv->bv_offset +
> + total_len) & queue_virt_boundary(q))) || !len)
> + break;

Checking queue_virt_boundary(q) != 0 is superfluous, and the len check
could just control the loop, i.e.,

while (len && new_nsegs + *nsegs < queue_max_segments(q)) {
seg_size = min(queue_max_segment_size(q), len);

new_nsegs++;
total_len += seg_size;
len -= seg_size;

if ((bv->bv_offset + total_len) & queue_virt_boundary(q))
break;
}

And if you rewrite it this way, I _think_ you can get rid of this
special case:

if ((*nsegs >= queue_max_segments(q)) || !len)
return need_split;

above.

> + }
> +
> + /* split in the middle of the bvec */
> + if (len)
> + need_split = true;

need_split is unnecessary, just return len != 0.

> +
> + /* update front segment size */
> + if (!*nsegs) {
> + unsigned first_seg_size = seg_size;
> +
> + if (new_nsegs > 1)
> + 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;
> +
> + return need_split;
> +}
> +
> static struct bio *blk_bio_segment_split(struct request_queue *q,
> struct bio *bio,
> struct bio_set *bs,
> @@ -173,7 +229,7 @@ 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.
> @@ -188,8 +244,12 @@ 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;
> }
> @@ -214,11 +274,12 @@ static struct bio *blk_bio_segment_split(struct request_queue *q,
> if (nsegs == 1 && seg_size > front_seg_size)
> front_seg_size = seg_size;

Hm, do we still need to check this here now that we're updating
front_seg_size inside of bvec_split_segs()?

>
> - 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))

What happened to the indent alignment here?

> + goto split;
>
> }
>
> @@ -296,6 +357,7 @@ static unsigned int __blk_recalc_rq_segments(struct request_queue *q,
> struct bio_vec bv, bvprv = { NULL };
> int cluster, prev = 0;
> unsigned int seg_size, nr_phys_segs;
> + unsigned front_seg_size = bio->bi_seg_front_size;
> struct bio *fbio, *bbio;
> struct bvec_iter iter;
>
> @@ -316,7 +378,7 @@ 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 +398,20 @@ 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;
> + if (nr_phys_segs == 1 && seg_size > front_seg_size)
> + front_seg_size = seg_size;

Same comment as in blk_bio_segment_split(), do we still need to check
this if we're updating front_seg_size in bvec_split_segs()?

>
> - nr_phys_segs++;
> bvprv = bv;
> prev = 1;
> - 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;
> + if (nr_phys_segs == 1 && seg_size > front_seg_size)
> + front_seg_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
>