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

Ming Lei
2018-11-19T07:50:57Z

On Thu, Nov 15, 2018 at 12:20:28PM -0800, Omar Sandoval wrote:
> 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.

Good point, will do in next version.

>
> > + }
> > +
> > + /* split in the middle of the bvec */
> > + if (len)
> > + need_split = true;
>
> need_split is unnecessary, just return len != 0.

OK.

>
> > +
> > + /* 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()?

Right, the check & update can be removed.

>
> >
> > - 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?

Will fix it in next version.

>
> > + 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()?

I think we can remove it too.

Thanks,
Ming