Re: [PATCH V10 04/19] block: use bio_for_each_bvec() to map sg

Omar Sandoval
2018-11-15T22:34:07Z

On Thu, Nov 15, 2018 at 04:52:51PM +0800, Ming Lei wrote:
> It is more efficient to use bio_for_each_bvec() to map sg, meantime
> we have to consider splitting multipage bvec as done in blk_bio_segment_split().
>
> 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

Reviewed-by: Omar Sandoval <osandov@fb.com>

> Signed-off-by: Ming Lei <ming.lei@redhat.com>
> ---
> block/blk-merge.c | 72 +++++++++++++++++++++++++++++++++++++++----------------
> 1 file changed, 52 insertions(+), 20 deletions(-)
>
> diff --git a/block/blk-merge.c b/block/blk-merge.c
> index 6f7deb94a23f..cb9f49bcfd36 100644
> --- a/block/blk-merge.c
> +++ b/block/blk-merge.c
> @@ -473,6 +473,56 @@ static int blk_phys_contig_segment(struct request_queue *q, struct bio *bio,
> return biovec_phys_mergeable(q, &end_bv, &nxt_bv);
> }
>
> +static struct scatterlist *blk_next_sg(struct scatterlist **sg,
> + struct scatterlist *sglist)
> +{
> + if (!*sg)
> + return sglist;
> + else {
> + /*
> + * If the driver previously mapped a shorter
> + * list, we could see a termination bit
> + * prematurely unless it fully inits the sg
> + * table on each mapping. We KNOW that there
> + * must be more entries here or the driver
> + * would be buggy, so force clear the
> + * termination bit to avoid doing a full
> + * sg_init_table() in drivers for each command.
> + */
> + sg_unmark_end(*sg);
> + return sg_next(*sg);
> + }
> +}
> +
> +static unsigned blk_bvec_map_sg(struct request_queue *q,
> + struct bio_vec *bvec, struct scatterlist *sglist,
> + struct scatterlist **sg)
> +{
> + unsigned nbytes = bvec->bv_len;
> + unsigned nsegs = 0, total = 0;
> +
> + while (nbytes > 0) {
> + unsigned seg_size;
> + struct page *pg;
> + unsigned offset, idx;
> +
> + *sg = blk_next_sg(sg, sglist);
> +
> + seg_size = min(nbytes, queue_max_segment_size(q));
> + offset = (total + bvec->bv_offset) % PAGE_SIZE;
> + idx = (total + bvec->bv_offset) / PAGE_SIZE;
> + pg = nth_page(bvec->bv_page, idx);
> +
> + sg_set_page(*sg, pg, seg_size, offset);
> +
> + total += seg_size;
> + nbytes -= seg_size;
> + nsegs++;
> + }
> +
> + return nsegs;
> +}
> +
> static inline void
> __blk_segment_map_sg(struct request_queue *q, struct bio_vec *bvec,
> struct scatterlist *sglist, struct bio_vec *bvprv,
> @@ -490,25 +540,7 @@ __blk_segment_map_sg(struct request_queue *q, struct bio_vec *bvec,
> (*sg)->length += nbytes;
> } else {
> new_segment:
> - if (!*sg)
> - *sg = sglist;
> - else {
> - /*
> - * If the driver previously mapped a shorter
> - * list, we could see a termination bit
> - * prematurely unless it fully inits the sg
> - * table on each mapping. We KNOW that there
> - * must be more entries here or the driver
> - * would be buggy, so force clear the
> - * termination bit to avoid doing a full
> - * sg_init_table() in drivers for each command.
> - */
> - sg_unmark_end(*sg);
> - *sg = sg_next(*sg);
> - }
> -
> - sg_set_page(*sg, bvec->bv_page, nbytes, bvec->bv_offset);
> - (*nsegs)++;
> + (*nsegs) += blk_bvec_map_sg(q, bvec, sglist, sg);
> }
> *bvprv = *bvec;
> }
> @@ -530,7 +562,7 @@ static int __blk_bios_map_sg(struct request_queue *q, struct bio *bio,
> int cluster = blk_queue_cluster(q), nsegs = 0;
>
> for_each_bio(bio)
> - bio_for_each_segment(bvec, bio, iter)
> + bio_for_each_bvec(bvec, bio, iter)
> __blk_segment_map_sg(q, &bvec, sglist, &bvprv, sg,
> &nsegs, &cluster);
>
> --
> 2.9.5
>