Re: [RFC 1/3] block: Introduce blk_bio_map_sg() to map one bio

[Ming Lei <ming.lei@canonical.com>]

On Wed, May 25, 2016 at 2:12 PM, Baolin Wang <baolin.wang@linaro.org> wrote:
> In dm-crypt, it need to map one bio to scatterlist for improving the
> hardware engine encryption efficiency. Thus this patch introduces the
> blk_bio_map_sg() function to map one bio with scatterlists.
>
> Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
> ---
>  block/blk-merge.c      |   45 +++++++++++++++++++++++++++++++++++++++++++++
>  include/linux/blkdev.h |    3 +++
>  2 files changed, 48 insertions(+)
>
> diff --git a/block/blk-merge.c b/block/blk-merge.c
> index 2613531..9b92af4 100644
> --- a/block/blk-merge.c
> +++ b/block/blk-merge.c
> @@ -417,6 +417,51 @@ single_segment:
>  }
>
>  /*
> + * map a bio to scatterlist, return number of sg entries setup.
> + */
> +int blk_bio_map_sg(struct request_queue *q, struct bio *bio,
> +                  struct scatterlist *sglist,
> +                  struct scatterlist **sg)
> +{
> +       struct bio_vec bvec, bvprv = { NULL };
> +       struct bvec_iter iter;
> +       int nsegs, cluster;
> +
> +       nsegs = 0;
> +       cluster = blk_queue_cluster(q);
> +
> +       if (bio->bi_rw & REQ_DISCARD) {
> +               /*
> +                * This is a hack - drivers should be neither modifying the
> +                * biovec, nor relying on bi_vcnt - but because of
> +                * blk_add_request_payload(), a discard bio may or may not have
> +                * a payload we need to set up here (thank you Christoph) and
> +                * bi_vcnt is really the only way of telling if we need to.
> +                */
> +
> +               if (bio->bi_vcnt)
> +                       goto single_segment;
> +
> +               return 0;
> +       }
> +
> +       if (bio->bi_rw & REQ_WRITE_SAME) {
> +single_segment:
> +               *sg = sglist;
> +               bvec = bio_iovec(bio);
> +               sg_set_page(*sg, bvec.bv_page, bvec.bv_len, bvec.bv_offset);
> +               return 1;
> +       }
> +
> +       bio_for_each_segment(bvec, bio, iter)
> +               __blk_segment_map_sg(q, &bvec, sglist, &bvprv, sg,
> +                                    &nsegs, &cluster);
> +
> +       return nsegs;
> +}
> +EXPORT_SYMBOL(blk_bio_map_sg);

You can use __blk_bios_map_sg() to implement blk_bio_map_sg(),
then code duplication may be avoided.

> +
> +/*
>   * map a request to scatterlist, return number of sg entries setup. Caller
>   * must make sure sg can hold rq->nr_phys_segments entries
>   */
> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> index 1fd8fdf..e5de4f8 100644
> --- a/include/linux/blkdev.h
> +++ b/include/linux/blkdev.h
> @@ -1013,6 +1013,9 @@ extern void blk_queue_write_cache(struct request_queue *q, bool enabled, bool fu
>  extern struct backing_dev_info *blk_get_backing_dev_info(struct block_device *bdev);
>
>  extern int blk_rq_map_sg(struct request_queue *, struct request *, struct scatterlist *);
> +extern int blk_bio_map_sg(struct request_queue *q, struct bio *bio,
> +                         struct scatterlist *sglist,
> +                         struct scatterlist **sg);
>  extern void blk_dump_rq_flags(struct request *, char *);
>  extern long nr_blockdev_pages(void);
>
> --
> 1.7.9.5
>