[dm-devel] [RFC PATCH v3 1/2] block: add simple copy support

[Selva Jove]

On Fri, Dec 11, 2020 at 11:35 PM Keith Busch <kbusch at kernel.org> wrote:
>
> On Fri, Dec 11, 2020 at 07:21:38PM +0530, SelvaKumar S wrote:
> > +int blk_copy_emulate(struct block_device *bdev, struct blk_copy_payload *payload,
> > +             gfp_t gfp_mask)
> > +{
> > +     struct request_queue *q = bdev_get_queue(bdev);
> > +     struct bio *bio;
> > +     void *buf = NULL;
> > +     int i, nr_srcs, max_range_len, ret, cur_dest, cur_size;
> > +
> > +     nr_srcs = payload->copy_range;
> > +     max_range_len = q->limits.max_copy_range_sectors << SECTOR_SHIFT;
>
> The default value for this limit is 0, and this is the function for when
> the device doesn't support copy. Are we expecting drivers to set this
> value to something else for that case?

Sorry. Missed that. Will add a fix.

>
> > +     cur_dest = payload->dest;
> > +     buf = kvmalloc(max_range_len, GFP_ATOMIC);
> > +     if (!buf)
> > +             return -ENOMEM;
> > +
> > +     for (i = 0; i < nr_srcs; i++) {
> > +             bio = bio_alloc(gfp_mask, 1);
> > +             bio->bi_iter.bi_sector = payload->range[i].src;
> > +             bio->bi_opf = REQ_OP_READ;
> > +             bio_set_dev(bio, bdev);
> > +
> > +             cur_size = payload->range[i].len << SECTOR_SHIFT;
> > +             ret = bio_add_page(bio, virt_to_page(buf), cur_size,
> > +                                                offset_in_page(payload));
>
> 'buf' is vmalloc'ed, so we don't necessarily have congituous pages. I
> think you need to allocate the bio with bio_map_kern() or something like
> that instead with that kind of memory.
>

Sure. Will use bio_map_kern().

> > +             if (ret != cur_size) {
> > +                     ret = -ENOMEM;
> > +                     goto out;
> > +             }
> > +
> > +             ret = submit_bio_wait(bio);
> > +             bio_put(bio);
> > +             if (ret)
> > +                     goto out;
> > +
> > +             bio = bio_alloc(gfp_mask, 1);
> > +             bio_set_dev(bio, bdev);
> > +             bio->bi_opf = REQ_OP_WRITE;
> > +             bio->bi_iter.bi_sector = cur_dest;
> > +             ret = bio_add_page(bio, virt_to_page(buf), cur_size,
> > +                                                offset_in_page(payload));
> > +             if (ret != cur_size) {
> > +                     ret = -ENOMEM;
> > +                     goto out;
> > +             }
> > +
> > +             ret = submit_bio_wait(bio);
> > +             bio_put(bio);
> > +             if (ret)
> > +                     goto out;
> > +
> > +             cur_dest += payload->range[i].len;
> > +     }
>
> I think this would be a faster implementation if the reads were
> asynchronous with a payload buffer allocated specific to that read, and
> the callback can enqueue the write part. This would allow you to
> accumulate all the read data and write it in a single call.

Sounds like a better approach. Will add this implementation in v4.