[RFC PATCH 1/3] block: add copy offload support

[Keith Busch]

On Tue, Feb 01, 2022 at 01:32:29PM -0500, Mikulas Patocka wrote:
> +int blkdev_issue_copy(struct block_device *bdev1, sector_t sector1,
> +		      struct block_device *bdev2, sector_t sector2,
> +		      sector_t nr_sects, sector_t *copied, gfp_t gfp_mask)
> +{
> +	struct page *token;
> +	sector_t m;
> +	int r = 0;
> +	struct completion comp;
> +
> +	*copied = 0;
> +
> +	m = min(bdev_max_copy_sectors(bdev1), bdev_max_copy_sectors(bdev2));
> +	if (!m)
> +		return -EOPNOTSUPP;
> +	m = min(m, (sector_t)round_down(UINT_MAX, PAGE_SIZE) >> 9);
> +
> +	if (unlikely(bdev_read_only(bdev2)))
> +		return -EPERM;
> +
> +	token = alloc_page(gfp_mask);
> +	if (unlikely(!token))
> +		return -ENOMEM;
> +
> +	while (nr_sects) {
> +		struct bio *read_bio, *write_bio;
> +		sector_t this_step = min(nr_sects, m);
> +
> +		read_bio = bio_alloc(gfp_mask, 1);
> +		if (unlikely(!read_bio)) {
> +			r = -ENOMEM;
> +			break;
> +		}
> +		bio_set_op_attrs(read_bio, REQ_OP_COPY_READ_TOKEN, REQ_NOMERGE);
> +		bio_set_dev(read_bio, bdev1);
> +		__bio_add_page(read_bio, token, PAGE_SIZE, 0);

You have this "token" payload as driver specific data, but there's no
check that bdev1 and bdev2 subscribe to the same driver specific format.

I thought we discussed defining something like a "copy domain" that
establishes which block devices can offload copy operations to/from each
other, and that should be checked before proceeding with the copy
operation.