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

[Bart Van Assche]

On 2/1/22 10:32, Mikulas Patocka wrote:
>   /**
> + * blk_queue_max_copy_sectors - set maximum copy offload sectors for the queue
> + * @q:  the request queue for the device
> + * @size:  the maximum copy offload sectors
> + */
> +void blk_queue_max_copy_sectors(struct request_queue *q, unsigned int size)
> +{
> +	q->limits.max_copy_sectors = size;
> +}
> +EXPORT_SYMBOL_GPL(blk_queue_max_copy_sectors);

Please either change the unit of 'size' into bytes or change its type 
into sector_t.

> +extern 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);
> +

Only supporting copying between contiguous LBA ranges seems restrictive 
to me. I expect garbage collection by filesystems for UFS devices to 
perform better if multiple LBA ranges are submitted as a single SCSI 
XCOPY command.

A general comment about the approach: encoding the LBA range information 
in a bio payload is not compatible with bio splitting. How can the dm 
driver implement copy offloading without the ability to split copy 
offload bio's?

> +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;

Consider using DECLARE_COMPLETION_ONSTACK() instead of a separate 
declaration and init_completion() call.

Thanks,

Bart.