[PATCH v5 02/10] block: Add copy offload support infrastructure

[Ming Lei]

On Wed, Nov 23, 2022 at 11:28:19AM +0530, Nitesh Shetty wrote:
> Introduce blkdev_issue_copy which supports source and destination bdevs,
> and an array of (source, destination and copy length) tuples.
> Introduce REQ_COPY copy offload operation flag. Create a read-write
> bio pair with a token as payload and submitted to the device in order.
> Read request populates token with source specific information which
> is then passed with write request.
> This design is courtesy Mikulas Patocka's token based copy

I thought this patchset is just for enabling copy command which is
supported by hardware. But turns out it isn't, because blk_copy_offload()
still submits read/write bios for doing the copy.

I am just wondering why not let copy_file_range() cover this kind of copy,
and the framework has been there.

When I was researching pipe/splice code for supporting ublk zero copy[1], I
have got idea for async copy_file_range(), such as: io uring based
direct splice, user backed intermediate buffer, still zero copy, if these
ideas are finally implemented, we could get super-fast generic offload copy,
and bdev copy is really covered too.

[1] https://lore.kernel.org/linux-block/20221103085004.1029763-1-ming.lei@redhat.com/

thanks,
Ming