Re: [PATCH v3 00/10] Add Copy offload support

[Dave Chinner <david@fromorbit.com>]

On Mon, Feb 14, 2022 at 01:29:50PM +0530, Nitesh Shetty wrote:
> The patch series covers the points discussed in November 2021 virtual call
> [LSF/MM/BFP TOPIC] Storage: Copy Offload[0].
> We have covered the Initial agreed requirements in this patchset.
> Patchset borrows Mikulas's token based approach for 2 bdev
> implementation.
> 
> Overall series supports –
> 
> 1. Driver
> - NVMe Copy command (single NS), including support in nvme-target (for
> 	block and file backend)
> 
> 2. Block layer
> - Block-generic copy (REQ_COPY flag), with interface accommodating
> 	two block-devs, and multi-source/destination interface
> - Emulation, when offload is natively absent
> - dm-linear support (for cases not requiring split)
> 
> 3. User-interface
> - new ioctl
> 
> 4. In-kernel user
> - dm-kcopyd

The biggest missing piece - and arguably the single most useful
piece of this functionality for users - is hooking this up to the
copy_file_range() syscall so that user file copies can be offloaded
to the hardware efficiently.

This seems like it would relatively easy to do with an fs/iomap iter
loop that maps src + dst file ranges and issues block copy offload
commands on the extents. We already do similar "read from source,
write to destination" operations in iomap, so it's not a huge
stretch to extent the iomap interfaces to provide an copy offload
mechanism using this infrastructure.

Also, hooking this up to copy-file-range() will also get you
immediate data integrity testing right down to the hardware via fsx
in fstests - it uses copy_file_range() as one of it's operations and
it will find all the off-by-one failures in both the linux IO stack
implementation and the hardware itself.

And, in reality, I wouldn't trust a block copy offload mechanism
until it is integrated with filesystems, the page cache and has
solid end-to-end data integrity testing available to shake out all
the bugs that will inevitably exist in this stack....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com