Re: [PATCH v20 02/12] Add infrastructure for copy offload in block and request layer.

[Christoph Hellwig <hch@lst.de>]

On Mon, Jun 03, 2024 at 10:12:48AM -0700, Bart Van Assche wrote:
> Consider the following use case:
> * Task A calls blk_start_plug()
> * Task B calls blk_start_plug()
> * Task A submits a REQ_OP_COPY_DST bio and a REQ_OP_COPY_SRC bio.
> * Task B submits a REQ_OP_COPY_DST bio and a REQ_OP_COPY_SRC bio.
> * The stacking driver to which all REQ_OP_COPY_* operations have been
>   submitted processes bios asynchronusly.
> * Task A calls blk_finish_plug()
> * Task B calls blk_finish_plug()
> * The REQ_OP_COPY_DST bio from task A and the REQ_OP_COPY_SRC bio from
>   task B are combined into a single request.
> * The REQ_OP_COPY_DST bio from task B and the REQ_OP_COPY_SRC bio from
>   task A are combined into a single request.
>
> This results in silent and hard-to-debug data corruption. Do you agree
> that we should not restrict copy offloading to stacking drivers that
> process bios synchronously and also that this kind of data corruption
> should be prevented?

There is no requirement to process them synchronously, there is just
a requirement to preserve the order.  Note that my suggestion a few
arounds ago also included a copy id to match them up.  If we don't
need that I'm happy to leave it away.  If need it it to make stacking
drivers' lifes easier that suggestion still stands.

>
> Thanks,
>
> Bart.
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