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

[Nitesh Shetty]

On 04/06/24 04:44AM, Bart Van Assche wrote:
>On 6/3/24 21:40, Christoph Hellwig wrote:
>>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.
>
>Including an ID in REQ_OP_COPY_DST and REQ_OP_COPY_SRC operations sounds
>much better to me than abusing the merge infrastructure for combining
>these two operations into a single request. With the ID-based approach
>stacking drivers are allowed to process copy bios asynchronously and it
>is no longer necessary to activate merging for copy operations if
>merging is disabled (QUEUE_FLAG_NOMERGES).
>
Single request, with bio merging approach:
The current approach is to send a single request to driver,
which contains both destination and source information inside separate bios.
Do you have any different approach in mind ?

If we want to proceed with this single request based approach,
we need to merge the destination request with source BIOs at some point.
a. We chose to do it via plug approach.
b. Alternative I see is scheduler merging, but here we need some way to
hold the request which has destination info, until source bio is also
submitted.
c. Is there any other way, which I am missing here ?

Limitation of current plug based approach:
I missed the possibility of asynchronous submission by stacked device.
Since we enabled only dm-linear, we had synchronous submission till now
and our test cases worked fine.
But in future if we start enabling dm targets with asynchronous submission,
the current plug based approach won't work.
The case where Bart mentioned possibility of 2 different tasks sending
copy[1] and they are getting merged wrongly is valid in this case.
There will be corruption, copy ID approach can solve this wrong merging.

Copy ID based merging might preserve the order, but we still need the
copy destination request to wait for copy source bio to merge.

Copy ID approach:
We see 3 possibilities here:
1. No merging: If we include copy-id in src and dst bio, the bio's will get
submitted separately and reach to the driver as separate requests.
How do we plan to form a copy command in driver ?
2. Merging BIOs:
At some point we need to match the src bio with the dst bio and send this
information together to the driver. The current implementation.
This still does not solve the asynchronous submission problem, mentioned
above.
3. Chaining BIOs:
This won't work with stacked devices as there will be cloning, and hence
chain won't be maintained.

[1] https://lore.kernel.org/all/d7ae00c8-c038-4bed-937e-222251bc627a@acm.org/

Thank You,
Nitesh Shetty