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Date: Thu, 23 May 2024 11:34:54 +0000
From: Nitesh Shetty <nj.shetty@samsung.com>
To: Bart Van Assche <bvanassche@acm.org>
Cc: Jens Axboe <axboe@kernel.dk>, Jonathan Corbet <corbet@lwn.net>, Alasdair
	Kergon <agk@redhat.com>, Mike Snitzer <snitzer@kernel.org>, Mikulas Patocka
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	<hch@lst.de>, Sagi Grimberg <sagi@grimberg.me>, Chaitanya Kulkarni
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Subject: Re: [PATCH v20 02/12] Add infrastructure for copy offload in block
 and request layer.
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On 22/05/24 11:05AM, Bart Van Assche wrote:
>On 5/20/24 03:20, Nitesh Shetty wrote:
>>We add two new opcode REQ_OP_COPY_DST, REQ_OP_COPY_SRC.
>>Since copy is a composite operation involving src and dst sectors/lba,
>>each needs to be represented by a separate bio to make it compatible
>>with device mapper.
>>We expect caller to take a plug and send bio with destination information,
>>followed by bio with source information.
>>Once the dst bio arrives we form a request and wait for source
>>bio. Upon arrival of source bio we merge these two bio's and send
>>corresponding request down to device driver.
>>Merging non copy offload bio is avoided by checking for copy specific
>>opcodes in merge function.
>
>Plugs are per task. Can the following happen?
We rely on per-context plugging to avoid this.
>* 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.
Lets say this forms request A and stored in plug A
>* Task B submits a REQ_OP_COPY_DST bio and a REQ_OP_COPY_SRC bio.
Lets say this forms request B and stored in plug B
>* 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.
Here task A picks plug A and hence request A
>* 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.
same as above, request B

So we expect this case not to happen.

Thank you,
Nitesh Shetty

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