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Date: Wed, 22 May 2024 11:05:24 -0700
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Subject: Re: [PATCH v20 02/12] Add infrastructure for copy offload in block
 and request layer.
To: Nitesh Shetty <nj.shetty@samsung.com>, Jens Axboe <axboe@kernel.dk>,
 Jonathan Corbet <corbet@lwn.net>, Alasdair Kergon <agk@redhat.com>,
 Mike Snitzer <snitzer@kernel.org>, Mikulas Patocka <mpatocka@redhat.com>,
 Keith Busch <kbusch@kernel.org>, Christoph Hellwig <hch@lst.de>,
 Sagi Grimberg <sagi@grimberg.me>, Chaitanya Kulkarni <kch@nvidia.com>,
 Alexander Viro <viro@zeniv.linux.org.uk>,
 Christian Brauner <brauner@kernel.org>, Jan Kara <jack@suse.cz>
Cc: martin.petersen@oracle.com, david@fromorbit.com, hare@suse.de,
 damien.lemoal@opensource.wdc.com, anuj20.g@samsung.com, joshi.k@samsung.com,
 nitheshshetty@gmail.com, gost.dev@samsung.com, linux-block@vger.kernel.org,
 linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
 dm-devel@lists.linux.dev, linux-nvme@lists.infradead.org,
 linux-fsdevel@vger.kernel.org
References: <20240520102033.9361-1-nj.shetty@samsung.com>
 <CGME20240520102842epcas5p4949334c2587a15b8adab2c913daa622f@epcas5p4.samsung.com>
 <20240520102033.9361-3-nj.shetty@samsung.com>
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From: Bart Van Assche <bvanassche@acm.org>
In-Reply-To: <20240520102033.9361-3-nj.shetty@samsung.com>
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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?
* 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.
* 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.

Thanks,

Bart.