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

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Date: Fri, 28 Jun 2024 06:53:32 -0700
From: Bart Van Assche <bvanassche@....org>
To: Christoph Hellwig <hch@....de>, Damien Le Moal <dlemoal@...nel.org>
Cc: Nitesh Shetty <nj.shetty@...sung.com>, Jens Axboe <axboe@...nel.dk>,
 Jonathan Corbet <corbet@....net>, Alasdair Kergon <agk@...hat.com>,
 Mike Snitzer <snitzer@...nel.org>, Mikulas Patocka <mpatocka@...hat.com>,
 Keith Busch <kbusch@...nel.org>, Sagi Grimberg <sagi@...mberg.me>,
 Chaitanya Kulkarni <kch@...dia.com>, Alexander Viro
 <viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>,
 Jan Kara <jack@...e.cz>, martin.petersen@...cle.com, david@...morbit.com,
 hare@...e.de, damien.lemoal@...nsource.wdc.com, anuj20.g@...sung.com,
 joshi.k@...sung.com, nitheshshetty@...il.com, gost.dev@...sung.com,
 linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-doc@...r.kernel.org, dm-devel@...ts.linux.dev,
 linux-nvme@...ts.infradead.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v20 02/12] Add infrastructure for copy offload in block
 and request layer.

On 6/25/24 10:22 PM, Christoph Hellwig wrote:
> It's not just dm.  You also need it in the partition remapping code
> (mandatory), md (nice to have), etc.
> 
> And then we have the whole mess of what is in the payload for the I/O
> stack vs what is in the payload for the on the wire protocol, which
> will have different formatting and potentially also different sizes.

Drivers like dm-linear rely on bio splitting. If the COPY_SRC and
COPY_DST operations travel independently down a stacked block driver
hierarchy, a separate data structure is needed to keep track of which
operations have been split and to combine the split operations into
requests. Isn't this an argument in favor of storing the source and
destination parameters in a single bio?

Thanks,

Bart.