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

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Date: Sat, 1 Jun 2024 07:59:31 +0200
From: Christoph Hellwig <hch@....de>
To: Bart Van Assche <bvanassche@....org>
Cc: Nitesh Shetty <nj.shetty@...sung.com>,
	Damien Le Moal <dlemoal@...nel.org>, 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>, Christoph Hellwig <hch@....de>,
	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 Thu, May 30, 2024 at 10:11:15AM -0700, Bart Van Assche wrote:
> This new approach has the following two disadvantages:
> * Without plug, REQ_OP_COPY_SRC and REQ_OP_COPY_DST are not combined. These two
>   operation types are the only operation types for which not using a plug causes
>   an I/O failure.

So?  We can clearly document that and even fail submission with a helpful
message trivially to enforce that.

> * A loop is required to combine the REQ_OP_COPY_SRC and REQ_OP_COPY_DST operations.

I don't see why that is a problem.  Especiallly once we allow multiple
sources which is really useful for GC workloads we'll need to do that
anyway.

> Please switch to the approach Hannes suggested, namely bio chaining. Chaining
> REQ_OP_COPY_SRC and REQ_OP_COPY_DST bios before these are submitted eliminates the
> two disadvantages mentioned above.

No.