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

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Message-ID: <20240605082028.GC18688@lst.de>
Date: Wed, 5 Jun 2024 10:20:28 +0200
From: Christoph Hellwig <hch@....de>
To: Bart Van Assche <bvanassche@....org>
Cc: Christoph Hellwig <hch@....de>, 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>, 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 Tue, Jun 04, 2024 at 04:44:34AM -0700, 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).

Again, we can decided on QUEUE_FLAG_NOMERGES per request type.  In fact
I think we should not use it for discards as that just like copy
is a very different kind of "merge".

I'm in fact much more happy about avoiding the copy_id IFF we can.  It
it a fair amout of extra overhead, so we should only add it if there
is a real need for it