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

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Message-ID: <Znn6C-C73Tps3WJk@kbusch-mbp.dhcp.thefacebook.com>
Date: Mon, 24 Jun 2024 16:58:19 -0600
From: Keith Busch <kbusch@...nel.org>
To: Nitesh Shetty <nj.shetty@...sung.com>
Cc: Christoph Hellwig <hch@....de>, Bart Van Assche <bvanassche@....org>,
	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>,
	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 Mon, Jun 24, 2024 at 04:14:07PM +0530, Nitesh Shetty wrote:
> c. List/ctx based approach:
> A new member is added to bio, bio_copy_ctx, which will a union with
> bi_integrity. Idea is once a copy bio reaches blk_mq_submit_bio, it will
> add the bio to this list.

Is there a reason to tie this to CONFIG_BLK_DEV_INTEGRITY? Why not use
the bio_io_vec? There's no user data here, so that's unused, right?

> 1. Send the destination BIO, once this reaches blk_mq_submit_bio, this
> will add the destination BIO to the list inside bi_copy_ctx and return
> without forming any request.
> 2. Send source BIO, once this reaches blk_mq_submit_bio, this will
> retrieve the destination BIO from bi_copy_ctx and form a request with
> destination BIO and source BIO. After this request will be sent to
> driver.

Like Damien, I also don't see the point of the 2-bio requirement. Treat
it like discard, and drivers can allocate "special".