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

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Date: Fri, 24 May 2024 06:54:49 +0000
From: Nitesh Shetty <nj.shetty@...sung.com>
To: Hannes Reinecke <hare@...e.de>
Cc: 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, bvanassche@....org, david@...morbit.com,
	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 21/05/24 09:01AM, Hannes Reinecke wrote:
>On 5/20/24 12:20, 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.
>>
>>Signed-off-by: Nitesh Shetty <nj.shetty@...sung.com>
>>Signed-off-by: Anuj Gupta <anuj20.g@...sung.com>
>>---
>>  block/blk-core.c          |  7 +++++++
>>  block/blk-merge.c         | 41 +++++++++++++++++++++++++++++++++++++++
>>  block/blk.h               | 16 +++++++++++++++
>>  block/elevator.h          |  1 +
>>  include/linux/bio.h       |  6 +-----
>>  include/linux/blk_types.h | 10 ++++++++++
>>  6 files changed, 76 insertions(+), 5 deletions(-)
>>
>I am a bit unsure about leveraging 'merge' here. As Bart pointed out, 
>this is arguably as mis-use of the 'merge' functionality as we don't
>actually merge bios, but rather use the information from these bios to
>form the actual request.
>Wouldn't it be better to use bio_chain here, and send out the eventual
>request from the end_io function of the bio chain?
>
With above bio chain approach, I see a difficulty while dealing with
stacked/partitioned devices, as they might get remapped.
Or am I missing something here ?

Bart, Hannes,
Regarding merge, does it looks any better, if we use single request
operation such as REQ_OP_COPY and use op_flags(REQ_COPY_DST/REQ_COPY_SRC)
to identify dst and src bios ?


Regards,
Nitesh Shetty