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

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Date: Wed, 26 Jun 2024 06:18:18 +0900
From: Damien Le Moal <dlemoal@...nel.org>
To: Bart Van Assche <bvanassche@....org>,
 Nitesh Shetty <nj.shetty@...sung.com>, Christoph Hellwig <hch@....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>,
 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 6/26/24 03:18, Bart Van Assche wrote:
> On 6/24/24 2:55 PM, Damien Le Moal wrote:
>> I am still a little confused as to why we need 2 BIOs, one for src and one for
>> dst... Is it because of the overly complex scsi extended copy support ?
>>
>> Given that the main use case is copy offload for data within the same device,
>> using a single BIO which somehow can carry a list of LBA sources and a single
>> destination LBA would be far simpler and perfectly matching nvme simple copy and
>> ATA write gathered. And I think that this would also match the simplest case for
>> scsi extended copy as well.
> 
> Hi Damien,
> 
> What are the implications for the device mapper code if the copy source
> and destination LBAs are encoded in the bio payload instead of in
> bio->bi_sector?

DM can deal with "abnormal" BIOs on its own. There is code for that.
See is_abnormal_io() and __process_abnormal_io(). Sure, that will need more code
compared to a bio sector+size based simple split, but I do not think it is a big
deal given the potential benefits of the offloading.

-- 
Damien Le Moal
Western Digital Research