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

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Message-ID: <a866d5b5-5b01-44a2-9ccb-63bf30aa8a51@acm.org>
Date: Wed, 29 May 2024 15:41:51 -0700
From: Bart Van Assche <bvanassche@....org>
To: Damien Le Moal <dlemoal@...nel.org>, Nitesh Shetty <nj.shetty@...sung.com>
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, 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 5/29/24 12:48 AM, Damien Le Moal wrote:
> On 5/29/24 15:17, Nitesh Shetty wrote:
>> On 24/05/24 01:33PM, Bart Van Assche wrote:
>>> On 5/20/24 03:20, 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.
>>>
>>> In this patch I don't see any changes for blk_attempt_bio_merge(). Does
>>> this mean that combining REQ_OP_COPY_DST and REQ_OP_COPY_SRC will never
>>> happen if the QUEUE_FLAG_NOMERGES request queue flag has been set?
>>>
>> Yes, in this case copy won't work, as both src and dst bio reach driver
>> as part of separate requests.
>> We will add this as part of documentation.
> 
> So that means that 2 major SAS HBAs which set this flag (megaraid and mpt3sas)
> will not get support for copy offload ? Not ideal, by far.

QUEUE_FLAG_NOMERGES can also be set through sysfs (see also
queue_nomerges_store()). This is one of the reasons why using the merge
infrastructure for combining REQ_OP_COPY_DST and REQ_OP_COPY_SRC is
unacceptable.

Thanks,

Bart.