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Message-ID: <a866d5b5-5b01-44a2-9ccb-63bf30aa8a51@acm.org>
Date: Wed, 29 May 2024 15:41:51 -0700
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Subject: Re: [PATCH v20 02/12] Add infrastructure for copy offload in block
 and request layer.
To: Damien Le Moal <dlemoal@kernel.org>, Nitesh Shetty <nj.shetty@samsung.com>
Cc: Jens Axboe <axboe@kernel.dk>, Jonathan Corbet <corbet@lwn.net>,
 Alasdair Kergon <agk@redhat.com>, Mike Snitzer <snitzer@kernel.org>,
 Mikulas Patocka <mpatocka@redhat.com>, Keith Busch <kbusch@kernel.org>,
 Christoph Hellwig <hch@lst.de>, Sagi Grimberg <sagi@grimberg.me>,
 Chaitanya Kulkarni <kch@nvidia.com>, Alexander Viro
 <viro@zeniv.linux.org.uk>, Christian Brauner <brauner@kernel.org>,
 Jan Kara <jack@suse.cz>, martin.petersen@oracle.com, david@fromorbit.com,
 hare@suse.de, damien.lemoal@opensource.wdc.com, anuj20.g@samsung.com,
 joshi.k@samsung.com, nitheshshetty@gmail.com, gost.dev@samsung.com,
 linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
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References: <20240520102033.9361-1-nj.shetty@samsung.com>
 <CGME20240520102842epcas5p4949334c2587a15b8adab2c913daa622f@epcas5p4.samsung.com>
 <20240520102033.9361-3-nj.shetty@samsung.com>
 <eda6c198-3a29-4da4-94db-305cfe28d3d6@acm.org>
 <20240529061736.rubnzwkkavgsgmie@nj.shetty@samsung.com>
 <9f1ec1c1-e1b8-48ac-b7ff-8efb806a1bc8@kernel.org>
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From: Bart Van Assche <bvanassche@acm.org>
In-Reply-To: <9f1ec1c1-e1b8-48ac-b7ff-8efb806a1bc8@kernel.org>
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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.