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Date: Fri, 24 May 2024 06:54:49 +0000
From: Nitesh Shetty <nj.shetty@samsung.com>
To: Hannes Reinecke <hare@suse.de>
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
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	martin.petersen@oracle.com, bvanassche@acm.org, david@fromorbit.com,
	damien.lemoal@opensource.wdc.com, anuj20.g@samsung.com, joshi.k@samsung.com,
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Subject: Re: [PATCH v20 02/12] Add infrastructure for copy offload in block
 and request layer.
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	<CGME20240520102842epcas5p4949334c2587a15b8adab2c913daa622f@epcas5p4.samsung.com>
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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@samsung.com>
>>Signed-off-by: Anuj Gupta <anuj20.g@samsung.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

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