linux-kernel - Re: [PATCH v20 04/12] block: add emulation for copy

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Message-ID: <cf6929e1-0dea-4216-bbc5-c00d963372f7@suse.de>
Date: Tue, 21 May 2024 09:06:43 +0200
From: Hannes Reinecke <hare@...e.de>
To: Nitesh Shetty <nj.shetty@...sung.com>, 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>
Cc: 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,
 Vincent Fu <vincent.fu@...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 04/12] block: add emulation for copy

On 5/20/24 12:20, Nitesh Shetty wrote:
> For the devices which does not support copy, copy emulation is added.
> It is required for in-kernel users like fabrics, where file descriptor is
> not available and hence they can't use copy_file_range.
> Copy-emulation is implemented by reading from source into memory and
> writing to the corresponding destination.
> At present in kernel user of emulation is fabrics.
> 
> Signed-off-by: Nitesh Shetty <nj.shetty@...sung.com>
> Signed-off-by: Vincent Fu <vincent.fu@...sung.com>
> Signed-off-by: Anuj Gupta <anuj20.g@...sung.com>
> ---
>   block/blk-lib.c        | 223 +++++++++++++++++++++++++++++++++++++++++
>   include/linux/blkdev.h |   4 +
>   2 files changed, 227 insertions(+)
> 
Again, I'm not sure if we need this.
After all, copy offload _is_optional, so we need to be prepared to 
handle systems where it's not supported. In the end, the caller might
decide to do something else entirely; having an in-kernel emulation 
would defeat that.
And with adding an emulation to nullblk we already have an emulation
target to try if people will want to start experimenting.
So I'd rather not have this but rather let the caller deal with the
fact that copy offload is optional.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                  Kernel Storage Architect
hare@...e.de                                +49 911 74053 688
SUSE Software Solutions GmbH, Frankenstr. 146, 90461 Nürnberg
HRB 36809 (AG Nürnberg), GF: I. Totev, A. McDonald, W. Knoblich