linux-kernel - Re: [PATCH v15 04/12] block: add emulation for copy

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Date:   Fri, 8 Sep 2023 08:06:38 +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>, dm-devel@...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>
Cc:     martin.petersen@...cle.com, mcgrof@...nel.org,
        gost.dev@...sung.com, Vincent Fu <vincent.fu@...sung.com>,
        Anuj Gupta <anuj20.g@...sung.com>, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-nvme@...ts.infradead.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v15 04/12] block: add emulation for copy

On 9/6/23 18:38, 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.
> Also emulation can be used, if copy offload fails or partially completes.
> At present in kernel user of emulation is NVMe fabrics.
> 
Leave out the last sentence; I really would like to see it enabled for 
SCSI, too (we do have copy offload commands for SCSI ...).

And it raises all the questions which have bogged us down right from the 
start: where is the point in calling copy offload if copy offload is not 
implemented or slower than copying it by hand?
And how can the caller differentiate whether copy offload bring a 
benefit to him?

IOW: wouldn't it be better to return -EOPNOTSUPP if copy offload is not 
available?

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                Kernel Storage Architect
hare@...e.de                              +49 911 74053 688
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