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

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Message-ID: <20230911070937.GB28177@green245>
Date:   Mon, 11 Sep 2023 12:39:37 +0530
From:   Nitesh Shetty <nj.shetty@...sung.com>
To:     Hannes Reinecke <hare@...e.de>
Cc:     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>,
        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 Fri, Sep 08, 2023 at 08:06:38AM +0200, Hannes Reinecke wrote:
> 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 ...).
> 
Sure, will do that

> 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?

Present approach treats copy as a background operation and the idea is to
maximize the chances of achieving copy by falling back to emulation.
Having said that, it should be possible to return -EOPNOTSUPP,
in case of offload IO failure or device not supporting offload.
We will update this in next version.

Thank you,
Nitesh Shetty