linux-kernel - Re: [PATCH v13 3/9] block: add emulation for copy

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Date:   Fri, 30 Jun 2023 16:52:27 +0530
From:   Nitesh Shetty <nj.shetty@...sung.com>
To:     Ming Lei <ming.lei@...hat.com>
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, linux-scsi@...r.kernel.org,
        willy@...radead.org, hare@...e.de, djwong@...nel.org,
        bvanassche@....org, dlemoal@...nel.org, nitheshshetty@...il.com,
        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 v13 3/9] block: add emulation for copy

On 23/06/29 04:33PM, Ming Lei wrote:
>Hi Nitesh,
>
>On Wed, Jun 28, 2023 at 12:06:17AM +0530, 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
>
>I can understand copy command does help for FS GC and fabrics storages,
>but still not very clear why copy emulation is needed for kernel users,
>is it just for covering both copy command and emulation in single
>interface? Or other purposes?
>
>I'd suggest to add more words about in-kernel users of copy emulation.
>

As you mentioned above, we need a single interface for covering both
copy command and emulation.
This is needed in fabrics cases, as we expose any non copy command
supported target device also as copy capable, so we fallback to emulation
once we recieve copy from host/initator.
Agreed, we will add more description to covey the same info.

>> 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 asynchronously.
>> Also emulation is used, if copy offload fails or partially completes.
>
>Per my understanding, this kind of emulation may not be as efficient
>as doing it in userspace(two linked io_uring SQEs, read & write with
>shared buffer). But it is fine if there are real in-kernel such users.
>

We do have plans for uring based copy interface in next phase,
once curent series is merged.
With current design we really see the advantage of emulation in fabrics case.

Thank you,
Nitesh Shetty