linux-kernel - Re: [dm-devel] [PATCH v14 00/11] Implement copy offload support

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Date:   Mon, 14 Aug 2023 22:25:54 +0530
From:   Nitesh Shetty <nitheshshetty@...il.com>
To:     Bart Van Assche <bvanassche@....org>
Cc:     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>,
        martin.petersen@...cle.com, linux-doc@...r.kernel.org,
        gost.dev@...sung.com, linux-kernel@...r.kernel.org,
        linux-nvme@...ts.infradead.org, linux-block@...r.kernel.org,
        mcgrof@...nel.org, dlemoal@...nel.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [dm-devel] [PATCH v14 00/11] Implement copy offload support

On Sat, Aug 12, 2023 at 3:42 AM Bart Van Assche <bvanassche@....org> wrote:
>
> On 8/11/23 03:52, Nitesh Shetty wrote:
> > We achieve copy offload by sending 2 bio's with source and destination
> > info and merge them to form a request. This request is sent to driver.
> > So this design works only for request based storage drivers.
>
> [ ... ]
>
> > Overall series supports:
> > ========================
> >       1. Driver
> >               - NVMe Copy command (single NS, TP 4065), including support
> >               in nvme-target (for block and file back end).
> >
> >       2. Block layer
> >               - Block-generic copy (REQ_OP_COPY_DST/SRC), operation with
> >                    interface accommodating two block-devs
> >                  - Merging copy requests in request layer
> >               - Emulation, for in-kernel user when offload is natively
> >                  absent
> >               - dm-linear support (for cases not requiring split)
> >
> >       3. User-interface
> >               - copy_file_range
>
> Is this sufficient? The combination of dm-crypt, dm-linear and the NVMe
> driver is very common. What is the plan for supporting dm-crypt?

Plan is to add offload support for other dm targets as part of subsequent
series once current patchset merges, dm targets can use emulation to
achieve the same at present.

> Shouldn't bio splitting be supported for dm-linear?
Handling split is tricky in this case, if we allow splitting, there is
no easy way
to match/merge different src/dst bio's. Once we have multi range support then
we feel at least src bio's can be split. But this series split won't work.

Thank you,
Nitesh Shetty