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Date:   Thu, 17 Feb 2022 18:32:15 +0530
From:   Nitesh Shetty <nj.shetty@samsung.com>
To:     Dave Chinner <david@fromorbit.com>
Cc:     javier@javigon.com, chaitanyak@nvidia.com,
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        Chaitanya Kulkarni <kch@nvidia.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
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Subject: Re: [PATCH v3 00/10] Add Copy offload support
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 Tue, Feb 15, 2022 at 09:08:12AM +1100, Dave Chinner wrote:
> On Mon, Feb 14, 2022 at 01:29:50PM +0530, Nitesh Shetty wrote:
> > The patch series covers the points discussed in November 2021 virtual call
> > [LSF/MM/BFP TOPIC] Storage: Copy Offload[0].
> > We have covered the Initial agreed requirements in this patchset.
> > Patchset borrows Mikulas's token based approach for 2 bdev
> > implementation.
> > 
> > Overall series supports –
> > 
> > 1. Driver
> > - NVMe Copy command (single NS), including support in nvme-target (for
> > 	block and file backend)
> > 
> > 2. Block layer
> > - Block-generic copy (REQ_COPY flag), with interface accommodating
> > 	two block-devs, and multi-source/destination interface
> > - Emulation, when offload is natively absent
> > - dm-linear support (for cases not requiring split)
> > 
> > 3. User-interface
> > - new ioctl
> > 
> > 4. In-kernel user
> > - dm-kcopyd
> 
> The biggest missing piece - and arguably the single most useful
> piece of this functionality for users - is hooking this up to the
> copy_file_range() syscall so that user file copies can be offloaded
> to the hardware efficiently.
> 
> This seems like it would relatively easy to do with an fs/iomap iter
> loop that maps src + dst file ranges and issues block copy offload
> commands on the extents. We already do similar "read from source,
> write to destination" operations in iomap, so it's not a huge
> stretch to extent the iomap interfaces to provide an copy offload
> mechanism using this infrastructure.
> 
> Also, hooking this up to copy-file-range() will also get you
> immediate data integrity testing right down to the hardware via fsx
> in fstests - it uses copy_file_range() as one of it's operations and
> it will find all the off-by-one failures in both the linux IO stack
> implementation and the hardware itself.
> 
> And, in reality, I wouldn't trust a block copy offload mechanism
> until it is integrated with filesystems, the page cache and has
> solid end-to-end data integrity testing available to shake out all
> the bugs that will inevitably exist in this stack....
> 
> Cheers,
> 
> Dave.
> -- 
> Dave Chinner
> david@fromorbit.com
>

We had planned copy_file_range (CFR) in next phase of copy offload patch series.
Thinking that we will get to CFR when everything else is robust.
But if that is needed to make things robust, will start looking into that.

--
Nitesh Shetty

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