linux-kernel - Re: [RFC] extending splice for copy offloading

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Message-ID: <CAELBmZBGD4rph=gjLCPKCdEj+nzEQ-F=DExoL+h3vRm7qF7dCQ@mail.gmail.com>
Date:	Fri, 20 Sep 2013 11:49:56 +0200
From:	Szeredi Miklos <miklos@...redi.hu>
To:	Zach Brown <zab@...hat.com>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-nfs@...r.kernel.org,
	Trond Myklebust <Trond.Myklebust@...app.com>,
	Bryan Schumaker <bjschuma@...app.com>,
	"Martin K. Petersen" <mkp@....net>, Jens Axboe <axboe@...nel.dk>,
	Mark Fasheh <mfasheh@...e.com>,
	Joel Becker <jlbec@...lplan.org>,
	Eric Wong <normalperson@...t.net>
Subject: Re: [RFC] extending splice for copy offloading

On Wed, Sep 11, 2013 at 7:06 PM, Zach Brown <zab@...hat.com> wrote:
>
> When I first started on this stuff I followed the lead of previous
> work and added a new syscall for the copy operation:
>
> https://lkml.org/lkml/2013/5/14/618
>
> Towards the end of that thread Eric Wong asked why we didn't just
> extend splice.  I immediately replied with some dumb dismissive
> answer.  Once I sat down and looked at it, though, it does make a
> lot of sense.  So good job, Eric.  +10 Dummie points for me.
>
> Extending splice avoids all the noise of adding a new syscall and
> naturally falls back to buffered copying as that's what the direct
> splice path does for sendfile() today.

Nice idea.

>
> So that's what this patch series demonstrates.  It adds a flag that
> lets splice get at the same direct splicing that sendfile() does.
> We then add a file system file_operations method to accelerate the
> copy which has access to both files.
>
> Some things to talk about:
> - I really don't care about the naming here.  If you do, holler.
> - We might want different flags for file-to-file splicing and acceleration

Yes, I think "copy" and "reflink" needs to be differentiated.

> - We might want flags to require or forbid acceleration
> - We might want to provide all these flags to sendfile, too
>
> Thoughts?  Objections?

Can filesystem support "whole file copy" only?  Or arbitrary
block-to-block copy should be mandatory?

Splice has size_t argument for the size, which is limited to 4G on 32
bit.  Won't this be an issue for whole-file-copy?  We could have
special value (-1) for whole file, but that's starting to be hackish.

We are talking about copying large amounts of data in a single
syscall, which will possibly take a long time.  Will the syscall be
interruptible?  Restartable?

Thanks,
Miklos
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