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

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Date:	Wed, 18 Dec 2013 12:26:11 -0500
From:	Anna Schumaker <schumaker.anna@...il.com>
To:	Zach Brown <zab@...hat.com>, Christoph Hellwig <hch@...radead.org>
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 12/18/2013 12:10 PM, Zach Brown wrote:
> On Wed, Dec 18, 2013 at 04:41:26AM -0800, Christoph Hellwig wrote:
>> On Wed, Sep 11, 2013 at 10:06:47AM -0700, Zach Brown 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.
>> Given the convolute mess that the splice code already is I'd rather
>> prefer not overloading it even further.
> I agree after trying to weave the copy offloading API into the splice
> interface.  There are also weird cases that we haven't really discussed
> so far (preserving unwritten allocations between the copied files?) that
> would muddy the waters even further.
>
> The further the APIs drift from each other, the more I'm prefering
> giving copy offloading its own clean syscall.  Even if the argument
> types superficially match the splice() ABI.
>
>> We can still fall back to the splice code as a fallback if no option
>> is provided as a last resort, but I think making the splice code handle
>> even more totally different cases is the wrong direction.
> I'm with you.  I'll have another version out sometime after the US
> holiday break.. say in a few weeks?

That'll work for me, I'll update my NFS code once your new patches are out.

Anna

>
> - z
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