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

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Date:	Mon, 30 Sep 2013 10:52:22 -0400
From:	Ric Wheeler <rwheeler@...hat.com>
To:	Miklos Szeredi <miklos@...redi.hu>
CC:	"J. Bruce Fields" <bfields@...ldses.org>,
	"Myklebust, Trond" <Trond.Myklebust@...app.com>,
	Zach Brown <zab@...hat.com>,
	Anna Schumaker <schumaker.anna@...il.com>,
	Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux-Fsdevel <linux-fsdevel@...r.kernel.org>,
	"linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>,
	"Schumaker, Bryan" <Bryan.Schumaker@...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 09/30/2013 10:51 AM, Miklos Szeredi wrote:
> On Mon, Sep 30, 2013 at 4:34 PM, J. Bruce Fields <bfields@...ldses.org> wrote:
>>> My other worry is about interruptibility/restartability.  Ideas?
>>>
>>> What happens on splice(from, to, 4G) and it's a non-reflink copy?
>>> Can the page cache copy be made restartable?   Or should splice() be
>>> allowed to return a short count?  What happens on (non-reflink) remote
>>> copies and huge request sizes?
>> If I were writing an application that required copies to be restartable,
>> I'd probably use the largest possible range in the reflink case but
>> break the copy into smaller chunks in the splice case.
>>
> The app really doesn't want to care about that.  And it doesn't want
> to care about restartability, etc..  It's something the *kernel* has
> to care about.   You just can't have uninterruptible syscalls that
> sleep for a "long" time, otherwise first you'll just have annoyed
> users pressing ^C in vain; then, if the sleep is even longer, warnings
> about task sleeping too long.
>
> One idea is letting splice() return a short count, and so the app can
> safely issue SIZE_MAX requests and the kernel can decide if it can
> copy the whole file in one go or if it wants to do it in smaller
> chunks.
>
> Thanks,
> Miklos

You cannot rely on a short count. That implies that an offloaded copy starts at 
byte 0 and the short count first bytes are all valid.

I don't believe that is in fact required by all (any?) versions of the spec :)

Best just to fail and restart the whole operation.

Ric

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