linux-kernel - Re: [patch 1/2] splice: dont steal

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Message-ID: <20070315122723.GQ15400@kernel.dk>
Date:	Thu, 15 Mar 2007 13:27:23 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Nick Piggin <npiggin@...e.de>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux Memory Management List <linux-mm@...ck.org>
Subject: Re: [patch 1/2] splice: dont steal

On Thu, Mar 15 2007, Nick Piggin wrote:
> On Thu, Mar 15, 2007 at 12:52:37PM +0100, Jens Axboe wrote:
> > On Wed, Mar 14 2007, Nick Piggin wrote:
> > > Here are a couple of splice patches I found when digging in the area.
> > > I could be wrong, so I'd appreciate confirmation.
> > > 
> > > Untested other than compile, because I don't have a good splice test
> > > setup.
> > > 
> > > Considering these are data corruption / information leak issues, then
> > > we could do worse than to merge them in 2.6.21 and earlier stable
> > > trees.
> > > 
> > > Does anyone really use splice stealing?
> > 
> > That's a damn shame, I'd greatly prefer if we can try and fix it
> > instead. Splice isn't really all that used yet to my knowledge, but
> > stealing is one of the niftier features I think. Otherwise you're just
> > copying data again.
> 
> We should be able to allow for it with the new a_ops API I'm working
> on.

"Should be" and in progress stuff, is it guarenteed to get there?

> Basically we can pass the page down to the filesystem, and tell it to
> attempt to install that page in-place.
> 
> The problem is that we can't just put this page here hoping the fs can
> take it, becaue it might fail allocating blocks, for example.
> 
> Anyway, we can still copy files with 1 less copy than read/write ;)

It's not about 1 vs 2 copies, it's more about 0 vs 1 copy. But yes, we
can file copy with less copies.

> It is a nifty feature, but I think it is more of a niche than simply
> saving that 1 copy, because you have to know that the source isn't
> going to be used again.

Well yes, same as when you free() a page. A little more tricky, but
that's mainly the vm assumptions/requirements for page stealing.

> But I'll try to support it with begin_write.

-- 
Jens Axboe

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