linux-kernel - Re: Unexpected splice "always copy" behavior observed

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Date:	Wed, 19 May 2010 17:56:36 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	miklos@...redi.hu, npiggin@...e.de, rostedt@...dmis.org,
	mathieu.desnoyers@...icios.com, peterz@...radead.org,
	fweisbec@...il.com, tardyp@...il.com, mingo@...e.hu,
	acme@...hat.com, tzanussi@...il.com, paulus@...ba.org,
	linux-kernel@...r.kernel.org, arjan@...radead.org,
	ziga.mahkovec@...il.com, davem@...emloft.net, linux-mm@...ck.org,
	akpm@...ux-foundation.org, kosaki.motohiro@...fujitsu.com,
	cl@...ux-foundation.org, tj@...nel.org, jens.axboe@...cle.com
Subject: Re: Unexpected splice "always copy" behavior observed

On Wed, 19 May 2010, Linus Torvalds wrote:
> On Wed, 19 May 2010, Miklos Szeredi wrote:
> > 
> > Another limitation I found while splicing from one file to another is
> > that stealing from the source file's page cache does not always
> > succeed.  This turned out to be because of a reference from the lru
> > cache for freshly read pages.  I'm not sure how this could be fixed.
> 
> It should be fixed by saying "you can't always just move the page".
> 
> Copying is not evil. Complexity  to avoid copies is evil.

And predictability is good.  The thing I don't like about the above is
that it makes it totally unpredictable which pages will get moved, if
any.

Another related thing: if splicing from a file knowing that it will
need to be stolen, then it makes zero sense to first insert the pages
into the page cache then remove them shortly to be inserted into
another file's cache.  So we could have a flag saying "don't cache
newly read pages, just put them in the pipe buffer", which would solve
the above problem as well as speeding up the operation.

Miklos
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