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

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Message-ID: <20100519154431.GC2516@laptop>
Date:	Thu, 20 May 2010 01:44:31 +1000
From:	Nick Piggin <npiggin@...e.de>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Pierre Tardy <tardyp@...il.com>, Ingo Molnar <mingo@...e.hu>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Tom Zanussi <tzanussi@...il.com>,
	Paul Mackerras <paulus@...ba.org>,
	linux-kernel@...r.kernel.org, arjan@...radead.org,
	ziga.mahkovec@...il.com, davem <davem@...emloft.net>,
	linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Tejun Heo <tj@...nel.org>, Jens Axboe <jens.axboe@...cle.com>
Subject: Re: Unexpected splice "always copy" behavior observed

On Wed, May 19, 2010 at 08:30:10AM -0700, Linus Torvalds wrote:
> 
> 
> On Thu, 20 May 2010, Nick Piggin wrote:
> > 
> > Well I mean a full invalidate -- invalidate_mapping_pages -- so there is
> > literally no pagecache there at all.
> 
> Umm. That won't work. Think mapped pages. You can't handle them 
> atomically, so somebody will page-fault them in.
> 
> So you'd have to have a "invalidate_and_replace()" to do it atomically 
> while holding the mapping spinlock or something. 
> 
> And WHAT IS THE POINT? That will be about a million times slower than 
> just doing the effing copy in the first place!
> 
> Memory copies are _not_ slow. Not compared to taking locks and doing TLB 
> invalidates.

No I never thought it would be a good idea to try to avoid all races
or anything. Obviously some cases *cannot* be easily invalidated, if
there is a ref on the page or whatever, so the fallback code has to
be there anyway.

So you would just invalidate and try to insert your page. 99.something%
of the time it will work fine. If the insert fails, fall back to
copying.

And hey you *may* even want a heuristic that avoids trying to invalidate
if the page is mapped, due to cost of TLB flushing and faulting etc.
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