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Message-ID: <alpine.LFD.2.00.1005190828050.23538@i5.linux-foundation.org>
Date: Wed, 19 May 2010 08:30:10 -0700 (PDT)
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Nick Piggin <npiggin@...e.de>
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 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.
Linus
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