From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1752903Ab0ESPde (ORCPT <rfc822;w@1wt.eu>);
	Wed, 19 May 2010 11:33:34 -0400
Received: from smtp1.linux-foundation.org ([140.211.169.13]:37678 "EHLO
	smtp1.linux-foundation.org" rhost-flags-OK-OK-OK-OK)
	by vger.kernel.org with ESMTP id S1752611Ab0ESPdc (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Wed, 19 May 2010 11:33:32 -0400
Date: Wed, 19 May 2010 08:30:10 -0700 (PDT)
From: Linus Torvalds <torvalds@linux-foundation.org>
To: Nick Piggin <npiggin@suse.de>
cc: Steven Rostedt <rostedt@goodmis.org>,
       Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
       Peter Zijlstra <peterz@infradead.org>,
       Frederic Weisbecker <fweisbec@gmail.com>,
       Pierre Tardy <tardyp@gmail.com>, Ingo Molnar <mingo@elte.hu>,
       Arnaldo Carvalho de Melo <acme@redhat.com>,
       Tom Zanussi <tzanussi@gmail.com>, Paul Mackerras <paulus@samba.org>,
       linux-kernel@vger.kernel.org, arjan@infradead.org,
       ziga.mahkovec@gmail.com, davem <davem@davemloft.net>,
       linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
       KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
       Christoph Lameter <cl@linux-foundation.org>, Tejun Heo <tj@kernel.org>,
       Jens Axboe <jens.axboe@oracle.com>
Subject: Re: Unexpected splice "always copy" behavior observed
In-Reply-To: <20100519151726.GB2516@laptop>
Message-ID: <alpine.LFD.2.00.1005190828050.23538@i5.linux-foundation.org>
References: <20100518153440.GB7748@Krystal> <1274197993.26328.755.camel@gandalf.stny.rr.com> <1274199039.26328.758.camel@gandalf.stny.rr.com> <alpine.LFD.2.00.1005180918300.4195@i5.linux-foundation.org> <20100519063116.GR2516@laptop>
 <alpine.LFD.2.00.1005190736370.23538@i5.linux-foundation.org> <20100519151726.GB2516@laptop>
User-Agent: Alpine 2.00 (LFD 1167 2008-08-23)
MIME-Version: 1.0
Content-Type: TEXT/PLAIN; charset=US-ASCII
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org



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