linux-kernel - Re: [PATCH] Fix dirty page accounting in redirty_page_for

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Message-ID: <20090430143819.GF14696@elte.hu>
Date:	Thu, 30 Apr 2009 16:38:19 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>, thomas.pi@...or.dea,
	Yuriy Lalym <ylalym@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	ltt-dev@...ts.casi.polymtl.ca, Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH] Fix dirty page accounting in
	redirty_page_for_writepage()


* Christoph Lameter <cl@...ux.com> wrote:

> On Thu, 30 Apr 2009, Ingo Molnar wrote:
> 
> > The patch below makes the fallback/slowpath irq safe.
> 
> Yes but sometimes you are already irq safe and such a fallback 
> would create significant irq/enable/disable stack operations etc 
> overhead for architectures that are using the fallback.

It's a fallback slowpath - non-x86 architectures should still fill 
in a real implementation of course.

> I think we really need another __xxx op here. Especially since 
> these operations are often in critical code paths.

That's a receipe for fragility: as using __xxx will still be 
irq-safe on x86, and 95% of the testing is done on x86, so this 
opens up the path to non-x86 bugs.

So we first have to see the list of architectures that _cannot_ 
implement an irq-safe op here via a single machine instruction.
x86, ia64 and powerpc should be fine.

	Ingo
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