linux-kernel - Re: [RFC][PATCH] HWPOISON: remove the unsafe __set_page

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Date:	Sat, 26 Sep 2009 21:12:01 +0200
From:	Nick Piggin <npiggin@...e.de>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Wu Fengguang <fengguang.wu@...el.com>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH] HWPOISON: remove the unsafe __set_page_locked()

On Sat, Sep 26, 2009 at 05:05:55PM +0200, Andi Kleen wrote:
> > However we may well end up to accept the fact that "we just cannot do
> > hwpoison 100% correct", and settle with a simple and 99% correct code.
> 
> I would prefer to avoid any oopses, but if they are unlikely enough
> and too hard to fix that's bearable. The race window here is certainly rather 
> small. 

Well, several places non-atomically modify page flags, including
within preempt-enabled regions... It's nasty to introduce these
oopses in the hwposion code! I'm ashamed I didn't pick up on this
problem seeing as I introduced several of them.

 
> On the other hand if you cannot detect a difference in benchmarks I see
> no reason not to add the additional steps, as long as the code isn't
> complicated or ugly. These changes are neither.

The patch to add atomics back into the fastpaths? I don't think that's
acceptable at all. A config option doesn't go far enough either because
distros will have to turn it on.

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