linux-kernel - Re: [PATCH 0/2 x86] fix some page faults in nmi if kmemcheck is enabled

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Date:	Tue, 21 Feb 2012 11:17:17 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Li Zhong <zhong@...ux.vnet.ibm.com>
Cc:	LKML <linux-kernel@...r.kernel.org>, tglx@...utronix.de,
	mingo@...hat.com, hpa@...or.com, x86@...nel.org, paulus@...ba.org,
	mingo@...e.hu, acme@...stprotocols.net
Subject: Re: [PATCH 0/2 x86] fix some page faults in nmi if kmemcheck is
 enabled

On Tue, 2012-02-21 at 09:42 +0800, Li Zhong wrote:

> > Hell no, these are some of the ugliest patches I've seen in a while. Not
> > to mention that their changelogs are utter crap since they don't even
> > explain why they're doing what they're doing.
> > 
> Hi Peter, 
> 
> I agree that the fix is ugly. I'm willing to change if there are some
> better ways. 

There are always better ways..

> The problem here is: 
> 1. It seems x86 doesn't allow page faults in nmi, and there are checks
> in the code, like WARN_ON_ONCE(in_nmi()).

I bet that's not x86 only..

> 2. If CONFIG_KMEMCHECK is enabled, the pages allocated through slab will
> be marked as non-present, to capture uninitialized memory access. More
> information in Documentation/kmemcheck.txt .

So then kmemcheck is buggy, since the nmiaction structure is initialized
in register_nmi_handler(), so it should most definitely not be marked
non-present.

> 3. From the log, there are some memories accessed in nmi, which are in
> pages marked as non-present by kmemcheck, as they are allocated by
> something like kmalloc(). 

So figure out why and fix that instead of writing ugly ass patches that
seemingly work around the problem without actually thinking about it.
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