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

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Date:	Fri, 2 Mar 2012 14:44:54 -0500
From:	Don Zickus <dzickus@...hat.com>
To:	Li Zhong <zhong@...ux.vnet.ibm.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	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,
	Vegard Nossum <vegardno@....uio.no>
Subject: Re: [PATCH 0/2 x86] fix some page faults in nmi if kmemcheck is
 enabled

On Tue, Feb 28, 2012 at 10:45:41AM +0800, Li Zhong wrote:
> > > I'm not sure whether I understand it correctly. Do you mean that
> > > nmiaction is initialized in register_nmi_handler(), which indicates it
> > > will be used in nmi, so it shouldn't be marked non-present?
> > 
> > No, you said that it marks memory non-present to detect uninitialized
> > stuff, but since it is initialized, it shouldn't then be non-present,
> > right?
> 
> From my understanding of kmemcheck, the checking is based on the
> non-present page. So while handling page fault, if the memory hasn't
> been written before read, kmemcheck knows that it is uninitialized. 
> 
> I think it is used to find code errors, so it need mark all non-present,
> to check if there are any access to uninitialized memory. 

I am not sure if this is what your tool is catching, but someone pointed
out to me privately that when panic'ing in an NMI, either the shutdown
path I modified or the kdump path will register an NMI handler in an NMI
context.  Thus they will try to allocate memory in the NMI context.

So I will have to fix that.  Wonder if that popular lockless memory
allocator can help me there... otherwise I have to go back to pass in
structs like the notifier blocks do.

Cheers,
Don
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