linux-kernel - Re: [PATCH v2 x86 1/2] fix page faults by nmiaction in nmi if kmemcheck is enabled

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Message-ID: <1330970118.11248.256.camel@twins>
Date:	Mon, 05 Mar 2012 18:55:18 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Don Zickus <dzickus@...hat.com>
Cc:	Li Zhong <zhong@...ux.vnet.ibm.com>,
	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>, tony.luck@...el.com,
	bp@...64.org, robert.richter@....com, lenb@...nel.org,
	minyard@....org, wim@...ana.be, linux-edac@...r.kernel.org,
	oprofile-list@...ts.sf.net, linux-acpi@...r.kernel.org,
	openipmi-developer@...ts.sourceforge.net,
	linux-watchdog@...r.kernel.org
Subject: Re: [PATCH v2 x86 1/2]  fix page faults by nmiaction in nmi if
 kmemcheck is enabled

On Mon, 2012-03-05 at 10:54 -0500, Don Zickus wrote:
> This is one way of doing this.  I was trying to avoid this when I rewrote the
> nmi handlers, because everyone kept screwing up the structs.  I thought it
> would be safer to have callers pass in data based on an api instead.

Apparently kmemcheck marks pages as non-present and does magic in the
fault handler. Having the action thing allocated meant kmemcheck also
marks that thing as non-present in the page-tables, the list iteration
from NMI context would then fault and things would go funny.

There's two ways out, help kmemcheck with a new annotation (which of
course starts with checking if there isn't already such a thing).

Or this one, avoid the action things from being allocated, this
side-steps kmemcheck and avoids the problem thusly.

Sadly this patch doesn't at all mention the first possibility and why
that isn't a feasible approach. A well...
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