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Subject: Re: [patch 1/2] x86_64 page fault NMI-safe
From: Steven Rostedt <rostedt@goodmis.org>
To: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Andi Kleen <andi@firstfloor.org>, Ingo Molnar <mingo@elte.hu>,
        LKML <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Steven Rostedt <rostedt@rostedt.homelinux.com>,
        Thomas Gleixner <tglx@linutronix.de>, Christoph Hellwig <hch@lst.de>,
        Li Zefan <lizf@cn.fujitsu.com>, Lai Jiangshan <laijs@cn.fujitsu.com>,
        Johannes Berg <johannes.berg@intel.com>,
        Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>,
        Arnaldo Carvalho de Melo <acme@infradead.org>,
        Tom Zanussi <tzanussi@gmail.com>,
        KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Jeremy Fitzhardinge <jeremy@goop.org>,
        "Frank Ch. Eigler" <fche@redhat.com>, Tejun Heo <htejun@gmail.com>
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Date: Fri, 16 Jul 2010 08:54:58 -0400
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On Fri, 2010-07-16 at 14:00 +0200, Frederic Weisbecker wrote:
> On Thu, Jul 15, 2010 at 07:51:55AM -0700, Linus Torvalds wrote:

> 
> Such new page table created that might race is only about top level page
> right? Otherwise it wouldn't race since the top level entries are shared
> and then updates inside lower level pages are naturally propagated, if
> I understood you well.
> 
> So, if only top level pages that gets added can generate such lazily
> mapping update, I wonder why I experienced this fault everytime with
> my patches.
> 
> I allocated 8192 bytes per cpu in a x86-32 system that has only 2 GB.
> I doubt there is a top level page table update there at this time with
> such a small amount of available memory. But still it faults once on
> access.
> 
> I have troubles to visualize the race and the problem here.
> 

A few trace_printks and a tracing_off() on fault would probably show
exactly what was happening ;-)

-- Steve


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