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Date: Fri, 16 Jul 2010 13:21:11 +0200
From: Frederic Weisbecker <fweisbec@gmail.com>
To: Andi Kleen <andi@firstfloor.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        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@goodmis.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>
Subject: Re: [patch 1/2] x86_64 page fault NMI-safe
Message-ID: <20100716112109.GB5377@nowhere>
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On Thu, Jul 15, 2010 at 04:35:18PM +0200, Andi Kleen wrote:
> > But then how did the previous tasks get this new mapping? You said
> > we don't walk through every process page tables for vmalloc.
> 
> No because those are always shared for the kernel and have been
> filled in for init_mm.
> 
> Also most updates only update the lower tables anyways, top level
> updates are extremly rare. In fact on PAE36 they should only happen
> at most once, if at all, and most likely at early boot anyways
> where you only  have a single task. 
> 
> On x86-64 they will only happen once every 512GB of vmalloc. 
> So for most systems also at most once at early boot.
> > 
> > I would understand this race if we were to walk on every processes page
> > tables and add the new mapping on them, but we missed one new task that
> > forked or so, because we didn't lock (or just rcu).
> 
> The new task will always get a copy of the reference init_mm, which
> was already updated.
> 
> -Andi


Ok, got it.

But then, in the example here with perf, I'm allocating 8192 bytes per cpu
and my total memory amount is of 2 GB.

And it always fault at least once on access, after the allocation.
I really doubt it's because we are adding a new top level page table,
considering the amount of memory I have.

It seems to me that the mapping of a newly allocated vmalloc area is
always inserted through the lazy way (update on fault). Or there is
something I'm missing.

Thanks.

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