linux-kernel - Re: [patch 1/2] x86_64 page fault NMI-safe

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Message-ID: <20100716112109.GB5377@nowhere>
Date:	Fri, 16 Jul 2010 13:21:11 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Steven Rostedt <rostedt@...tedt.homelinux.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Christoph Hellwig <hch@....de>, Li Zefan <lizf@...fujitsu.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Johannes Berg <johannes.berg@...el.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Tom Zanussi <tzanussi@...il.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	"Frank Ch. Eigler" <fche@...hat.com>, Tejun Heo <htejun@...il.com>
Subject: Re: [patch 1/2] x86_64 page fault NMI-safe

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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