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

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Date:	Wed, 14 Jul 2010 13:06:17 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Steven Rostedt <rostedt@...tedt.homelinux.com>,
	Frederic Weisbecker <fweisbec@...il.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>,
	Andi Kleen <andi@...stfloor.org>,
	"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

* Linus Torvalds (torvalds@...ux-foundation.org) wrote:
> On Wed, Jul 14, 2010 at 8:49 AM, Mathieu Desnoyers
> <mathieu.desnoyers@...icios.com> wrote:

(I was quoting Peter Anvin below) ;)

> >> I think you're vastly overestimating what is sane to do from an NMI
> >> context.  It is utterly and totally insane to assume vmalloc is available
> >> in NMI.
> 
> I agree that NMI handlers shouldn't touch vmalloc space. But now that
> percpu data is mapped through the VM, I do agree that other CPU's may
> potentially need to touch that data, and an interrupt (including an
> NMI) might be the first to create the mapping.
> 
[...]
> So please just document the sequence that actually needs the page
> table setup for the NMI/percpu case.
> 
> This patch (1/2) doesn't look horrible per se. I have no problems with
> it. I just want to understand why it is needed.

The problem originally addressed by this patch is the case where a NMI handler
try to access vmalloc'd per-cpu data, which goes as follow:

- One CPU does a fork(), which copies the basic kernel mappings.
- Perf allocates percpu memory for buffer control data structures.
  This mapping does not get copied.
- Tracing is activated.
- switch_to() to the newly forked process which missed the new percpu
  allocation.
- We take a NMI, which touches the vmalloc'd percpu memory in the Perf tracing
  handler, therefore leading to a page fault in NMI context. Here, we might be
  in the middle of switch_to(), where ->current might not be in sync with the
  current cr3 register.

The three choices we have to handle this that I am aware of are:
1) supporting page faults in NMI context, which imply removing ->current
   dependency and supporting iret-less return path.
2) duplicating the percpu alloc API with a variant that maps to kmalloc.
3) using vmalloc_sync_all() after creating the mapping. (only works for x86_64,
   not x86_32).

Choice 3 seems like a no-go on x86_32, choice 2 seems like a last-resort
(involves API duplication and reservation of a fixed-amount of per-cpu memory at
boot). Hence the proposal of choice 1.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com
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