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

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Message-ID: <20100714184642.GA9728@elte.hu>
Date:	Wed, 14 Jul 2010 20:46:42 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	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>,
	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:

> Ok. I was wondering why anybody would allocate core percpu variables so late 
> that this would ever be an issue, but I guess perf is a reasonable such 
> case. And reasonable to do from NMI.

Yeah.

Frederic (re-)discovered this problem via very hard to debug crashes when he 
extended perf call-graph tracing to have a bit larger buffer and used 
percpu_alloc() for it (which is entirely reasonable in itself).

> That said - grr. I really wish there was some other alternative than adding 
> yet more complexity to the exception return path. That "iret re-enables 
> NMI's unconditionally" thing annoys me.

Ok. We can solve it by allocating the space from the non-vmalloc percpu area - 
8K per CPU.

> In fact, I wonder if we couldn't just do a software NMI disable
> instead? Hav ea per-cpu variable (in the _core_ percpu areas that get
> allocated statically) that points to the NMI stack frame, and just
> make the NMI code itself do something like
> 
>  NMI entry:

I think at this point [NMI re-entry] we've corrupted the top of the NMI kernel 
stack already, due to entering via the IST stack mechanism, which is 
non-nesting and which enters at the same point - right?

We could solve that by copying that small stack frame off before entering the 
'generic' NMI routine - but it all feels a bit pulled in by the hair.

I feel uneasy about taking pagefaults from the NMI handler. Even if we 
implemented it all correctly, who knows what CPU erratas are waiting there to 
be discovered, etc ...

I think we should try to muddle through by preventing these situations from 
happening (and adding a WARN_ONCE() to the vmalloc page-fault handler would 
certainly help as well), and only go to more clever schemes if no other option 
looks sane anymore?

Thanks,

	Ingo
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