linux-kernel - Re: frequent lockups in 3.18rc4

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Message-ID: <CA+55aFwYCYoU5t1GU4i=1w5-5HzPfLb42MLANq6ghViSSenxsA@mail.gmail.com>
Date:	Fri, 21 Nov 2014 10:24:35 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Andy Lutomirski <luto@...capital.net>, Tejun Heo <tj@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Don Zickus <dzickus@...hat.com>, Dave Jones <davej@...hat.com>,
	"the arch/x86 maintainers" <x86@...nel.org>
Subject: Re: frequent lockups in 3.18rc4

On Fri, Nov 21, 2014 at 9:34 AM, Steven Rostedt <rostedt@...dmis.org> wrote:
>
> I see. You have issues with the use of current->active_mm instead of
> just doing a read_cr3() (and I'm sure other things).

Yes. And I have this memory of it actually mattering, where we'd get
get the page fault, but see that the (wrong) page table is already
populated, and say "ti wasn't a vmalloc fault", and then go down the
oops path.

Of course, the context switch itself has changed completely over the
years, but I think it would still be true with NMI. "active_mm" may
point to a different page table than the one the CPU is actually
using, and then the whole thing is bogus.

               Linus
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