linux-kernel - Re: frequent lockups in 3.18rc4

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Message-ID: <CA+55aFxi5mNNXFH20AwrgOVsT1HyuU1a63VYm6m+j0jSVr4dGQ@mail.gmail.com>
Date:	Wed, 19 Nov 2014 16:40:49 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	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 Wed, Nov 19, 2014 at 4:30 PM, Andy Lutomirski <luto@...capital.net> wrote:
>
> This is why I think that the grsec kernels will crash on very large
> memory systems.  They don't seem to get this right for the kernel
> stack, and a page fault trying to access the stack is a big no-no.

For something like a stack, that's trivial, you could just probe it
before the actual task switch.

So I wouldn't worry about the kernel stack itself (although I think
vmallocing it isn't likely worth it), I'd worry more about some other
random dynamic percpu allocation. Although they arguably shouldn't
happen for low-level code that cannot handle the dynamic
pgd-population. And they generally don't.

It's really tracing that tends to be a special case not because of any
particular low-level code issue, but because instrumenting itself
recursively tends to be a bad idea.

                    Linus
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