Re: frequent lockups in 3.18rc4

From: Linus Torvalds
Date: Wed Nov 19 2014 - 19:40:55 EST

On Wed, Nov 19, 2014 at 4:30 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> 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.

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