Re: [PATCH v5 14/32] x86/mm/64: Enable vmapped stacks

From: Ingo Molnar
Date: Wed Jul 13 2016 - 03:53:50 EST

* Andy Lutomirski <luto@xxxxxxxxxx> wrote:

> This allows x86_64 kernels to enable vmapped stacks. There are a
> couple of interesting bits.

> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -92,6 +92,7 @@ config X86
> select HAVE_EBPF_JIT if X86_64
> + select HAVE_ARCH_VMAP_STACK if X86_64

So what is the performance impact?

Because I think we should consider enabling this feature by default on x86 - but
the way it's selected here it will be default-off.

On the plus side: the debuggability and reliability improvements are real and
making it harder for exploits to use kernel stack overflows is a nice bonus as
well. There's two performance effects:

- vmalloc now potentially moves into the thread pool create/destroy hot path.

- we use 4K TLBs for kernel stacks instead of 2MB TLBs.

The TLB effect should be relatively modest on modern CPUs, given that the kernel
stack size is limited and 4K TLBs are plenty. The vmalloc() part should be
measured I suspect.