Date: Wed, 13 Jul 2016 11:42:35 -0700 From: Andy Lutomirski <luto@...capital.net> To: Ingo Molnar <mingo@...nel.org> Cc: Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, linux-arch <linux-arch@...r.kernel.org>, Borislav Petkov <bp@...en8.de>, Nadav Amit <nadav.amit@...il.com>, Kees Cook <keescook@...omium.org>, Brian Gerst <brgerst@...il.com>, "kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>, Linus Torvalds <torvalds@...ux-foundation.org>, Josh Poimboeuf <jpoimboe@...hat.com>, Jann Horn <jann@...jh.net>, Heiko Carstens <heiko.carstens@...ibm.com> Subject: Re: [PATCH v5 14/32] x86/mm/64: Enable vmapped stacks On Wed, Jul 13, 2016 at 12:53 AM, Ingo Molnar <mingo@...nel.org> wrote: > > * Andy Lutomirski <luto@...nel.org> 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_ARCH_TRACEHOOK >> select HAVE_ARCH_TRANSPARENT_HUGEPAGE >> select HAVE_EBPF_JIT if X86_64 >> + select HAVE_ARCH_VMAP_STACK if X86_64 > > So what is the performance impact? Seems to be a very slight speedup (0.5 µs or so) on my silly benchmark (pthread_create, pthread_join in a loop). It should be a small slowdown on workloads that create many threads all at once, thus defeating the stack cache. It should be a *large* speedup on any workload that would trigger compaction on clone() to satisfy the high-order allocation. > > 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: Agreed. At the very least, I want to wait until after net-next gets pulled to flip the default to y. I'm also a bit concerned about more random driver issues that I haven't found yet. I suppose we could flip the default to y for a few -rc releases and see what, if anything, shakes loose. --Andy
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