Date: Thu, 14 Jul 2016 10:34:11 +0200 From: Ingo Molnar <mingo@...nel.org> To: Andy Lutomirski <luto@...capital.net> 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 * Andy Lutomirski <luto@...capital.net> wrote: > 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). [...] Music to my ears - although TBH there's probably two opposing forces: advantages from the cache versus (possibly very minor, if measurable at all) disadvantages from the 4K granularity. > [...] 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. So I'd prefer the following approach: to apply it to a v4.8-rc1 base in ~2 weeks and keep it default-y for much of the next development cycle. If no serious problems are found in those ~2 months then send it to Linus in that fashion. We can still turn it off by default (or re-spin the whole approach) if it turns out to be too risky. Exposing it as default-n for even a small amount of time will massively reduce the testing we'll get, as most people will just use the N setting (often without noticing). Plus this also gives net-next and other preparatory patches applied directly to maintainer trees time to trickle upstream. Thanks, Ingo
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