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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 ┬Ás 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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