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SubjectRe: x86: 4kstacks default
On Wed, Apr 23, 2008 at 03:27:01PM +1000, Benjamin Herrenschmidt wrote:
> On Sat, 2008-04-19 at 16:23 +0200, Ingo Molnar wrote:
> > * Andrew Morton <akpm@linux-foundation.org> wrote:
> >
> > > > config 4KSTACKS
> > > > bool "Use 4Kb for kernel stacks instead of 8Kb"
> > > > - depends on DEBUG_KERNEL
> > > > depends on X86_32
> > > > + default y
> > >
> > > This patch will cause kernels to crash.
> >
> > what mainline kernels crash and how will they crash? Fedora and other
> > distros have had 4K stacks enabled for years:
> >
> > $ grep 4K /boot/config-2.6.24-9.fc9
> > CONFIG_4KSTACKS=y
> >
> > and we've conducted tens of thousands of bootup tests with all sorts of
> > drivers and kernel options enabled and have yet to see a single crash
> > due to 4K stacks. So basically the kernel default just follows the
> > common distro default now. (distros and users can still disable it)
>
> Do we routinely test nasty scenarii such as a GFP_KERNEL allocation deep
> in a call stack trying to swap something out to NFS ?

I doubt it, because this is the place that a local XFS filesystem
typically blows a 4k stack (direct memory reclaim triggering
->writepage). Boot testing does nothing to exercise the potential
paths for stack overflows....

Cheers,

Dave.
--
Dave Chinner
Principal Engineer
SGI Australian Software Group


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