linux-kernel - Re: x86: 4kstacks default

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Message-ID: <20080423233652.GT103491721@sgi.com>
Date:	Thu, 24 Apr 2008 09:36:52 +1000
From:	David Chinner <dgc@....com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: 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@...ux-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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