linux-kernel - Re: x86: 4kstacks default

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Message-Id: <20080419104911.07d299ef.akpm@linux-foundation.org>
Date:	Sat, 19 Apr 2008 10:49:11 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	linux-kernel@...r.kernel.org, arjan@...radead.org,
	tglx@...utronix.de
Subject: Re: x86: 4kstacks default

> On Sat, 19 Apr 2008 16:23:29 +0200 Ingo Molnar <mingo@...e.hu> 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?

There has been a dribble of reports - I don't have the links handy, nor did
I search for them.

> 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.

I doubt if you're testing things like nfsd-on-xfs-on-md-on-porky-scsi-driver.

Enable CONFIG_DEBUG_STACK_USAGE.  Monitor the results.  It's so scary that
I wonder if the feature is busted.

> So basically the kernel default just follows the 
> common distro default now. (distros and users can still disable it)

Apparently not.  I wouldn't enable it if I had a distro.

Anyway.  We should be having this sort of discussion _before_ a patch
gets merged, no?
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