linux-kernel - Re: [-mm patch] enforce noreplace-smp in alternative

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Date:	Sat, 25 Aug 2007 14:23:24 +0200
From:	Frederik Deweerdt <deweerdt@...e.fr>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Andi Kleen <ak@...e.de>,
	kraxel@...hat.com
Subject: Re: [-mm patch] enforce noreplace-smp in alternative_instructions()

On Sat, Aug 25, 2007 at 10:07:29PM +1000, Rusty Russell wrote:
> On Fri, 2007-08-24 at 10:22 +0200, Frederik Deweerdt wrote:
> > [Added Gerd Hoffman and Rusty Russel to cc]
> > On Thu, Aug 23, 2007 at 11:46:52PM -0700, Jeremy Fitzhardinge wrote:
> > > Frederik Deweerdt wrote:
> > > > That means that even when you specify noreplace_smp, some replacing
> > > > takes place anyway. One of the consequences, besides noreplace_smp not
> > > > working as expected, is that lguest crashes when you feed it an SMP kernel
> > > > (I suspect that you can not replace alternatives for smp _and_ paravirt).
> > > >   
> > > 
> > > No, that should be fine.  Why does lguest crash?
> > It dies with:
> > [    0.131000] SMP alternatives: switching to UP code
> > lguest: bad stack page 0xc057a000
Hello Rusty,
> 
> How odd!  This means that the guest set the kernel to a stack which it
> hadn't mapped writable (or perhaps not mapped at all).  I always run SMP

I had time to investigate this a little further, it appears that in fact
0xc057a000 is the beginning of the __smp_locks section.

The crash responsible function call is in alternative_instructions():
		free_init_pages("SMP alternatives",
				(unsigned long)__smp_locks,
				(unsigned long)__smp_locks_end);

Ie, if I comment this out, I can boot lguest without passing
noreplace_smp.

BTW, to make things clear: the patch I sent does _not_ fix the
lguest/alternatives problem. It just makes noreplace_smp functional
again and hence allows working around the lguest/alternatives bug.

> kernels, and that seems a very strange side effect of a patching
> problem...
> 
> Nonetheless, I did have a previous problem with a bug in the patching
> code which didn't show up native and did show up under lguest.
> 
> Can you send your config? 
Here it is:
http://fdeweerdt.free.fr/lguest_smp/dot_config

> Do you need noreplace-smp even on 2.6.23-rc3,
> or only 2.6.23-rc3-mm1?
I'll try ASAP.

Thanks,
Frederik
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