linux-kernel - Re: Make sure we populate the initroot filesystem late enough

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Date:	Sun, 25 Feb 2007 20:13:49 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	David Woodhouse <dwmw2@...radead.org>
cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linuxppc-dev@...abs.org, john stultz <johnstul@...ibm.com>
Subject: Re: Make sure we populate the initroot filesystem late enough



On Sun, 25 Feb 2007, David Woodhouse wrote:
>
> > Can you try adding something like
> > 
> >         memset(start, 0xf0, end - start);
> 
> Yeah, I did that before giving up on it for the day and going in search
> of dinner. It changes the failure mode to a BUG() in
> cache_free_debugcheck(), at line 2876 of mm/slab.c

Ok, that's just strange. 

One obvious thing to do would be to remove all the "__initdata" entries in 
mm/slab.c.. But I'd also like to see the full backtrace for the  BUG_ON(), 
in case that gives any clues at all.

> It smells like the pages weren't actually reserved in the first place
> and we were blithely allocating them. The only problem with that theory
> is that the initrd doesn't seem to be getting corrupted -- and if we
> were handing out its pages like that then surely _something_ would have
> scribbled on it before we tried to read it.

Yeah, I don't think it's necessarily initrd itself, I'd be more inclined 
to think that the reason you see this change with the initrd unpacking is 
simply that it does a lot of allocations for the initrd files, so I think 
it is only indirectly involved - just because it ends up being a slab 
user.

> When I head back in tomorrow morning I'll instrument free_initrd_mem()
> to check that the PageReserved bit was actually set on each page, before
> clearing it. And I'll make the page allocation routines check whether
> they're giving out pages between initrd_start and initrd_end, etc.

Sounds like a sane plan.

			Linus
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