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

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Date:	Mon, 26 Feb 2007 11:24:57 -0500
From:	David Woodhouse <dwmw2@...radead.org>
To:	Linus Torvalds <torvalds@...ux-foundation.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, 2007-02-25 at 20:13 -0800, Linus Torvalds wrote:
> 
> 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. 

In this case I hadn't left the 'return' in free_initrd_mem(). I was
poisoning the pages and then returning them to the pool as usual.

If I poison the pages and _don't_ return them to the pool, it boots
fine. PageReserved is set on every page in the initrd region; total
page_count() is equal to the number of pages (which doesn't
_necessarily_ mean that page_count() for every page is equal to 1 but
it's a strong hint that that's the case).

Looking in /dev/mem after it boots, I see that my poison is still
present throughout the whole region.

> One obvious thing to do would be to remove all the "__initdata" entries in 
> mm/slab.c..

This is biting us long before we call free_initmem().

>  But I'd also like to see the full backtrace for the  BUG_ON(), 
> in case that gives any clues at all.

I'll see if I can find a camera. 

> > 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.

Whatever happens, initrd as a 'slab user' is fine. The crashes happen
_later_, when someone else is using the memory which used to belong to
the initrd. In that 'BUG at slab.c:2876' I mentioned above, r3 was
within the initrd region. As I said, I'll try to find a camera.

-- 
dwmw2

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