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

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Date:	Mon, 26 Feb 2007 10:44:20 -0600
From:	Milton Miller <miltonm@....com>
To:	David Woodhouse <dwmw2@...radead.org>
Cc:	LKML <linux-kernel@...r.kernel.org>, linuxppc-dev@...abs.org
Subject: Re: Make sure we populate the initroot filesystem late enough

On Feb 27, 2007, at 2:24 AM, David Woodhouse wrote:
> 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.


Just a thought,

Any chance you are using one of the unusal code paths, like the 
bootloader
moving the initrd or using a kernel-crash region?


milton

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