bug with 3.4.6, 3.5.3, 3.6.1

[Gilles Chanteperdrix]

On 10/11/2012 03:59 PM, Will Deacon wrote:
> On Thu, Oct 11, 2012 at 02:32:06PM +0100, Gilles Chanteperdrix wrote:
>> On 10/11/2012 12:36 PM, Will Deacon wrote:
>>> On Thu, Oct 11, 2012 at 06:46:35AM +0100, Gilles Chanteperdrix wrote:
>>>> Hi,
>>>
>>> Hi Gilles,
>>>
>>>> when booting Linux v3.4.6, v3.5.3, or v3.6.1 on a pandaboard with an 
>>>> OMAP4430 ES2.1, compiled with the following configuration:
>>>> http://xenomai.org/~gch/config-panda
>>>>
>>>> I get the bug below after mounting the root filesystem.
>>>>
>>>> CONFIG_VMSPLIT_2G and CONFIG_THUMB2_KERNEL disabled seems to be the 
>>>> combination which triggers the bug.
>>>>
>>>> With this configuration, it seems the init_mm.mm_count incrementation
>>>> done at the beginning of secondary_start_kernel() is "lost" after the
>>>> calls to cpu_switch_mm() and local_flush_tlb().
>>>>
>>>> Modifying the secondary_startup() function in head.S to pass the 
>>>> swapper pgdir instead of the idmap pgdir in r4 also avoids the issue.
>>>
>>> What's your PHYS_OFFSET? I suspect it's >= 2GB, in which case I have some
>>> ideas about this problem.
>>
>> Yes, according to /proc/iomem:
>> 80000000-bfefffff : System RAM
>>   80008000-80339fff : Kernel code
>>   80364000-803c52e7 : Kernel data
>>
>> So, PHYS_OFFSET is 0x80000000 that is, 2GB.
> 
> Argh, then there's something fishy with the interaction between the idmap
> and swapper. The overwritten entries *should* be identical, but something is
> causing us to corrupt the initial tables from pgd_alloc(). Perhaps something
> to do with us mapping in sections...
> 
> I'll have to do some digging and get back to you.

To satisfy my curiosity, what is the difference between VMSPLIT_3G and
VMSPLIT_2G? The fact that with VMSPLIT_2G with have physical == virtual?

-- 
					    Gilles.