flatmem broken for nommu? [Was: Re: does non-continuous RAM means I need to select the sparse memory model?]

[Stefan Hellermann]

Am 01.12.2011 16:39, schrieb Uwe Kleine-König:
> Hello,
> 
> On Thu, Dec 01, 2011 at 11:57:18AM +0100, Uwe Kleine-König wrote:
>> On Tue, Nov 29, 2011 at 10:39:10PM -0200, Fabio Estevam wrote:
>>> 2011/11/29 Uwe Kleine-König <u.kleine-koenig at pengutronix.de>:
>>>> Hello,
>>>>
>>>> I'm currently working on a new arch port and my current machine has RAM
>>>> at 0x10000000 and 0x80000000. So there is a big hole between the two
>>>> banks. When selecting the sparse memory model it works, but when
>>>> selecting flat the machine runs into a BUG in mark_bootmem() called by
>>>> free_unused_memmap() to free the space between the two banks.
>>>
>>> My understanding is that you have to select ARCH_HAS_HOLES_MEMORYMODEL.
>> I think that is not necessary.
>>  
>>>> Is that expected (meaning I cannot use the flat model)? I currently
>>>> don't have another machine handy that has >1 memory back to test that.
>>>
>>> In case you have access to a MX35PDK you can try on this board as it does have
>>> the memory hole.
>> No I havn't, but I just used a 128MB machine and changed that in the
>> .fixup callback to 64MB + 32MB with a 32MB hole in between and it works
>> fine without ARCH_HAS_HOLES_MEMORYMODEL.
>>
>> I debugged the problem a bit further and one symptom is that
>>
>> 	struct page *mem_map
>>
>> is NULL for me. That looks wrong. I guess this is just broken for nommu.
>> I will dig into that later today.
> The problem is that the memory for mem_map is allocated using:
> 
> 	map = alloc_bootmem_node_nopanic(pgdat, size);
> 
> without any error checking. The _nopanic was introduced by commit
> 
> 	8f389a99 (mm: use alloc_bootmem_node_nopanic() on really needed path)
> 
> I don't understand the commit's log and don't really see why it should
> be allowed to not panic if the allocation failes here but use a NULL
> pointer instead.
> I put the people involved in 8f389a99 on Cc, maybe someone can comment?
> 
> Apart from that it seems I cannot use flatmem as is on my machine. It
> has only 128kiB at 0x10000000 + 1MiB at 0x80000000 and needs 14MiB to hold the
> table of "struct page"s. :-(
> 
> Best regards
> Uwe
> 
The commit was made after an bug report from me. I have an old x86
tablet pc with only 8Mb Ram. This machine fails early on bootup without
this commit. I found an archived message of the bug report here:
http://comments.gmane.org/gmane.linux.kernel/1135909

Regards,
Stefan