Re: [memcg BUG] unable to handle kernel NULL pointer derefence at 00000000

[Balbir Singh <balbir@linux.vnet.ibm.com>]

KAMEZAWA Hiroyuki wrote:
> On Tue, 21 Oct 2008 13:19:51 +0200
> Ingo Molnar <mingo@elte.hu> wrote:
> 
>> * Li Zefan <lizf@cn.fujitsu.com> wrote:
>>
>>>> Oh! thanks...but it seems pc->page is NULL in the middle of ZONE_NORMAL..
>>>> ==
>>>>  Normal   0x00001000 -> 0x000373fe
>>>> ==
>>>> This is appearently in the range of page_cgroup initialization.
>>>> (if pgdat->node_page_cgroup is initalized correctly...)
>>>>
>>>> I think write to page_cgroup->page happens only at initialization.
>>>> Hmm ? not initilization failure but curruption ?
>>>>
>>> Yes, curruption. I didn't find informatation about initialization failure.
>>>
>>>> What happens if replacing __alloc_bootmem() with vmalloc() in page_cgroup.c init ?
>>>>
>>> So I did this change, and the box booted up without any problem.
>>>
>>> diff --git a/mm/page_cgroup.c b/mm/page_cgroup.c
>>> index 5d86550..82a30b1 100644
>>> --- a/mm/page_cgroup.c
>>> +++ b/mm/page_cgroup.c
>>> @@ -48,8 +48,7 @@ static int __init alloc_node_page_cgroup(int nid)
>>>  
>>>  	table_size = sizeof(struct page_cgroup) * nr_pages;
>>>  
>>> -	base = __alloc_bootmem_node_nopanic(NODE_DATA(nid),
>>> -			table_size, PAGE_SIZE, __pa(MAX_DMA_ADDRESS));
>>> +	base = vmalloc_node(table_size, nid);
>>>  	if (!base)
>>>  		return -ENOMEM;
>> i have this:
>>
>>   CONFIG_FAILSLAB=y
>>   CONFIG_FAIL_PAGE_ALLOC=y
>>   # CONFIG_FAIL_MAKE_REQUEST is not set
>>   CONFIG_FAIL_IO_TIMEOUT=y
>>
>> so the bug was perhaps that the __alloc_bootmem_node_nopanic() failed 
>> and this code continued silently? vmalloc_node() probably is more 
>> agressive about allocating memory.
>>
> Sorry. I think I cannot use alloc_bootmem() at this point because
> it's too late in init-path. (we can use usual page allocator)
> So, just replacing alloc_bootmem() with vmalloc_node() is a fix....

Kamezawa-San,

I would prefer to use alloc_bootmem() instead of vmalloc_node(). May be we can
shift cgroups, so that we use early_init for allocating page_cgroups.  What do
you think?

-- 
	Balbir

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