Re: [PATCH 2/3] memcgroup: fix typo in VM_BUG_ON()

[Li Zefan <lizf@cn.fujitsu.com>]

Balbir Singh wrote:
> Li Zefan wrote:
>> Balbir Singh wrote:
>>> YAMAMOTO Takashi wrote:
>>>>> Li Zefan wrote:
>>>>>> No need for VM_BUG_ON(pc), since 'pc' is the list entry. This should
>>>>>> be VM_BUG_ON(page).
>>>>>>
>>>>>> Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
>>>>>> Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
>>>>> pc is of type page_cgroup and we use list_for_each_entry_safe_reverse. Not sure
>>>>> why we can't bug on pc.
>>>> pc is dereferenced before this VM_BUG_ON.
>>>>
>>>> YAMAMOTO Takashi
>>>>
>>> OK, so the VM_BUG_ON needs to move to an earlier location. Agreed.
>>>
>> No, 'pc' has been dereferenced in list_for_each_entry_safe_reverse().
>>
>>
>> #define list_for_each_entry_safe_reverse(pos, n, head, member)		\
>> 	for (pos = list_entry((head)->prev, typeof(*pos), member),	\
>> 		n = list_entry(pos->member.prev, typeof(*pos), member);	\
>>                                ^^^^^^^^^^^
>> 	     &pos->member != (head); 					\
>>               ^^^^^^^^^^^
>> 	     pos = n, n = list_entry(n->member.prev, typeof(*n), member))
>>
> 
> Hmm.. We used to have a for loop with !list_empty() as a termination condition
> and VM_BUG_ON(!pc) is a spill over. With the new loop, VM_BUG_ON(!pc) does not
> make sense.
> 
> 

I see, and I'll post a new patch to just remove it.