linux-kernel - Re: [PATCH 2/3] memcgroup: fix typo in VM_BUG

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Message-ID: <47B93051.9020500@cn.fujitsu.com>
Date:	Mon, 18 Feb 2008 15:14:25 +0800
From:	Li Zefan <lizf@...fujitsu.com>
To:	balbir@...ux.vnet.ibm.com
CC:	containers@...ts.linux-foundation.org,
	YAMAMOTO Takashi <yamamoto@...inux.co.jp>,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] memcgroup: fix typo in VM_BUG_ON()

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@...fujitsu.com>
>>>>>> Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...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.
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