Re: [PATCH] hugetlb/cgroup: Simplify pre_destroy callback

[Kamezawa Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

(2012/07/19 18:41), Aneesh Kumar K.V wrote:
> Li Zefan <lizefan@huawei.com> writes:
>
>> on 2012/7/19 10:55, Aneesh Kumar K.V wrote:
>>
>>> Andrew Morton <akpm@linux-foundation.org> writes:
>>>
>>>> On Wed, 18 Jul 2012 11:04:09 +0530
>>>> "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com> wrote:
>>>>
>>>>> From: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
>>>>>
>>>>> Since we cannot fail in hugetlb_cgroup_move_parent, we don't really
>>>>> need to check whether cgroup have any change left after that. Also skip
>>>>> those hstates for which we don't have any charge in this cgroup.
>>>>>
>>>>> ...
>>>>>
>>>>> +	for_each_hstate(h) {
>>>>> +		/*
>>>>> +		 * if we don't have any charge, skip this hstate
>>>>> +		 */
>>>>> +		idx = hstate_index(h);
>>>>> +		if (res_counter_read_u64(&h_cg->hugepage[idx], RES_USAGE) == 0)
>>>>> +			continue;
>>>>> +		spin_lock(&hugetlb_lock);
>>>>> +		list_for_each_entry(page, &h->hugepage_activelist, lru)
>>>>> +			hugetlb_cgroup_move_parent(idx, cgroup, page);
>>>>> +		spin_unlock(&hugetlb_lock);
>>>>> +		VM_BUG_ON(res_counter_read_u64(&h_cg->hugepage[idx], RES_USAGE));
>>>>> +	}
>>>>>   out:
>>>>>   	return ret;
>>>>>   }
>>>>
>>>> This looks fishy.
>>>>
>>>> We test RES_USAGE before taking hugetlb_lock.  What prevents some other
>>>> thread from increasing RES_USAGE after that test?
>>>>
>>>> After walking the list we test RES_USAGE after dropping hugetlb_lock.
>>>> What prevents another thread from incrementing RES_USAGE before that
>>>> test, triggering the BUG?
>>>
>>> IIUC core cgroup will prevent a new task getting added to the cgroup
>>> when we are in pre_destroy. Since we already check that the cgroup doesn't
>>> have any task, the RES_USAGE cannot increase in pre_destroy.
>>>
>>
>>
>> You're wrong here. We release cgroup_lock before calling pre_destroy and retrieve
>> the lock after that, so a task can be attached to the cgroup in this interval.
>>
>
> But that means rmdir can be racy right ? What happens if the task got
> added, allocated few pages and then moved out ? We still would have task
> count 0 but few pages, which we missed to to move to parent cgroup.
>

That's a problem even if it's verrrry unlikely.
I'd like to look into it and fix the race in cgroup layer.
But I'm sorry I'm a bit busy in these days...

Thanks,
-Kame


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