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

[Aneesh Kumar K.V]

Li Zefan <lizefan@xxxxxxxxxx> writes:

> on 2012/7/19 10:55, Aneesh Kumar K.V wrote:
>
>> Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> writes:
>> 
>>> On Wed, 18 Jul 2012 11:04:09 +0530
>>> "Aneesh Kumar K.V" <aneesh.kumar@xxxxxxxxxxxxxxxxxx> wrote:
>>>
>>>> From: "Aneesh Kumar K.V" <aneesh.kumar@xxxxxxxxxxxxxxxxxx>
>>>>
>>>> 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. 

-aneesh

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