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SubjectRe: [PATCH] mm, oom_adj: avoid meaningless loop to find processes sharing mm
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On 2018/10/08 17:38, Yong-Taek Lee wrote:
>>
>> On 2018/10/08 15:14, Yong-Taek Lee wrote:
>>>> On 2018/10/08 10:19, Yong-Taek Lee wrote:
>>>>> @@ -1056,6 +1056,7 @@ static int __set_oom_adj(struct file *file, int oom_adj, bool legacy)
>>>>> struct mm_struct *mm = NULL;
>>>>> struct task_struct *task;
>>>>> int err = 0;
>>>>> + int mm_users = 0;
>>>>>
>>>>> task = get_proc_task(file_inode(file));
>>>>> if (!task)
>>>>> @@ -1092,7 +1093,8 @@ static int __set_oom_adj(struct file *file, int oom_adj, bool legacy)
>>>>> struct task_struct *p = find_lock_task_mm(task);
>>>>>
>>>>> if (p) {
>>>>> - if (atomic_read(&p->mm->mm_users) > 1) {
>>>>> + mm_users = atomic_read(&p->mm->mm_users);
>>>>> + if ((mm_users > 1) && (mm_users != get_nr_threads(p))) {
>>>>
>>>> How can this work (even before this patch)? When clone(CLONE_VM without CLONE_THREAD/CLONE_SIGHAND)
>>>> is requested, copy_process() calls copy_signal() in order to copy sig->oom_score_adj and
>>>> sig->oom_score_adj_min before calling copy_mm() in order to increment mm->mm_users, doesn't it?
>>>> Then, we will get two different "struct signal_struct" with different oom_score_adj/oom_score_adj_min
>>>> but one "struct mm_struct" shared by two thread groups.
>>>>
>>>
>>> Are you talking about race between __set_oom_adj and copy_process?
>>> If so, i agree with your opinion. It can not set oom_score_adj properly for copied process if __set_oom_adj
>>> check mm_users before copy_process calls copy_mm after copy_signal. Please correct me if i misunderstood anything.
>>
>> You understand it correctly.
>>
>> Reversing copy_signal() and copy_mm() is not sufficient either. We need to use a read/write lock
>> (read lock for copy_process() and write lock for __set_oom_adj()) in order to make sure that
>> the thread created by clone() becomes reachable from for_each_process() path in __set_oom_adj().
>>
>
> Thank you for your suggestion. But i think it would be better to seperate to 2 issues. How about think these
> issues separately because there are no dependency between race issue and my patch. As i already explained,
> for_each_process path is meaningless if there is only one thread group with many threads(mm_users > 1 but
> no other thread group sharing same mm). Do you have any other idea to avoid meaningless loop ?

Yes. I suggest reverting commit 44a70adec910d692 ("mm, oom_adj: make sure processes
sharing mm have same view of oom_score_adj") and commit 97fd49c2355ffded ("mm, oom:
kill all tasks sharing the mm").

>
>>>
>>>>> mm = p->mm;
>>>>> atomic_inc(&mm->mm_count);
>>>>> }
>>>
>>
>



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