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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 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().

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

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