linux-kernel - Re: [PATCH] mm, oom_adj: avoid meaningless loop to find processes sharing mm

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Date:   Mon, 8 Oct 2018 18:27:39 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     ytk.lee@...sung.com, "mhocko@...nel.org" <mhocko@...nel.org>,
        "mhocko@...e.com" <mhocko@...e.com>
Cc:     "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Oleg Nesterov <oleg@...hat.com>,
        David Rientjes <rientjes@...gle.com>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] mm, oom_adj: avoid meaningless loop to find processes
 sharing mm

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);
>>>>>                         }
>>>
>>
>