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

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Date:   Mon, 8 Oct 2018 15:22:37 +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>
Subject: Re: [PATCH] mm, oom_adj: avoid meaningless loop to find processes
 sharing mm

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);
>>>                         }
>