Re: [patch -mm] mm, oom: remove oom_lock from exit_mmap

[Tetsuo Handa]

On 2018/07/16 15:13, Michal Hocko wrote:
> On Sat 14-07-18 06:18:58, Tetsuo Handa wrote:
>>> @@ -3073,9 +3073,7 @@ void exit_mmap(struct mm_struct *mm)
>>>  		 * which clears VM_LOCKED, otherwise the oom reaper cannot
>>>  		 * reliably test it.
>>>  		 */
>>> -		mutex_lock(&oom_lock);
>>>  		__oom_reap_task_mm(mm);
>>> -		mutex_unlock(&oom_lock);
>>>  
>>>  		set_bit(MMF_OOM_SKIP, &mm->flags);
>>
>> David and Michal are using different version as a baseline here.
>> David is making changes using timeout based back off (in linux-next.git)
>> which is inappropriately trying to use MMF_UNSTABLE for two purposes.
>>
>> Michal is making changes using current code (in linux.git) which does not
>> address David's concern.
> 
> Yes I have based it on top of Linus tree because the point of this patch
> is to get rid of the locking which is no longer needed. I do not see
> what concern are you talking about.

I'm saying that applying your patch does not work on linux-next.git
because David's patch already did s/MMF_OOM_SKIP/MMF_UNSTABLE/ .

>>
>> My version ( https://marc.info/?l=linux-mm&m=153119509215026 ) is
>> making changes using current code which also provides oom-badness
>> based back off in order to address David's concern.
>>
>>>  		down_write(&mm->mmap_sem);
>>
>> Anyway, I suggest doing
>>
>>   mutex_lock(&oom_lock);
>>   set_bit(MMF_OOM_SKIP, &mm->flags);
>>   mutex_unlock(&oom_lock);
> 
> Why do we need it?
> 
>> like I mentioned at
>> http://lkml.kernel.org/r/201807130620.w6D6KiAJ093010@xxxxxxxxxxxxxxxxxxx
>> even if we make changes on top of linux-next's timeout based back off.
> 
> says
> : (3) Prevent from selecting new OOM victim when there is an !MMF_OOM_SKIP mm
> :     which current thread should wait for.
> [...]
> : Regarding (A), we can reduce the range oom_lock serializes from
> : "__oom_reap_task_mm()" to "setting MMF_OOM_SKIP", for oom_lock is useful for (3).
> 
> But why there is a lock needed for this? This doesn't make much sense to
> me. If we do not have MMF_OOM_SKIP set we still should have mm_is_oom_victim
> so no new task should be selected. If we race with the oom reaper than
> ok, we would just not select a new victim and retry later.
> 

How mm_is_oom_victim() helps? mm_is_oom_victim() is used by exit_mmap() whether
current thread should call __oom_reap_task_mm().

I'm talking about below sequence (i.e. after returning from __oom_reap_task_mm()).

  CPU 0                                   CPU 1
  
  mutex_trylock(&oom_lock) in __alloc_pages_may_oom() succeeds.
  get_page_from_freelist() fails.
  Enters out_of_memory().

                                          __oom_reap_task_mm() reclaims some memory.
                                          Sets MMF_OOM_SKIP.

  select_bad_process() selects new victim because MMF_OOM_SKIP is already set.
  Kills a new OOM victim without retrying last second allocation attempt.
  Leaves out_of_memory().
  mutex_unlock(&oom_lock) in __alloc_pages_may_oom() is called.

If setting MMF_OOM_SKIP is guarded by oom_lock, we can enforce
last second allocation attempt like below.

  CPU 0                                   CPU 1
  
  mutex_trylock(&oom_lock) in __alloc_pages_may_oom() succeeds.
  get_page_from_freelist() fails.
  Enters out_of_memory().

                                          __oom_reap_task_mm() reclaims some memory.
                                          mutex_lock(&oom_lock);

  select_bad_process() does not select new victim because MMF_OOM_SKIP is not yet set.
  Leaves out_of_memory().
  mutex_unlock(&oom_lock) in __alloc_pages_may_oom() is called.

                                          Sets MMF_OOM_SKIP.
                                          mutex_unlock(&oom_lock);

  get_page_from_freelist() likely succeeds before reaching __alloc_pages_may_oom() again.
  Saved one OOM victim from being needlessly killed.

That is, guarding setting MMF_OOM_SKIP works as if synchronize_rcu(); it waits for anybody
who already acquired (or started waiting for) oom_lock to release oom_lock, in order to
prevent select_bad_process() from needlessly selecting new OOM victim.