Re: crash during oom reaper

[Tetsuo Handa]

On 2016/12/16 22:14, Michal Hocko wrote:
> On Fri 16-12-16 16:07:30, Kirill A. Shutemov wrote:
>> On Fri, Dec 16, 2016 at 01:56:50PM +0100, Michal Hocko wrote:
>>> On Fri 16-12-16 15:35:55, Kirill A. Shutemov wrote:
>>>> On Fri, Dec 16, 2016 at 12:42:43PM +0100, Michal Hocko wrote:
>>>>> On Fri 16-12-16 13:44:38, Kirill A. Shutemov wrote:
>>>>>> On Fri, Dec 16, 2016 at 11:11:13AM +0100, Michal Hocko wrote:
>>>>>>> On Fri 16-12-16 10:43:52, Vegard Nossum wrote:
>>>>>>> [...]
>>>>>>>> I don't think it's a bug in the OOM reaper itself, but either of the
>>>>>>>> following two patches will fix the problem (without my understand how or
>>>>>>>> why):
>>>>>>>>
>>>>>>>> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
>>>>>>>> index ec9f11d4f094..37b14b2e2af4 100644
>>>>>>>> --- a/mm/oom_kill.c
>>>>>>>> +++ b/mm/oom_kill.c
>>>>>>>> @@ -485,7 +485,7 @@ static bool __oom_reap_task_mm(struct task_struct *tsk,
>>>>>>>> struct mm_struct *mm)
>>>>>>>>  	 */
>>>>>>>>  	mutex_lock(&oom_lock);
>>>>>>>>
>>>>>>>> -	if (!down_read_trylock(&mm->mmap_sem)) {
>>>>>>>> +	if (!down_write_trylock(&mm->mmap_sem)) {
>>>>>>>
>>>>>>> __oom_reap_task_mm is basically the same thing as MADV_DONTNEED and that
>>>>>>> doesn't require the exlusive mmap_sem. So this looks correct to me.
>>>>>>
>>>>>> BTW, shouldn't we filter out all VM_SPECIAL VMAs there? Or VM_PFNMAP at
>>>>>> least.
>>>>>>
>>>>>> MADV_DONTNEED doesn't touch VM_PFNMAP, but I don't see anything matching
>>>>>> on __oom_reap_task_mm() side.
>>>>>
>>>>> I guess you are right and we should match the MADV_DONTNEED behavior
>>>>> here. Care to send a patch?
>>>>
>>>> Below. Testing required.
>>>>
>>>>>> Other difference is that you use unmap_page_range() witch doesn't touch
>>>>>> mmu_notifiers. MADV_DONTNEED goes via zap_page_range(), which invalidates
>>>>>> the range. Not sure if it can make any difference here.
>>>>>
>>>>> Which mmu notifier would care about this? I am not really familiar with
>>>>> those users so I might miss something easily.
>>>>
>>>> No idea either.
>>>>
>>>> Is there any reason not to use zap_page_range here too?
>>>
>>> Yes, zap_page_range is much more heavy and performs operations which
>>> might lock AFAIR which I really would like to prevent from.
>>
>> What exactly can block there? I don't see anything with that potential.
> 
> I would have to rememeber all the details. This is mostly off-topic for
> this particular thread so I think it would be better if you could send a
> full patch separatelly and we can discuss it there?
> 

zap_page_range() calls mmu_notifier_invalidate_range_start().
mmu_notifier_invalidate_range_start() calls __mmu_notifier_invalidate_range_start().
__mmu_notifier_invalidate_range_start() calls srcu_read_lock()/srcu_read_unlock().
This means that zap_page_range() might sleep.

I don't know what individual notifier will do, but for example

  static const struct mmu_notifier_ops i915_gem_userptr_notifier = {
          .invalidate_range_start = i915_gem_userptr_mn_invalidate_range_start,
  };

i915_gem_userptr_mn_invalidate_range_start() calls flush_workqueue()
which means that we can OOM livelock if work item involves memory allocation.
Some of other notifiers call mutex_lock()/mutex_unlock().

Even if none of currently in-tree notifier users are blocked on memory
allocation, I think it is not guaranteed that future changes/users won't be
blocked on memory allocation.