Re: [RFC PATCH v2 3/3] mm, oom: hand over MMF_OOM_SKIP to exit path if it is guranteed to finish

[Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>]

On 2018/10/30 15:31, Michal Hocko wrote:
> On Tue 30-10-18 13:45:22, Tetsuo Handa wrote:
>> Michal Hocko wrote:
>>> @@ -3156,6 +3166,13 @@ void exit_mmap(struct mm_struct *mm)
>>>                 vma = remove_vma(vma);
>>>         }
>>>         vm_unacct_memory(nr_accounted);
>>> +
>>> +       /*
>>> +        * Now that the full address space is torn down, make sure the
>>> +        * OOM killer skips over this task
>>> +        */
>>> +       if (oom)
>>> +               set_bit(MMF_OOM_SKIP, &mm->flags);
>>>  }
>>>
>>>  /* Insert vm structure into process list sorted by address
>>
>> I don't like setting MMF_OOF_SKIP after remove_vma() loop. 50 users might
>> call vma->vm_ops->close() from remove_vma(). Some of them are doing fs
>> writeback, some of them might be doing GFP_KERNEL allocation from
>> vma->vm_ops->open() with a lock also held by vma->vm_ops->close().
>>
>> I don't think that waiting for completion of remove_vma() loop is safe.
> 
> What do you mean by 'safe' here?
> 

safe = "Does not cause OOM lockup."

remove_vma() is allowed to sleep, and some users might depend on memory
allocation when the OOM killer is waiting for remove_vma() to complete.