Re: WARNING in handle_mm_fault

[Dmitry Vyukov <dvyukov@google.com>]

On Wed, Nov 25, 2015 at 2:05 PM, Dmitry Vyukov <dvyukov@google.com> wrote:
> On Tue, Nov 24, 2015 at 11:31 PM, Johannes Weiner <hannes@cmpxchg.org> wrote:
>> Hi Dmitry,
>>
>> On Tue, Nov 24, 2015 at 02:50:26PM +0100, Dmitry Vyukov wrote:
>>> As a blind guess, I've added the following BUG into copy_process:
>>>
>>> diff --git a/kernel/fork.c b/kernel/fork.c
>>> index b4dc490..c5667e8 100644
>>> --- a/kernel/fork.c
>>> +++ b/kernel/fork.c
>>> @@ -1620,6 +1620,8 @@ static struct task_struct *copy_process(unsigned
>>> long clone_flags,
>>>         trace_task_newtask(p, clone_flags);
>>>         uprobe_copy_process(p, clone_flags);
>>>
>>> +       BUG_ON(p->memcg_may_oom);
>>> +
>>>         return p;
>>
>> Thanks for your report.
>>
>> I don't see how this could happen through the legitimate setters of
>> p->memcg_may_oom. Something must clobber it. What happens with the
>> following patch applied?
>>
>> diff --git a/include/linux/sched.h b/include/linux/sched.h
>> index edad7a4..42e1285 100644
>> --- a/include/linux/sched.h
>> +++ b/include/linux/sched.h
>> @@ -1463,9 +1463,11 @@ struct task_struct {
>>         unsigned sched_reset_on_fork:1;
>>         unsigned sched_contributes_to_load:1;
>>         unsigned sched_migrated:1;
>> +       unsigned dummy_a:1;
>>  #ifdef CONFIG_MEMCG
>>         unsigned memcg_may_oom:1;
>>  #endif
>> +       unsigned dummy_b:1;
>>  #ifdef CONFIG_MEMCG_KMEM
>>         unsigned memcg_kmem_skip_account:1;
>>  #endif
>> diff --git a/kernel/fork.c b/kernel/fork.c
>> index f97f2c4..ab6f7ba 100644
>> --- a/kernel/fork.c
>> +++ b/kernel/fork.c
>> @@ -1617,6 +1617,12 @@ static struct task_struct *copy_process(unsigned long clone_flags,
>>         trace_task_newtask(p, clone_flags);
>>         uprobe_copy_process(p, clone_flags);
>>
>> +       if (p->dummy_a || p->dummy_b || p->memcg_may_oom) {
>> +               printk(KERN_ALERT "dummy_a:%d dummy_b:%d memcg_may_oom:%d\n",
>> +                      p->dummy_a, p->dummy_b, p->memcg_may_oom);
>> +               BUG();
>> +       }
>> +
>>         return p;
>>
>>  bad_fork_cancel_cgroup:
>
>
> I cannot reproduce the condition again, either with your patch or with
> mine patch... Will try harder.

I mean that I still can reproduce the original warning, but I can't
catch memcg_may_oom=1 in copy_process.

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