Re: [PATCH v2] memcg: Fix memcg_kmem_bypass() for remote memcg charging

[Zefan Li <lizefan@huawei.com>]

On 2020/5/15 14:56, Michal Hocko wrote:
> On Thu 14-05-20 15:52:59, Roman Gushchin wrote:
>> On Thu, May 14, 2020 at 09:16:29AM +0800, Zefan Li wrote:
>>> On 2020/5/14 0:11, Roman Gushchin wrote:
>>>> On Wed, May 13, 2020 at 07:47:49PM +0800, Zefan Li wrote:
>>>>> While trying to use remote memcg charging in an out-of-tree kernel module
>>>>> I found it's not working, because the current thread is a workqueue thread.
>>>>>
>>>>> As we will probably encounter this issue in the future as the users of
>>>>> memalloc_use_memcg() grow, it's better we fix it now.
>>>>>
>>>>> Signed-off-by: Zefan Li <lizefan@huawei.com>
>>>>> ---
>>>>>
>>>>> v2: add a comment as sugguested by Michal. and add changelog to explain why
>>>>> upstream kernel needs this fix.
>>>>>
>>>>> ---
>>>>>
>>>>>  mm/memcontrol.c | 3 +++
>>>>>  1 file changed, 3 insertions(+)
>>>>>
>>>>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>>>>> index a3b97f1..43a12ed 100644
>>>>> --- a/mm/memcontrol.c
>>>>> +++ b/mm/memcontrol.c
>>>>> @@ -2802,6 +2802,9 @@ static void memcg_schedule_kmem_cache_create(struct mem_cgroup *memcg,
>>>>>  
>>>>>  static inline bool memcg_kmem_bypass(void)
>>>>>  {
>>>>> +	/* Allow remote memcg charging in kthread contexts. */
>>>>> +	if (unlikely(current->active_memcg))
>>>>> +		return false;
>>>>>  	if (in_interrupt() || !current->mm || (current->flags & PF_KTHREAD))
>>>>>  		return true;
>>>>
>>>> Shakeel is right about interrupts. How about something like this?
>>>>
>>>> static inline bool memcg_kmem_bypass(void)
>>>> {
>>>> 	if (in_interrupt())
>>>> 		return true;
>>>>
>>>> 	if ((!current->mm || current->flags & PF_KTHREAD) && !current->active_memcg)
>>>> 		return true;
>>>>
>>>> 	return false;
>>>> }
>>>>
>>>
>>> I thought the user should ensure not do this, but now I think it makes sense to just bypass
>>> the interrupt case.
>>
>> I think now it's mostly a legacy of the opt-out kernel memory accounting.
>> Actually we can relax this requirement by forcibly overcommit the memory cgroup
>> if the allocation is happening from the irq context, and punish it afterwards.
>> Idk how much we wanna this, hopefully nobody is allocating large non-temporarily
>> objects from an irq.
> 
> I do not think we want to pretend that remote charging from the IRQ
> context is supported. Why don't we simply WARN_ON(in_interrupt()) there?
> 

How about:

static inline bool memcg_kmem_bypass(void)
{
        if (in_interrupt()) {
                WARN_ON(current->active_memcg);
                return true;
        }

        /* Allow remote memcg charging in kthread contexts. */
        if ((!current->mm || (current->flags & PF_KTHREAD)) && !current->active_memcg)
                return true;
        return false;
}