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

[Zefan Li <lizefan@huawei.com>]

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.