Re: [PATCH v2 1/3] mm: Fix dropped memcg from mem cgroup soft limit tree

[Tim Chen]

On 3/5/21 1:11 AM, Michal Hocko wrote:
> On Thu 04-03-21 09:35:08, Tim Chen wrote:
>>
>>
>> On 2/18/21 11:13 AM, Michal Hocko wrote:
>>
>>>
>>> Fixes: 4e41695356fb ("memory controller: soft limit reclaim on contention")
>>> Acked-by: Michal Hocko <mhocko@xxxxxxxx>
>>>
>>> Thanks!
>>>> ---
>>>>  mm/memcontrol.c | 6 +++++-
>>>>  1 file changed, 5 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>>>> index ed5cc78a8dbf..a51bf90732cb 100644
>>>> --- a/mm/memcontrol.c
>>>> +++ b/mm/memcontrol.c
>>>> @@ -3505,8 +3505,12 @@ unsigned long mem_cgroup_soft_limit_reclaim(pg_data_t *pgdat, int order,
>>>>  			loop > MEM_CGROUP_MAX_SOFT_LIMIT_RECLAIM_LOOPS))
>>>>  			break;
>>>>  	} while (!nr_reclaimed);
>>>> -	if (next_mz)
>>>> +	if (next_mz) {
>>>> +		spin_lock_irq(&mctz->lock);
>>>> +		__mem_cgroup_insert_exceeded(next_mz, mctz, excess);
>>>> +		spin_unlock_irq(&mctz->lock);
>>>>  		css_put(&next_mz->memcg->css);
>>>> +	}
>>>>  	return nr_reclaimed;
>>>>  }
>>>>  
>>>> -- 
>>>> 2.20.1
>>>
>>
>> Mel,
>>
>> Reviewing this patch a bit more, I realize that there is a chance that the removed
>> next_mz could be inserted back to the tree from a memcg_check_events
>> that happen in between.  So we need to make sure that the next_mz
>> is indeed off the tree and update the excess value before adding it
>> back.  Update the patch to the patch below.
> 
> This scenario is certainly possible but it shouldn't really matter much
> as __mem_cgroup_insert_exceeded bails out when the node is on the tree
> already.
> 

Makes sense. We should still update the excess value with

+		excess = soft_limit_excess(next_mz->memcg);
+		__mem_cgroup_insert_exceeded(next_mz, mctz, excess);

before doing insertion.  The excess value was recorded from previous
mz in the loop and needs to be updated to that of next_mz.

Tim