Re: [PATCH v5 29/32] mm: memcontrol: prepare for reparenting non-hierarchical stats

[Qi Zheng <qi.zheng@linux.dev>]

On 2/26/26 9:41 AM, Shakeel Butt wrote:
> On Wed, Feb 25, 2026 at 03:53:12PM +0800, Qi Zheng wrote:
>> From: Qi Zheng <zhengqi.arch@bytedance.com>
>>
>> To resolve the dying memcg issue, we need to reparent LRU folios of child
>> memcg to its parent memcg. This could cause problems for non-hierarchical
>> stats.
>>
>> As Yosry Ahmed pointed out:
>>
>> ```
>> In short, if memory is charged to a dying cgroup at the time of
>> reparenting, when the memory gets uncharged the stats updates will occur
>> at the parent. This will update both hierarchical and non-hierarchical
>> stats of the parent, which would corrupt the parent's non-hierarchical
>> stats (because those counters were never incremented when the memory was
>> charged).
>> ```
>>
>> Now we have the following two types of non-hierarchical stats, and they
>> are only used in CONFIG_MEMCG_V1:
>>
>> a. memcg->vmstats->state_local[i]
>> b. pn->lruvec_stats->state_local[i]
>>
>> To ensure that these non-hierarchical stats work properly, we need to
>> reparent these non-hierarchical stats after reparenting LRU folios. To
>> this end, this commit makes the following preparations:
>>
>> 1. implement reparent_state_local() to reparent non-hierarchical stats
>> 2. make css_killed_work_fn() to be called in rcu work, and implement
>>     get_non_dying_memcg_start() and get_non_dying_memcg_end() to avoid race
>>     between mod_memcg_state()/mod_memcg_lruvec_state()
>>     and reparent_state_local()
>> 3. change these non-hierarchical stats to atomic_long_t type to avoid race
>>     between mem_cgroup_stat_aggregate() and reparent_state_local()
>>
>> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
> 
> [...]
> 
>> --- a/mm/memcontrol-v1.h
>> +++ b/mm/memcontrol-v1.h
>> @@ -45,6 +45,7 @@ static inline bool do_memsw_account(void)
>>   
>>   unsigned long memcg_events_local(struct mem_cgroup *memcg, int event);
>>   unsigned long memcg_page_state_local(struct mem_cgroup *memcg, int idx);
>> +void mod_memcg_page_state_local(struct mem_cgroup *memcg, int idx, unsigned long val);
> 
> The above seems like addition by mistake.

Yes, my mistake, need to be deleted directly. ;)

> 
> After fixing this, you can add:
> 
> Acked-by: Shakeel Butt <shakeel.butt@linux.dev>

Thanks!

>