Re: [PATCH v6 31/33] mm: memcontrol: convert objcg to be per-memcg per-node type

[Usama Arif <usama.arif@linux.dev>]

On 07/03/2026 08:51, Qi Zheng wrote:
> Hi Usama,
> 
> On 3/7/26 4:29 AM, Usama Arif wrote:
>> On Thu,  5 Mar 2026 19:52:49 +0800 Qi Zheng <qi.zheng@linux.dev> wrote:
>>
>>> From: Qi Zheng <zhengqi.arch@bytedance.com>
>>>
>>> Convert objcg to be per-memcg per-node type, so that when reparent LRU
>>> folios later, we can hold the lru lock at the node level, thus avoiding
>>> holding too many lru locks at once.
>>>
>>> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
>>> Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
>>> ---
>>>   include/linux/memcontrol.h | 23 +++++------
>>>   include/linux/sched.h      |  2 +-
>>>   mm/memcontrol.c            | 79 +++++++++++++++++++++++---------------
>>>   3 files changed, 62 insertions(+), 42 deletions(-)
>>>
>>
>> [...]
>>
>>> @@ -4087,7 +4100,13 @@ static int mem_cgroup_css_online(struct cgroup_subsys_state *css)
>>>       xa_store(&mem_cgroup_private_ids, memcg->id.id, memcg, GFP_KERNEL);
>>>         return 0;
>>> -free_shrinker:
>>> +free_objcg:
>>> +    for_each_node(nid) {
>>> +        struct mem_cgroup_per_node *pn = memcg->nodeinfo[nid];
>>> +
>>> +        if (pn && pn->orig_objcg)
>>> +            obj_cgroup_put(pn->orig_objcg);
>>
>> Is it possible that you might call obj_cgroup_put twice on the same cgroup?
> 
> Oh, I think you are right. Here pn->orig_objcg was not reset to NULL, so
> obj_cgroup_put() will be called in __mem_cgroup_free() again.
> 
>>
>> If css_create fails, css_free_rwork_fn is queued, which ends up calling
>> mem_cgroup_css_free which calls obj_cgroup_put again?
>>
>> Maybe adding pn->orig_objcg = NULL overhere after obj_cgroup_put
>> is enough to prevent the double put from causing issues?
> 
> Agree.
> 
> Like this?
> 

Yes below looks good! Might be good to add a comment as well why setting
it to NULL.

> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 992a3f5caa62b..e0795aec4356b 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -4140,8 +4140,10 @@ static int mem_cgroup_css_online(struct cgroup_subsys_state *css)
>         for_each_node(nid) {
>                 struct mem_cgroup_per_node *pn = memcg->nodeinfo[nid];
> 
> -               if (pn && pn->orig_objcg)
> +               if (pn && pn->orig_objcg) {
>                         obj_cgroup_put(pn->orig_objcg);
> +                       pn->orig_objcg = NULL;
> +               }
>         }
>         free_shrinker_info(memcg);
>  offline_kmem:
> 
> If there are no problems, I will send a fix patch later.
> 
> Thanks,
> Qi
> 
>>
>>> +    }
>>>       free_shrinker_info(memcg);
>>>   offline_kmem:
>>>       memcg_offline_kmem(memcg);
>>> -- 
>>> 2.20.1
>>>
>>>
>