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

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

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?

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
>>
>>