Re: [PATCH v1 06/26] mm: memcontrol: return root object cgroup for root memory cgroup

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

Hi Harry,

On 11/17/25 5:17 PM, Harry Yoo wrote:
> On Tue, Oct 28, 2025 at 09:58:19PM +0800, Qi Zheng wrote:
>> From: Muchun Song <songmuchun@bytedance.com>
>>
>> Memory cgroup functions such as get_mem_cgroup_from_folio() and
>> get_mem_cgroup_from_mm() return a valid memory cgroup pointer,
>> even for the root memory cgroup. In contrast, the situation for
>> object cgroups has been different.
>>
>> Previously, the root object cgroup couldn't be returned because
>> it didn't exist. Now that a valid root object cgroup exists, for
>> the sake of consistency, it's necessary to align the behavior of
>> object-cgroup-related operations with that of memory cgroup APIs.
>>
>> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
>> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
>> ---
>>   include/linux/memcontrol.h | 29 +++++++++++++++++-------
>>   mm/memcontrol.c            | 45 ++++++++++++++++++++------------------
>>   mm/percpu.c                |  2 +-
>>   3 files changed, 46 insertions(+), 30 deletions(-)
>>
>> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
>> index 6185d8399a54e..9fdbd4970021d 100644
>> --- a/include/linux/memcontrol.h
>> +++ b/include/linux/memcontrol.h
>> @@ -332,6 +332,7 @@ struct mem_cgroup {
>>   #define MEMCG_CHARGE_BATCH 64U
>>   
>>   extern struct mem_cgroup *root_mem_cgroup;
>> +extern struct obj_cgroup *root_obj_cgroup;
>>   
>>   enum page_memcg_data_flags {
>>   	/* page->memcg_data is a pointer to an slabobj_ext vector */
>> @@ -549,6 +550,11 @@ static inline bool mem_cgroup_is_root(struct mem_cgroup *memcg)
>>   	return (memcg == root_mem_cgroup);
>>   }
>>   
>> +static inline bool obj_cgroup_is_root(const struct obj_cgroup *objcg)
>> +{
>> +	return objcg == root_obj_cgroup;
>> +}
> 
> After reparenting, an objcg may satisfy objcg->memcg == root_mem_cgroup
> while objcg != root_obj_cgroup. Should they be considered as
> root objcgs?

Indeed, it's pointless to charge to root_mem_cgroup (objcg->memcg).

So it should be:

static inline bool obj_cgroup_is_root(const struct obj_cgroup *objcg)
{
	return (objcg == root_obj_cgroup) || (objcg->memcg == root_mem_cgroup);
}

> 
>>   static inline bool mem_cgroup_disabled(void)
>>   {
>>   	return !cgroup_subsys_enabled(memory_cgrp_subsys);
>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>> index 2afd7f99ca101..d484b632c790f 100644
>> --- a/mm/memcontrol.c
>> +++ b/mm/memcontrol.c
>> @@ -2871,7 +2865,7 @@ int __memcg_kmem_charge_page(struct page *page, gfp_t gfp, int order)
>>   	int ret = 0;
>>   
>>   	objcg = current_obj_cgroup();
>> -	if (objcg) {
>> +	if (!obj_cgroup_is_root(objcg)) {
> 
> Now that we support the page and slab allocators support allocating memory
> in NMI contexts (on some archs), current_obj_cgroup() can return NULL
> if (IS_ENABLED(CONFIG_MEMCG_NMI_UNSAFE) && in_nmi()) returns true
> (then it leads to a NULL-pointer-deref bug).
> 
> But IIUC this is applied to kmem charging only (as they use this_cpu ops
> for stats update), and we don't have to apply the same restriction to
> charging LRU pages with objcg.
> 
> Maybe Shakeel has more insight on this.
> 
> Link: https://lore.kernel.org/all/20250519063142.111219-1-shakeel.butt@linux.dev

Thanks for this information, and it seems there's nothing wrong here.

Thanks,
Qi

>