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

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Message-ID: <938180ff-7e24-4d73-87d5-fca4bbaa4ced@linux.dev>
Date: Wed, 19 Nov 2025 15:42:51 +0800
From: Qi Zheng <qi.zheng@...ux.dev>
To: Harry Yoo <harry.yoo@...cle.com>
Cc: hannes@...xchg.org, hughd@...gle.com, mhocko@...e.com,
 roman.gushchin@...ux.dev, shakeel.butt@...ux.dev, muchun.song@...ux.dev,
 david@...hat.com, lorenzo.stoakes@...cle.com, ziy@...dia.com,
 imran.f.khan@...cle.com, kamalesh.babulal@...cle.com,
 axelrasmussen@...gle.com, yuanchu@...gle.com, weixugc@...gle.com,
 akpm@...ux-foundation.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 cgroups@...r.kernel.org, Muchun Song <songmuchun@...edance.com>,
 Qi Zheng <zhengqi.arch@...edance.com>
Subject: Re: [PATCH v1 06/26] mm: memcontrol: return root object cgroup for
 root memory cgroup



On 11/19/25 3:24 PM, Harry Yoo wrote:
> On Tue, Nov 18, 2025 at 08:11:04PM +0800, Qi Zheng wrote:
>>
>>
>> On 11/18/25 7:28 PM, Qi Zheng wrote:
>>> 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@...edance.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@...edance.com>
>>>>> Signed-off-by: Qi Zheng <zhengqi.arch@...edance.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);
>>> }
>>
>> Oh, we can't do that because we still need to consider this objcg when
>> uncharging. Some pages may be charged before reparenting.
> 
> Ouch, right. We don't know if it's charged before reparenting and so
> it can break statistics in a few places if we skip uncharging it after
> repareting.

Right.

> 
> And I think we don't charge new pages to objcgs that satisfy
> (objcg->memcg == root_mem_cgroup) && (objcg != root_obj_cgroup)
> after they're reparented anyway...

The charge and uncharge operations must be symmetrical, so we cannot
control the charge operation independently.

Otherwise:

charge
======

if ((objcg->memcg == root_mem_cgroup))
	skip charge this page

uncharge
========

we can't decide whether to skip this page.

Thanks,
Qi


>