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

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Message-ID: <17ce88c0-a5d5-4315-8074-6c0a4a4e7d64@linux.dev>
Date: Tue, 18 Nov 2025 19:31:01 +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 11/17/25 5:41 PM, Harry Yoo wrote:
> On Mon, Nov 17, 2025 at 06:17:47PM +0900, Harry Yoo wrote:
>> On Tue, Oct 28, 2025 at 09:58:19PM +0800, Qi Zheng wrote:
>>> 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).
> 
> This is a real issue, but
> 
>> 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.
> 
> actually this should be fine for now since we use get_mem_cgroup_from_mm()
> and obj_cgroup_from_memcg() instead of current_obj_cgroup() when charging
> LRU pages.
> 
> But it is not immediately obvious that there are multiple ways to get
> an objcg, each with different restrictions depending on what you are
> going to charge :/

Perhaps some comments need to be added. :)

> 
>> Maybe Shakeel has more insight on this.
>>
>> Link: https://lore.kernel.org/all/20250519063142.111219-1-shakeel.butt@linux.dev
>