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

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Message-ID: <aRroO9ypxvHsAjug@hyeyoo>
Date: Mon, 17 Nov 2025 18:17:47 +0900
From: Harry Yoo <harry.yoo@...cle.com>
To: Qi Zheng <qi.zheng@...ux.dev>
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 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?

>  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

-- 
Cheers,
Harry / Hyeonggon