Re: [PATCH v2 2/3] mm: memcg: redirect stats updates of dying memcgs for all hierarchies

[Shakeel Butt <shakeel.butt@linux.dev>]

On Mon, Aug 31, 2026 at 05:46:10PM +0800, Hui Zhu wrote:
> From: Hui Zhu <zhuhui@kylinos.cn>
> 
> get_non_dying_memcg_start() redirects the stat updates of a dying memcg to
> its closest non-dying ancestor, but only on cgroup v1; on cgroup v2 the
> stats keep being accounted to the dying memcg itself.
> 
> The previous patch restored lruvec_page_state_local() in
> count_shadow_nodes(), which reads those state_locals on cgroup v2 too, so
> apply the redirection to all hierarchies.  Offlining is rare, so the added
> cost on the stat update fast path is limited to an rcu_read_lock() and a
> css_is_dying() check; the upward walk happens only while a memcg is dying.
> 
> Signed-off-by: Hui Zhu <zhuhui@kylinos.cn>

If we want to backport the first patch to stable trees then we need to backport
this one too.

> ---
>  mm/memcontrol.c | 30 +++++-------------------------
>  1 file changed, 5 insertions(+), 25 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 8319ad8c5c23..b3d1ac3fe0aa 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -805,20 +805,14 @@ static long memcg_state_val_in_pages(int idx, long val)
>  	return val < 0 ? -res : res;
>  }
>  
> -#ifdef CONFIG_MEMCG_V1
>  /*
> - * Used in mod_memcg_state() and mod_memcg_lruvec_state() to avoid race with
> - * reparenting of non-hierarchical state_locals.
> + * Used in mod_memcg_state() and mod_memcg_lruvec_state() to avoid race
> + * with reparenting of non-hierarchical state_locals.  Offlining a
> + * memcg is rare, so do the redirection for all cgroup hierarchies.
>   */
> -static inline struct mem_cgroup *get_non_dying_memcg_start(struct mem_cgroup *memcg,
> -							   bool *rcu_locked)
> +static inline struct mem_cgroup *
> +get_non_dying_memcg_start(struct mem_cgroup *memcg, bool *rcu_locked)
>  {
> -	/* Rebinding can cause this value to be changed at runtime */
> -	if (cgroup_subsys_on_dfl(memory_cgrp_subsys)) {
> -		*rcu_locked = false;
> -		return memcg;
> -	}
> -
>  	rcu_read_lock();
>  	*rcu_locked = true;
>  
> @@ -830,22 +824,8 @@ static inline struct mem_cgroup *get_non_dying_memcg_start(struct mem_cgroup *me
>  
>  static inline void get_non_dying_memcg_end(bool rcu_locked)
>  {
> -	if (!rcu_locked)
> -		return;
> -
>  	rcu_read_unlock();
>  }
> -#else
> -static inline struct mem_cgroup *get_non_dying_memcg_start(struct mem_cgroup *memcg,
> -							   bool *rcu_locked)
> -{
> -	return memcg;
> -}
> -
> -static inline void get_non_dying_memcg_end(bool rcu_locked)
> -{
> -}
> -#endif
>  
>  static void __mod_memcg_state(struct mem_cgroup *memcg,
>  			      enum memcg_stat_item idx, long val)
> -- 
> 2.53.0
>