Re: [PATCH bpf-next 13/15] mm, memcg: Add new helper get_obj_cgroup_from_cgroup

[Roman Gushchin <roman.gushchin@linux.dev>]

On Wed, Aug 10, 2022 at 03:18:38PM +0000, Yafang Shao wrote:
> Introduce new helper get_obj_cgroup_from_cgroup() to get obj_cgroup from
> a specific cgroup.
> 
> Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
> ---
>  include/linux/memcontrol.h |  1 +
>  mm/memcontrol.c            | 41 +++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 42 insertions(+)
> 
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 2f0a611..901a921 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -1713,6 +1713,7 @@ static inline void set_shrinker_bit(struct mem_cgroup *memcg,
>  int __memcg_kmem_charge_page(struct page *page, gfp_t gfp, int order);
>  void __memcg_kmem_uncharge_page(struct page *page, int order);
>  
> +struct obj_cgroup *get_obj_cgroup_from_cgroup(struct cgroup *cgrp);
>  struct obj_cgroup *get_obj_cgroup_from_current(void);
>  struct obj_cgroup *get_obj_cgroup_from_page(struct page *page);
>  
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 618c366..762cffa 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2908,6 +2908,47 @@ static struct obj_cgroup *__get_obj_cgroup_from_memcg(struct mem_cgroup *memcg)
>  	return objcg;
>  }
>  
> +static struct obj_cgroup *get_obj_cgroup_from_memcg(struct mem_cgroup *memcg)
> +{
> +	struct obj_cgroup *objcg;
> +
> +	if (memcg_kmem_bypass())
> +		return NULL;
> +
> +	rcu_read_lock();
> +	objcg = __get_obj_cgroup_from_memcg(memcg);
> +	rcu_read_unlock();
> +	return objcg;

This code doesn't make sense to me. What does rcu read lock protect here?