Re: [PATCH bpf-next v1 3/6] mm: introduce bpf_get_root_mem_cgroup() BPF kfunc

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

On Thu, Dec 18, 2025 at 05:57:47PM -0800, Roman Gushchin wrote:
> Introduce a BPF kfunc to get a trusted pointer to the root memory
> cgroup. It's very handy to traverse the full memcg tree, e.g.
> for handling a system-wide OOM.
> 
> It's possible to obtain this pointer by traversing the memcg tree
> up from any known memcg, but it's sub-optimal and makes BPF programs
> more complex and less efficient.
> 
> bpf_get_root_mem_cgroup() has a KF_ACQUIRE | KF_RET_NULL semantics,
> however in reality it's not necessarily

necessary*

> to bump the corresponding
> reference counter - root memory cgroup is immortal, reference counting
> is skipped, see css_get(). Once set, root_mem_cgroup is always a valid
> memcg pointer. It's safe to call bpf_put_mem_cgroup() for the pointer
> obtained with bpf_get_root_mem_cgroup(), it's effectively a no-op.
> 
> Signed-off-by: Roman Gushchin <roman.gushchin@linux.dev>
> ---
>  mm/bpf_memcontrol.c | 15 +++++++++++++++
>  1 file changed, 15 insertions(+)
> 
> diff --git a/mm/bpf_memcontrol.c b/mm/bpf_memcontrol.c
> index 8aa842b56817..6d0d73bf0dd1 100644
> --- a/mm/bpf_memcontrol.c
> +++ b/mm/bpf_memcontrol.c
> @@ -10,6 +10,20 @@
>  
>  __bpf_kfunc_start_defs();
>  
> +/**
> + * bpf_get_root_mem_cgroup - Returns a pointer to the root memory cgroup
> + *
> + * The function has KF_ACQUIRE semantics, even though the root memory
> + * cgroup is never destroyed after being created and doesn't require
> + * reference counting. And it's perfectly safe to pass it to
> + * bpf_put_mem_cgroup()
> + */
> +__bpf_kfunc struct mem_cgroup *bpf_get_root_mem_cgroup(void)
> +{
> +	/* css_get() is not needed */
> +	return root_mem_cgroup;

I think we need mem_cgroup_disabled() check here as well because I think
root_mem_cgroup can get allocated before memcg can be disabled due to
boot param.

> +}
> +
>  /**
>   * bpf_get_mem_cgroup - Get a reference to a memory cgroup
>   * @css: pointer to the css structure
> @@ -64,6 +78,7 @@ __bpf_kfunc void bpf_put_mem_cgroup(struct mem_cgroup *memcg)
>  __bpf_kfunc_end_defs();
>  
>  BTF_KFUNCS_START(bpf_memcontrol_kfuncs)
> +BTF_ID_FLAGS(func, bpf_get_root_mem_cgroup, KF_ACQUIRE | KF_RET_NULL)
>  BTF_ID_FLAGS(func, bpf_get_mem_cgroup, KF_TRUSTED_ARGS | KF_ACQUIRE | KF_RET_NULL | KF_RCU)
>  BTF_ID_FLAGS(func, bpf_put_mem_cgroup, KF_TRUSTED_ARGS | KF_RELEASE)
>  
> -- 
> 2.52.0
>