Re: [PATCH bpf-next v2 3/7] mm: introduce bpf_get_root_mem_cgroup() BPF kfunc

[Shakeel Butt]

On Fri, Dec 19, 2025 at 08:12:46PM -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 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@xxxxxxxxx>

Acked-by: Shakeel Butt <shakeel.butt@xxxxxxxxx>