Re: [PATCH v1 05/14] mm: introduce bpf_get_root_mem_cgroup() bpf kfunc

[Kumar Kartikeya Dwivedi <memxor@gmail.com>]

On Mon, 18 Aug 2025 at 19:02, Roman Gushchin <roman.gushchin@linux.dev> 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 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 66f2a359af7e..a8faa561bcba 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;
> +}
> +
>  /**
>   * bpf_get_mem_cgroup - Get a reference to a memory cgroup
>   * @css: pointer to the css structure
> @@ -122,6 +136,7 @@ __bpf_kfunc void bpf_mem_cgroup_flush_stats(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)

Same suggestion here (re: trusted args).

>  BTF_ID_FLAGS(func, bpf_get_mem_cgroup, KF_ACQUIRE | KF_RET_NULL)
>  BTF_ID_FLAGS(func, bpf_put_mem_cgroup, KF_RELEASE)
>
> --
> 2.50.1
>