linux-kernel - Re: [PATCH bpf-next v2 3/7] mm: introduce bpf_get_root_mem

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Message-ID: <he4iziqotq6ecucq5ipqukwnetonwyzl2gm5ej7ii3ltjc3lue@xpqxrvcblquj>
Date: Fri, 19 Dec 2025 21:21:32 -0800
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Roman Gushchin <roman.gushchin@...ux.dev>
Cc: bpf@...r.kernel.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
	JP Kobryn <inwardvessel@...il.com>, Alexei Starovoitov <ast@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, Michal Hocko <mhocko@...nel.org>, 
	Johannes Weiner <hannes@...xchg.org>
Subject: Re: [PATCH bpf-next v2 3/7] mm: introduce bpf_get_root_mem_cgroup()
 BPF kfunc

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@...ux.dev>

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