From: Shakeel Butt <shakeel.butt@linux.dev>
To: Roman Gushchin <roman.gushchin@linux.dev>
Cc: bpf@vger.kernel.org, linux-mm@kvack.org,
linux-kernel@vger.kernel.org, JP Kobryn <inwardvessel@gmail.com>,
Alexei Starovoitov <ast@kernel.org>,
Daniel Borkmann <daniel@iogearbox.net>,
Michal Hocko <mhocko@kernel.org>,
Johannes Weiner <hannes@cmpxchg.org>
Subject: Re: [PATCH bpf-next v2 3/7] mm: introduce bpf_get_root_mem_cgroup() BPF kfunc
Date: Fri, 19 Dec 2025 21:21:32 -0800 [thread overview]
Message-ID: <he4iziqotq6ecucq5ipqukwnetonwyzl2gm5ej7ii3ltjc3lue@xpqxrvcblquj> (raw)
In-Reply-To: <20251220041250.372179-4-roman.gushchin@linux.dev>
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@linux.dev>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
next prev parent reply other threads:[~2025-12-20 5:21 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-12-20 4:12 [PATCH bpf-next v2 0/7] mm: bpf kfuncs to access memcg data Roman Gushchin
2025-12-20 4:12 ` [PATCH bpf-next v2 1/7] mm: declare memcg_page_state_output() in memcontrol.h Roman Gushchin
2025-12-20 4:12 ` [PATCH bpf-next v2 2/7] mm: introduce BPF kfuncs to deal with memcg pointers Roman Gushchin
2025-12-20 5:20 ` Shakeel Butt
2025-12-22 0:39 ` Alexei Starovoitov
2025-12-20 4:12 ` [PATCH bpf-next v2 3/7] mm: introduce bpf_get_root_mem_cgroup() BPF kfunc Roman Gushchin
2025-12-20 5:21 ` Shakeel Butt [this message]
2025-12-20 4:12 ` [PATCH bpf-next v2 4/7] mm: introduce BPF kfuncs to access memcg statistics and events Roman Gushchin
2025-12-20 4:29 ` bot+bpf-ci
2025-12-20 4:39 ` Roman Gushchin
2025-12-20 5:22 ` Shakeel Butt
2025-12-20 4:12 ` [PATCH bpf-next v2 5/7] mm: introduce BPF kfunc to access memory events Roman Gushchin
2025-12-20 4:29 ` bot+bpf-ci
2025-12-20 4:41 ` Roman Gushchin
2025-12-20 13:19 ` Chris Mason
2025-12-20 18:43 ` Roman Gushchin
2025-12-20 19:59 ` Chris Mason
2025-12-22 22:23 ` Roman Gushchin
2025-12-23 14:09 ` Chris Mason
2025-12-22 0:49 ` Alexei Starovoitov
2025-12-22 0:51 ` Alexei Starovoitov
2025-12-20 4:12 ` [PATCH bpf-next v2 6/7] bpf: selftests: selftests for memcg stat kfuncs Roman Gushchin
2025-12-20 5:23 ` Shakeel Butt
2025-12-20 4:12 ` [PATCH bpf-next v2 7/7] MAINTAINERS: add an entry for MM BPF extensions Roman Gushchin
2025-12-20 5:26 ` Shakeel Butt
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