linux-kernel - [PATCH v2 09/23] mm: introduce bpf_get_root_mem

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Message-ID: <20251027231727.472628-10-roman.gushchin@linux.dev>
Date: Mon, 27 Oct 2025 16:17:12 -0700
From: Roman Gushchin <roman.gushchin@...ux.dev>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-kernel@...r.kernel.org,
	Alexei Starovoitov <ast@...nel.org>,
	Suren Baghdasaryan <surenb@...gle.com>,
	Michal Hocko <mhocko@...nel.org>,
	Shakeel Butt <shakeel.butt@...ux.dev>,
	Johannes Weiner <hannes@...xchg.org>,
	Andrii Nakryiko <andrii@...nel.org>,
	JP Kobryn <inwardvessel@...il.com>,
	linux-mm@...ck.org,
	cgroups@...r.kernel.org,
	bpf@...r.kernel.org,
	Martin KaFai Lau <martin.lau@...nel.org>,
	Song Liu <song@...nel.org>,
	Kumar Kartikeya Dwivedi <memxor@...il.com>,
	Tejun Heo <tj@...nel.org>,
	Roman Gushchin <roman.gushchin@...ux.dev>
Subject: [PATCH v2 09/23] mm: introduce bpf_get_root_mem_cgroup() BPF kfunc

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@...ux.dev>
---
 mm/bpf_memcontrol.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/mm/bpf_memcontrol.c b/mm/bpf_memcontrol.c
index 1e46097745cf..76c342318256 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
@@ -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_ACQUIRE | KF_RET_NULL | KF_RCU)
 BTF_ID_FLAGS(func, bpf_put_mem_cgroup, KF_RELEASE)
 
-- 
2.51.0