Commit 5c7db323 authored Dec 22, 2025 by Roman Gushchin Committed by Alexei Starovoitov Dec 22, 2025

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 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>
Link: https://lore.kernel.org/r/20251223044156.208250-4-roman.gushchin@linux.dev


Signed-off-by: Alexei Starovoitov <ast@kernel.org>

parent 5904db98

mm/bpf_memcontrol.c

+20 −0

Original line number	Diff line number	Diff line
		@@ -10,6 +10,25 @@

		__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()
		*
		* Return: A pointer to the root memory cgroup.
		*/
		__bpf_kfunc struct mem_cgroup *bpf_get_root_mem_cgroup(void)
		{
		if (mem_cgroup_disabled())
		return NULL;

		/* 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 +83,7 @@ __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)