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mm: introduce bpf_get_root_mem_cgroup() BPF kfunc

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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>
This commit is contained in:
Roman Gushchin 2025-12-22 20:41:53 -08:00 committed by Alexei Starovoitov
parent 5904db9891
commit 5c7db3239c
1 changed files with 20 additions and 0 deletions
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20
mm/bpf_memcontrol.c
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@ -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)