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 --- mm/bpf_memcontrol.c | 15 +++++++++++++++ 1 file changed, 15 insertions(+) diff --git a/mm/bpf_memcontrol.c b/mm/bpf_memcontrol.c index 66f2a359af7e..a8faa561bcba 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 @@ -122,6 +136,7 @@ __bpf_kfunc void bpf_mem_cgroup_flush_stats(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) BTF_ID_FLAGS(func, bpf_put_mem_cgroup, KF_RELEASE) -- 2.50.1