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@linux.dev>
---
 mm/bpf_memcontrol.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/mm/bpf_memcontrol.c b/mm/bpf_memcontrol.c
index dacdf53735e5..94bc6c17d80b 100644
--- a/mm/bpf_memcontrol.c
+++ b/mm/bpf_memcontrol.c
@@ -10,6 +10,12 @@
 
 __bpf_kfunc_start_defs();
 
+__bpf_kfunc struct mem_cgroup *bpf_get_root_mem_cgroup(void)
+{
+	/* css_get() is not needed */
+	return root_mem_cgroup;
+}
+
 __bpf_kfunc struct mem_cgroup *
 bpf_get_mem_cgroup(struct cgroup_subsys_state *css)
 {
@@ -72,6 +78,7 @@ __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)
 
-- 
2.49.0.901.g37484f566f-goog