mm: memcontrol: let v2 cgroups follow changes in system swappiness

Cgroup2 currently doesn't have a per-cgroup swappiness setting. We
might want to add one later - that's a different discussion - but until
we do, the cgroups should always follow the system setting. Otherwise
it will be unchangeably set to whatever the ancestor inherited from the
system setting at the time of cgroup creation.

Signed-off-by: Johannes Weiner <[e-mail]>
Acked-by: Michal Hocko <[e-mail]>
Acked-by: Vladimir Davydov <[e-mail]>
Cc: <[e-mail]> [4.5]
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/swap.h

···
 #ifdef CONFIG_MEMCG
 static inline int mem_cgroup_swappiness(struct mem_cgroup *memcg)
 {
+	/* Cgroup2 doesn't have per-cgroup swappiness */
+	if (cgroup_subsys_on_dfl(memory_cgrp_subsys))
+		return vm_swappiness;
+
 	/* root ? */
 	if (mem_cgroup_disabled() || !memcg->css.parent)
 		return vm_swappiness;