mm/memcontrol.c: convert mem_cgroup_id::ref to refcount_t type

This will allow to use generic refcount_t interfaces to check counters
overflow instead of currently existing VM_BUG_ON().  The only difference
after the patch is VM_BUG_ON() may cause BUG(), while refcount_t fires
with WARN().  But this seems not to be significant here, since such the
problems are usually caught by syzbot with panic-on-warn enabled.

Link: http://lkml.kernel.org/r/153910718919.7006.13400779039257185427.stgit@localhost.localdomain
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
Cc: Andrea Parri <andrea.parri@amarulasolutions.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/memcontrol.h

@@ -78,7 +78,7 @@ struct mem_cgroup_reclaim_cookie {
 
 struct mem_cgroup_id {
 	int id;
-	atomic_t ref;
+	refcount_t ref;
 };
 
 /*

mm/memcontrol.c

@@ -4299,14 +4299,12 @@ static void mem_cgroup_id_remove(struct mem_cgroup *memcg)
 
 static void mem_cgroup_id_get_many(struct mem_cgroup *memcg, unsigned int n)
 {
-	VM_BUG_ON(atomic_read(&memcg->id.ref) <= 0);
-	atomic_add(n, &memcg->id.ref);
+	refcount_add(n, &memcg->id.ref);
 }
 
 static void mem_cgroup_id_put_many(struct mem_cgroup *memcg, unsigned int n)
 {
-	VM_BUG_ON(atomic_read(&memcg->id.ref) < n);
-	if (atomic_sub_and_test(n, &memcg->id.ref)) {
+	if (refcount_sub_and_test(n, &memcg->id.ref)) {
 		mem_cgroup_id_remove(memcg);
 
 		/* Memcg ID pins CSS */
@@ -4523,7 +4521,7 @@ static int mem_cgroup_css_online(struct cgroup_subsys_state *css)
 	}
 
 	/* Online state pins memcg ID, memcg ID pins CSS */
-	atomic_set(&memcg->id.ref, 1);
+	refcount_set(&memcg->id.ref, 1);
 	css_get(css);
 	return 0;
 }
@@ -6357,7 +6355,7 @@ subsys_initcall(mem_cgroup_init);
 #ifdef CONFIG_MEMCG_SWAP
 static struct mem_cgroup *mem_cgroup_id_get_online(struct mem_cgroup *memcg)
 {
-	while (!atomic_inc_not_zero(&memcg->id.ref)) {
+	while (!refcount_inc_not_zero(&memcg->id.ref)) {
 		/*
 		 * The root cgroup cannot be destroyed, so it's refcount must
 		 * always be >= 1.