mm: memcontrol: use mem_cgroup_is_root() helper

Replace the checks for memcg is root memcg, with mem_cgroup_is_root()
helper.

Link: https://lkml.kernel.org/r/20220930134433.338103-1-kamalesh.babulal@oracle.comSigned-off-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>
Reviewed-by: Muchun Song <songmuchun@bytedance.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kamalesh Babulal <kamalesh.babulal@oracle.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Tom Hromatka <tom.hromatka@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/memcontrol.c

@@ -1219,7 +1219,7 @@ static void invalidate_reclaim_iterators(struct mem_cgroup *dead_memcg)
 	 * cgroup root (root_mem_cgroup). So we have to handle
 	 * dead_memcg from cgroup root separately.
 	 */
-	if (last != root_mem_cgroup)
+	if (!mem_cgroup_is_root(last))
 		__invalidate_reclaim_iterators(root_mem_cgroup,
 						dead_memcg);
 }
@@ -1243,7 +1243,7 @@ int mem_cgroup_scan_tasks(struct mem_cgroup *memcg,
 	struct mem_cgroup *iter;
 	int ret = 0;
 
-	BUG_ON(memcg == root_mem_cgroup);
+	BUG_ON(mem_cgroup_is_root(memcg));
 
 	for_each_mem_cgroup_tree(iter, memcg) {
 		struct css_task_iter it;
@@ -1272,7 +1272,7 @@ void lruvec_memcg_debug(struct lruvec *lruvec, struct folio *folio)
 	memcg = folio_memcg(folio);
 
 	if (!memcg)
-		VM_BUG_ON_FOLIO(lruvec_memcg(lruvec) != root_mem_cgroup, folio);
+		VM_BUG_ON_FOLIO(!mem_cgroup_is_root(lruvec_memcg(lruvec)), folio);
 	else
 		VM_BUG_ON_FOLIO(lruvec_memcg(lruvec) != memcg, folio);
 }
@@ -2036,7 +2036,7 @@ struct mem_cgroup *mem_cgroup_get_oom_group(struct task_struct *victim,
 	rcu_read_lock();
 
 	memcg = mem_cgroup_from_task(victim);
-	if (memcg == root_mem_cgroup)
+	if (mem_cgroup_is_root(memcg))
 		goto out;
 
 	/*
@@ -2995,7 +2995,7 @@ static struct obj_cgroup *__get_obj_cgroup_from_memcg(struct mem_cgroup *memcg)
 {
 	struct obj_cgroup *objcg = NULL;
 
-	for (; memcg != root_mem_cgroup; memcg = parent_mem_cgroup(memcg)) {
+	for (; !mem_cgroup_is_root(memcg); memcg = parent_mem_cgroup(memcg)) {
 		objcg = rcu_dereference(memcg->objcg);
 		if (objcg && obj_cgroup_tryget(objcg))
 			break;
@@ -7163,7 +7163,7 @@ void mem_cgroup_sk_alloc(struct sock *sk)
 
 	rcu_read_lock();
 	memcg = mem_cgroup_from_task(current);
-	if (memcg == root_mem_cgroup)
+	if (mem_cgroup_is_root(memcg))
 		goto out;
 	if (!cgroup_subsys_on_dfl(memory_cgrp_subsys) && !memcg->tcpmem_active)
 		goto out;
@@ -7298,7 +7298,7 @@ static struct mem_cgroup *mem_cgroup_id_get_online(struct mem_cgroup *memcg)
 		 * The root cgroup cannot be destroyed, so it's refcount must
 		 * always be >= 1.
 		 */
-		if (WARN_ON_ONCE(memcg == root_mem_cgroup)) {
+		if (WARN_ON_ONCE(mem_cgroup_is_root(memcg))) {
 			VM_BUG_ON(1);
 			break;
 		}
@@ -7462,7 +7462,7 @@ long mem_cgroup_get_nr_swap_pages(struct mem_cgroup *memcg)
 
 	if (mem_cgroup_disabled() || do_memsw_account())
 		return nr_swap_pages;
-	for (; memcg != root_mem_cgroup; memcg = parent_mem_cgroup(memcg))
+	for (; !mem_cgroup_is_root(memcg); memcg = parent_mem_cgroup(memcg))
 		nr_swap_pages = min_t(long, nr_swap_pages,
 				      READ_ONCE(memcg->swap.max) -
 				      page_counter_read(&memcg->swap));
@@ -7484,7 +7484,7 @@ bool mem_cgroup_swap_full(struct folio *folio)
 	if (!memcg)
 		return false;
 
-	for (; memcg != root_mem_cgroup; memcg = parent_mem_cgroup(memcg)) {
+	for (; !mem_cgroup_is_root(memcg); memcg = parent_mem_cgroup(memcg)) {
 		unsigned long usage = page_counter_read(&memcg->swap);
 
 		if (usage * 2 >= READ_ONCE(memcg->swap.high) ||
@@ -7648,7 +7648,7 @@ bool obj_cgroup_may_zswap(struct obj_cgroup *objcg)
 		return true;
 
 	original_memcg = get_mem_cgroup_from_objcg(objcg);
-	for (memcg = original_memcg; memcg != root_mem_cgroup;
+	for (memcg = original_memcg; !mem_cgroup_is_root(memcg);
 	     memcg = parent_mem_cgroup(memcg)) {
 		unsigned long max = READ_ONCE(memcg->zswap_max);
 		unsigned long pages;