memcg: clean up checking of the disabled flag

Those checks are unnecessary, because when the subsystem is disabled
it can't be mounted, so those functions won't get called.

The check is needed in functions which will be called in other places
except cgroup.

[[e-mail]: further checking of disabled flag]
Signed-off-by: Li Zefan <[e-mail]>
Acked-by: Balbir Singh <[e-mail]>
Acked-by: KAMEZAWA Hiroyuki <[e-mail]>
Acked-by: KOSAKI Motohiro <[e-mail]>
Signed-off-by: Hugh Dickins <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/memcontrol.c

···
 	struct mem_cgroup_per_zone *mz;
 	unsigned long flags;
 
+	if (mem_cgroup_subsys.disabled)
+		return;
+
 	/*
 	 * We cannot lock_page_cgroup while holding zone's lru_lock,
 	 * because other holders of lock_page_cgroup can be interrupted
···
 	unsigned long nr_retries = MEM_CGROUP_RECLAIM_RETRIES;
 	struct mem_cgroup_per_zone *mz;
 
-	if (mem_cgroup_subsys.disabled)
-		return 0;
-
 	pc = kmem_cache_alloc(page_cgroup_cache, gfp_mask);
 	if (unlikely(pc == NULL))
 		goto err;
···
 
 int mem_cgroup_charge(struct page *page, struct mm_struct *mm, gfp_t gfp_mask)
 {
+	if (mem_cgroup_subsys.disabled)
+		return 0;
+
 	/*
 	 * If already mapped, we don't have to account.
 	 * If page cache, page->mapping has address_space.
···
 int mem_cgroup_cache_charge(struct page *page, struct mm_struct *mm,
 				gfp_t gfp_mask)
 {
+	if (mem_cgroup_subsys.disabled)
+		return 0;
+
 	/*
 	 * Corner case handling. This is called from add_to_page_cache()
 	 * in usual. But some FS (shmem) precharges this page before calling it
···
 	int progress = 0;
 	int retry = MEM_CGROUP_RECLAIM_RETRIES;
 
+	if (mem_cgroup_subsys.disabled)
+		return 0;
+
 	rcu_read_lock();
 	mem = mem_cgroup_from_task(rcu_dereference(mm->owner));
 	css_get(&mem->css);
···
 {
 	int ret = -EBUSY;
 	int node, zid;
-
-	if (mem_cgroup_subsys.disabled)
-		return 0;
 
 	css_get(&mem->css);
 	/*
···
 static int mem_cgroup_populate(struct cgroup_subsys *ss,
 				struct cgroup *cont)
 {
-	if (mem_cgroup_subsys.disabled)
-		return 0;
 	return cgroup_add_files(cont, ss, mem_cgroup_files,
 					ARRAY_SIZE(mem_cgroup_files));
 }
···
 {
 	struct mm_struct *mm;
 	struct mem_cgroup *mem, *old_mem;
-
-	if (mem_cgroup_subsys.disabled)
-		return;
 
 	mm = get_task_mm(p);
 	if (mm == NULL)