memcg: zap memcg_can_account_kmem

memcg_can_account_kmem() returns true iff

    !mem_cgroup_disabled() && !mem_cgroup_is_root(memcg) &&
                                   memcg_kmem_is_active(memcg);

To begin with the !mem_cgroup_is_root(memcg) check is useless, because one
can't enable kmem accounting for the root cgroup (mem_cgroup_write()
returns EINVAL on an attempt to set the limit on the root cgroup).

Furthermore, the !mem_cgroup_disabled() check also seems to be redundant.
The point is memcg_can_account_kmem() is called from three places:
mem_cgroup_salbinfo_read(), __memcg_kmem_get_cache(), and
__memcg_kmem_newpage_charge().  The latter two functions are only invoked
if memcg_kmem_enabled() returns true, which implies that the memory cgroup
subsystem is enabled.  And mem_cgroup_slabinfo_read() shows the output of
memory.kmem.slabinfo, which won't exist if the memory cgroup is completely
disabled.

So let's substitute all the calls to memcg_can_account_kmem() with plain
memcg_kmem_is_active(), and kill the former.

Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -2762,12 +2762,6 @@ static DEFINE_MUTEX(memcg_slab_mutex);
 
 static DEFINE_MUTEX(activate_kmem_mutex);
 
-static inline bool memcg_can_account_kmem(struct mem_cgroup *memcg)
-{
-	return !mem_cgroup_disabled() && !mem_cgroup_is_root(memcg) &&
-		memcg_kmem_is_active(memcg);
-}
-
 /*
  * This is a bit cumbersome, but it is rarely used and avoids a backpointer
  * in the memcg_cache_params struct.
@@ -2787,7 +2781,7 @@ static int mem_cgroup_slabinfo_read(struct seq_file *m, void *v)
 	struct mem_cgroup *memcg = mem_cgroup_from_css(seq_css(m));
 	struct memcg_cache_params *params;
 
-	if (!memcg_can_account_kmem(memcg))
+	if (!memcg_kmem_is_active(memcg))
 		return -EIO;
 
 	print_slabinfo_header(m);
@@ -3164,7 +3158,7 @@ struct kmem_cache *__memcg_kmem_get_cache(struct kmem_cache *cachep,
 	rcu_read_lock();
 	memcg = mem_cgroup_from_task(rcu_dereference(current->mm->owner));
 
-	if (!memcg_can_account_kmem(memcg))
+	if (!memcg_kmem_is_active(memcg))
 		goto out;
 
 	memcg_cachep = cache_from_memcg_idx(cachep, memcg_cache_id(memcg));
@@ -3249,7 +3243,7 @@ __memcg_kmem_newpage_charge(gfp_t gfp, struct mem_cgroup **_memcg, int order)
 
 	memcg = get_mem_cgroup_from_mm(current->mm);
 
-	if (!memcg_can_account_kmem(memcg)) {
+	if (!memcg_kmem_is_active(memcg)) {
 		css_put(&memcg->css);
 		return true;
 	}