mm: kmem: rename memcg_kmem_(un)charge() into memcg_kmem_(un)charge_page()

Rename (__)memcg_kmem_(un)charge() into (__)memcg_kmem_(un)charge_page()
to better reflect what they are actually doing:

1) call __memcg_kmem_(un)charge_memcg() to actually charge or uncharge
   the current memcg

2) set or clear the PageKmemcg flag

Signed-off-by: Roman Gushchin <guro@fb.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
Link: http://lkml.kernel.org/r/20200109202659.752357-4-guro@fb.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/kernel/fork.c b/kernel/fork.c
index d90af13..e37e12c 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -281,7 +281,7 @@
 					     MEMCG_KERNEL_STACK_KB,
 					     -(int)(PAGE_SIZE / 1024));
 
-			memcg_kmem_uncharge(vm->pages[i], 0);
+			memcg_kmem_uncharge_page(vm->pages[i], 0);
 		}
 
 		for (i = 0; i < NR_CACHED_STACKS; i++) {
@@ -413,12 +413,13 @@
 
 		for (i = 0; i < THREAD_SIZE / PAGE_SIZE; i++) {
 			/*
-			 * If memcg_kmem_charge() fails, page->mem_cgroup
-			 * pointer is NULL, and both memcg_kmem_uncharge()
+			 * If memcg_kmem_charge_page() fails, page->mem_cgroup
+			 * pointer is NULL, and both memcg_kmem_uncharge_page()
 			 * and mod_memcg_page_state() in free_thread_stack()
 			 * will ignore this page. So it's safe.
 			 */
-			ret = memcg_kmem_charge(vm->pages[i], GFP_KERNEL, 0);
+			ret = memcg_kmem_charge_page(vm->pages[i], GFP_KERNEL,
+						     0);
 			if (ret)
 				return ret;