mm: memcontrol: allow to disable kmem accounting for cgroup2

Kmem accounting might incur overhead that some users can't put up with.
Besides, the implementation is still considered unstable.  So let's
provide a way to disable it for those users who aren't happy with it.

To disable kmem accounting for cgroup2, pass cgroup.memory=nokmem at
boot time.

Signed-off-by: Vladimir Davydov <vdavydov@virtuozzo.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 3ea869d..cfb2c0f 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -611,6 +611,7 @@
 	cgroup.memory=	[KNL] Pass options to the cgroup memory controller.
 			Format: <string>
 			nosocket -- Disable socket memory accounting.
+			nokmem -- Disable kernel memory accounting.
 
 	checkreqprot	[SELINUX] Set initial checkreqprot flag value.
 			Format: { "0" | "1" }