cgroup: Disable cgroup "memory" by default

Some Raspberry Pis have limited RAM and most users won't use the
cgroup memory support so it is disabled by default. Enable with:

    cgroup_enable=memory

See: #1950

Signed-off-by: Phil Elwell <phil@raspberrypi.org>

kernel/cgroup/cgroup.c

@@ -6051,6 +6051,9 @@ int __init cgroup_init_early(void)
 	return 0;
 }
 
+static u16 cgroup_enable_mask __initdata;
+static int __init cgroup_disable(char *str);
+
 /**
  * cgroup_init - cgroup initialization
  *
@@ -6084,6 +6087,12 @@ int __init cgroup_init(void)
 
 	cgroup_unlock();
 
+	/*
+	 * Apply an implicit disable, knowing that an explicit enable will
+	 * prevent if from doing anything.
+	 */
+	cgroup_disable("memory");
+
 	for_each_subsys(ss, ssid) {
 		if (ss->early_init) {
 			struct cgroup_subsys_state *css =
@@ -6724,6 +6733,10 @@ static int __init cgroup_disable(char *str)
 			    strcmp(token, ss->legacy_name))
 				continue;
 
+			/* An explicit cgroup_enable overrides a disable */
+			if (cgroup_enable_mask & (1 << i))
+				continue;
+
 			static_branch_disable(cgroup_subsys_enabled_key[i]);
 			pr_info("Disabling %s control group subsystem\n",
 				ss->name);
@@ -6742,6 +6755,31 @@ static int __init cgroup_disable(char *str)
 }
 __setup("cgroup_disable=", cgroup_disable);
 
+static int __init cgroup_enable(char *str)
+{
+	struct cgroup_subsys *ss;
+	char *token;
+	int i;
+
+	while ((token = strsep(&str, ",")) != NULL) {
+		if (!*token)
+			continue;
+
+		for_each_subsys(ss, i) {
+			if (strcmp(token, ss->name) &&
+			    strcmp(token, ss->legacy_name))
+				continue;
+
+			cgroup_enable_mask |= 1 << i;
+			static_branch_enable(cgroup_subsys_enabled_key[i]);
+			pr_info("Enabling %s control group subsystem\n",
+				ss->name);
+		}
+	}
+	return 1;
+}
+__setup("cgroup_enable=", cgroup_enable);
+
 void __init __weak enable_debug_cgroup(void) { }
 
 static int __init enable_cgroup_debug(char *str)