cgroup: Add named hierarchy disabling to cgroup_no_v1 boot param

It can be useful to inhibit all cgroup1 hierarchies especially during
transition and for debugging.  cgroup_no_v1 can block hierarchies with
controllers which leaves out the named hierarchies.  Expand it to
cover the named hierarchies so that "cgroup_no_v1=all,named" disables
all cgroup1 hierarchies.
Signed-off-by: Tejun Heo <tj@kernel.org>
Suggested-by: Marcin Pawlowski <mpawlowski@fb.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

Documentation/admin-guide/kernel-parameters.txt

@@ -486,10 +486,14 @@
 			cut the overhead, others just disable the usage. So
 			only cgroup_disable=memory is actually worthy}
 
-	cgroup_no_v1=	[KNL] Disable one, multiple, all cgroup controllers in v1
-			Format: { controller[,controller...] | "all" }
+	cgroup_no_v1=	[KNL] Disable cgroup controllers and named hierarchies in v1
+			Format: { { controller | "all" | "named" }
+			          [,{ controller | "all" | "named" }...] }
 			Like cgroup_disable, but only applies to cgroup v1;
 			the blacklisted controllers remain available in cgroup2.
+			"all" blacklists all controllers and "named" disables
+			named mounts. Specifying both "all" and "named" disables
+			all v1 hierarchies.
 
 	cgroup.memory=	[KNL] Pass options to the cgroup memory controller.
 			Format: <string>

kernel/cgroup/cgroup-v1.c

@@ -27,6 +27,9 @@
 /* Controllers blocked by the commandline in v1 */
 static u16 cgroup_no_v1_mask;
 
+/* disable named v1 mounts */
+static bool cgroup_no_v1_named;
+
 /*
  * pidlist destructions need to be flushed on cgroup destruction.  Use a
  * separate workqueue as flush domain.
@@ -963,6 +966,10 @@ static int parse_cgroupfs_options(char *data, struct cgroup_sb_opts *opts)
 		}
 		if (!strncmp(token, "name=", 5)) {
 			const char *name = token + 5;
+
+			/* blocked by boot param? */
+			if (cgroup_no_v1_named)
+				return -ENOENT;
 			/* Can't specify an empty name */
 			if (!strlen(name))
 				return -EINVAL;
@@ -1292,7 +1299,12 @@ static int __init cgroup_no_v1(char *str)
 
 		if (!strcmp(token, "all")) {
 			cgroup_no_v1_mask = U16_MAX;
-			break;
+			continue;
+		}
+
+		if (!strcmp(token, "named")) {
+			cgroup_no_v1_named = true;
+			continue;
 		}
 
 		for_each_subsys(ss, i) {