3 files changed, 27 insertions, 7 deletions

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 4afe544d3547..8a111dd42d7a 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -305,6 +305,8 @@ enum {
          *   the flag is not created.
          *
          * - blkcg: blk-throttle becomes properly hierarchical.
+         *
+         * - debug: disallowed on the default hierarchy.
          */
         CGRP_ROOT_SANE_BEHAVIOR = (1 << 0),
 
diff --git a/include/linux/cgroup_subsys.h b/include/linux/cgroup_subsys.h
index 768fe44e19f0..98c4f9b12b03 100644
--- a/include/linux/cgroup_subsys.h
+++ b/include/linux/cgroup_subsys.h
@@ -7,10 +7,6 @@
 SUBSYS(cpuset)
 #endif
 
-#if IS_ENABLED(CONFIG_CGROUP_DEBUG)
-SUBSYS(debug)
-#endif
-
 #if IS_ENABLED(CONFIG_CGROUP_SCHED)
 SUBSYS(cpu)
 #endif
@@ -50,6 +46,13 @@ SUBSYS(net_prio)
 #if IS_ENABLED(CONFIG_CGROUP_HUGETLB)
 SUBSYS(hugetlb)
 #endif
+
+/*
+ * The following subsystems are not supported on the default hierarchy.
+ */
+#if IS_ENABLED(CONFIG_CGROUP_DEBUG)
+SUBSYS(debug)
+#endif
 /*
  * DO NOT ADD ANY SUBSYSTEM WITHOUT EXPLICIT ACKS FROM CGROUP MAINTAINERS.
  */
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 082bb842b11a..a5f75ac4e793 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -148,6 +148,13 @@ struct cgroup_root cgrp_dfl_root;
  */
 static bool cgrp_dfl_root_visible;
 
+/* some controllers are not supported in the default hierarchy */
+static const unsigned int cgrp_dfl_root_inhibit_ss_mask = 0
+#ifdef CONFIG_CGROUP_DEBUG
+        | (1 << debug_cgrp_id)
+#endif
+        ;
+
 /* The list of hierarchy roots */
 
 static LIST_HEAD(cgroup_roots);
@@ -1126,6 +1133,7 @@ static void cgroup_clear_dir(struct cgroup *cgrp, unsigned int subsys_mask)
 static int rebind_subsystems(struct cgroup_root *dst_root, unsigned int ss_mask)
 {
         struct cgroup_subsys *ss;
+        unsigned int tmp_ss_mask;
         int ssid, i, ret;
 
         lockdep_assert_held(&cgroup_mutex);
@@ -1143,7 +1151,12 @@ static int rebind_subsystems(struct cgroup_root *dst_root, unsigned int ss_mask)
                         return -EBUSY;
         }
 
-        ret = cgroup_populate_dir(&dst_root->cgrp, ss_mask);
+        /* skip creating root files on dfl_root for inhibited subsystems */
+        tmp_ss_mask = ss_mask;
+        if (dst_root == &cgrp_dfl_root)
+                tmp_ss_mask &= ~cgrp_dfl_root_inhibit_ss_mask;
+
+        ret = cgroup_populate_dir(&dst_root->cgrp, tmp_ss_mask);
         if (ret) {
                 if (dst_root != &cgrp_dfl_root)
                         return ret;
@@ -2426,7 +2439,8 @@ static int cgroup_root_controllers_show(struct seq_file *seq, void *v)
 {
         struct cgroup *cgrp = seq_css(seq)->cgroup;
 
-        cgroup_print_ss_mask(seq, cgrp->root->subsys_mask);
+        cgroup_print_ss_mask(seq, cgrp->root->subsys_mask &
+                             ~cgrp_dfl_root_inhibit_ss_mask);
         return 0;
 }
 
@@ -2564,7 +2578,8 @@ static ssize_t cgroup_subtree_control_write(struct kernfs_open_file *of,
                 if (tok[0] == '\0')
                         continue;
                 for_each_subsys(ss, ssid) {
-                        if (ss->disabled || strcmp(tok + 1, ss->name))
+                        if (ss->disabled || strcmp(tok + 1, ss->name) ||
+                            ((1 << ss->id) & cgrp_dfl_root_inhibit_ss_mask))
                                 continue;
 
                         if (*tok == '+') {