cgroup: make cgrp_dfl_root mountable

cgrp_dfl_root will be used as the default unified hierarchy.  This
patch makes cgrp_dfl_root mountable by making the following changes.

* cgroup_init_early() now initializes cgrp_dfl_root w/
  CGRP_ROOT_SANE_BEHAVIOR.  The default hierarchy is always sane.

* parse_cgroupfs_options() and cgroup_mount() are updated such that
  cgrp_dfl_root is mounted if sane_behavior is specified w/o any
  subsystems.

* rebind_subsystems() now populates the root directory of
  cgrp_dfl_root.  Note that the function still guarantees success of
  rebinding subsystems to cgrp_dfl_root.  If populating fails while
  rebinding to cgrp_dfl_root, it whines but ignores the error.

* For backward compatibility, the default hierarchy shows up in
  /proc/$PID/cgroup only after it's explicitly mounted so that
  userland which doesn't make use of it doesn't see any change.

* "current_css_set_cg_links" file of debug cgroup now treats the
  default hierarchy the same as other hierarchies.  This is visible to
  userland.  Given that it's for debug controller, this should be
  fine.

* While at it, implement cgroup_on_dfl() which tests whether a give
  cgroup is on the default hierarchy or not.

The above changes make cgrp_dfl_root mostly equivalent to other
controllers but the actual unified hierarchy behaviors are not
implemented yet.  Let's plug child cgroup creation in cgrp_dfl_root
from create_cgroup() for now.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

2 files changed, 71 insertions, 33 deletions

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 79993ac066c5..7e9fa505b7bb 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -250,6 +250,9 @@ enum {
          *
          * - "cgroup.clone_children" is removed.
          *
+         * - If mount is requested with sane_behavior but without any
+         *   subsystem, the default unified hierarchy is mounted.
+         *
          * - cpuset: tasks will be kept in empty cpusets when hotplug happens
          *   and take masks of ancestors with non-empty cpus/mems, instead of
          *   being moved to an ancestor.
@@ -468,6 +471,13 @@ struct cftype {
 #endif
 };
 
+extern struct cgroup_root cgrp_dfl_root;
+
+static inline bool cgroup_on_dfl(const struct cgroup *cgrp)
+{
+        return cgrp->root == &cgrp_dfl_root;
+}
+
 /*
  * See the comment above CGRP_ROOT_SANE_BEHAVIOR for details.  This
  * function can be called as long as @cgrp is accessible.
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index f5754910e80b..69b4939e9f6d 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -142,7 +142,13 @@ static const char *cgroup_subsys_name[] = {
  * unattached - it never has more than a single cgroup, and all tasks are
  * part of that cgroup.
  */
-static struct cgroup_root cgrp_dfl_root;
+struct cgroup_root cgrp_dfl_root;
+
+/*
+ * The default hierarchy always exists but is hidden until mounted for the
+ * first time.  This is for backward compatibility.
+ */
+static bool cgrp_dfl_root_visible;
 
 /* The list of hierarchy roots */
 
@@ -999,10 +1005,22 @@ static int rebind_subsystems(struct cgroup_root *dst_root,
                         return -EBUSY;
         }
 
-        if (dst_root != &cgrp_dfl_root) {
-                ret = cgroup_populate_dir(&dst_root->cgrp, ss_mask);
-                if (ret)
+        ret = cgroup_populate_dir(&dst_root->cgrp, ss_mask);
+        if (ret) {
+                if (dst_root != &cgrp_dfl_root)
                         return ret;
+
+                /*
+                 * Rebinding back to the default root is not allowed to
+                 * fail.  Using both default and non-default roots should
+                 * be rare.  Moving subsystems back and forth even more so.
+                 * Just warn about it and continue.
+                 */
+                if (cgrp_dfl_root_visible) {
+                        pr_warning("cgroup: failed to create files (%d) while rebinding 0x%lx to default root\n",
+                                   ret, ss_mask);
+                        pr_warning("cgroup: you may retry by moving them to a different hierarchy and unbinding\n");
+                }
         }
 
         /*
@@ -1011,7 +1029,7 @@ static int rebind_subsystems(struct cgroup_root *dst_root,
          */
         mutex_unlock(&cgroup_mutex);
         for_each_subsys(ss, ssid)
-                if ((ss_mask & (1 << ssid)) && ss->root != &cgrp_dfl_root)
+                if (ss_mask & (1 << ssid))
                         cgroup_clear_dir(&ss->root->cgrp, 1 << ssid);
         mutex_lock(&cgroup_mutex);
 
@@ -1039,8 +1057,7 @@ static int rebind_subsystems(struct cgroup_root *dst_root,
                         ss->bind(css);
         }
 
-        if (dst_root != &cgrp_dfl_root)
-                kernfs_activate(dst_root->cgrp.kn);
+        kernfs_activate(dst_root->cgrp.kn);
         return 0;
 }
 
@@ -1190,16 +1207,6 @@ static int parse_cgroupfs_options(char *data, struct cgroup_sb_opts *opts)
                         return -ENOENT;
         }
 
-        /*
-         * If the 'all' option was specified select all the subsystems,
-         * otherwise if 'none', 'name=' and a subsystem name options
-         * were not specified, let's default to 'all'
-         */
-        if (all_ss || (!one_ss && !opts->none && !opts->name))
-                for_each_subsys(ss, i)
-                        if (!ss->disabled)
-                                set_bit(i, &opts->subsys_mask);
-
         /* Consistency checks */
 
         if (opts->flags & CGRP_ROOT_SANE_BEHAVIOR) {
@@ -1211,6 +1218,23 @@ static int parse_cgroupfs_options(char *data, struct cgroup_sb_opts *opts)
                         pr_err("cgroup: sane_behavior: noprefix, xattr, clone_children, release_agent and name are not allowed\n");
                         return -EINVAL;
                 }
+        } else {
+                /*
+                 * If the 'all' option was specified select all the
+                 * subsystems, otherwise if 'none', 'name=' and a subsystem
+                 * name options were not specified, let's default to 'all'
+                 */
+                if (all_ss || (!one_ss && !opts->none && !opts->name))
+                        for_each_subsys(ss, i)
+                                if (!ss->disabled)
+                                        set_bit(i, &opts->subsys_mask);
+
+                /*
+                 * We either have to specify by name or by subsystems. (So
+                 * all empty hierarchies must have a name).
+                 */
+                if (!opts->subsys_mask && !opts->name)
+                        return -EINVAL;
         }
 
         /*
@@ -1226,13 +1250,6 @@ static int parse_cgroupfs_options(char *data, struct cgroup_sb_opts *opts)
         if (opts->subsys_mask && opts->none)
                 return -EINVAL;
 
-        /*
-         * We either have to specify by name or by subsystems. (So all
-         * empty hierarchies must have a name).
-         */
-        if (!opts->subsys_mask && !opts->name)
-                return -EINVAL;
-
         return 0;
 }
 
@@ -1487,6 +1504,14 @@ retry:
                 goto out_unlock;
 
         /* look for a matching existing root */
+        if (!opts.subsys_mask && !opts.none && !opts.name) {
+                cgrp_dfl_root_visible = true;
+                root = &cgrp_dfl_root;
+                cgroup_get(&root->cgrp);
+                ret = 0;
+                goto out_unlock;
+        }
+
         for_each_root(root) {
                 bool name_match = false;
 
@@ -3622,6 +3647,13 @@ static long cgroup_create(struct cgroup *parent, const char *name,
         struct cgroup_subsys *ss;
         struct kernfs_node *kn;
 
+        /*
+         * XXX: The default hierarchy isn't fully implemented yet.  Block
+         * !root cgroup creation on it for now.
+         */
+        if (root == &cgrp_dfl_root)
+                return -EINVAL;
+
         /* allocate the cgroup and its ID, 0 is reserved for the root */
         cgrp = kzalloc(sizeof(*cgrp), GFP_KERNEL);
         if (!cgrp)
@@ -4061,7 +4093,8 @@ static void __init cgroup_init_subsys(struct cgroup_subsys *ss)
  */
 int __init cgroup_init_early(void)
 {
-        static struct cgroup_sb_opts __initdata opts = { };
+        static struct cgroup_sb_opts __initdata opts =
+                { .flags = CGRP_ROOT_SANE_BEHAVIOR };
         struct cgroup_subsys *ss;
         int i;
 
@@ -4198,7 +4231,7 @@ int proc_cgroup_show(struct seq_file *m, void *v)
                 struct cgroup *cgrp;
                 int ssid, count = 0;
 
-                if (root == &cgrp_dfl_root)
+                if (root == &cgrp_dfl_root && !cgrp_dfl_root_visible)
                         continue;
 
                 seq_printf(m, "%d:", root->hierarchy_id);
@@ -4631,15 +4664,10 @@ static int current_css_set_cg_links_read(struct seq_file *seq, void *v)
         cset = rcu_dereference(current->cgroups);
         list_for_each_entry(link, &cset->cgrp_links, cgrp_link) {
                 struct cgroup *c = link->cgrp;
-                const char *name = "?";
-
-                if (c != &cgrp_dfl_root.cgrp) {
-                        cgroup_name(c, name_buf, NAME_MAX + 1);
-                        name = name_buf;
-                }
 
+                cgroup_name(c, name_buf, NAME_MAX + 1);
                 seq_printf(seq, "Root %d group %s\n",
-                           c->root->hierarchy_id, name);
+                           c->root->hierarchy_id, name_buf);
         }
         rcu_read_unlock();
         up_read(&css_set_rwsem);