Merge pull request #4670 from htejun/systemd-controller-on-unified-v2

Systemd controller on unified v2

17 files changed, 341 insertions, 238 deletions

diff --git a/src/basic/build.h b/src/basic/build.h
index 633c2aaccb..91312bd2a3 100644
--- a/src/basic/build.h
+++ b/src/basic/build.h
@@ -133,6 +133,8 @@
 #define _IDN_FEATURE_ "-IDN"
 #endif
 
+#define _CGROUP_HIEARCHY_ "default-hierarchy=" DEFAULT_HIERARCHY_NAME
+
 #define SYSTEMD_FEATURES                                                \
         _PAM_FEATURE_ " "                                               \
         _AUDIT_FEATURE_ " "                                             \
@@ -152,4 +154,5 @@
         _BLKID_FEATURE_ " "                                             \
         _ELFUTILS_FEATURE_ " "                                          \
         _KMOD_FEATURE_ " "                                              \
-        _IDN_FEATURE_
+        _IDN_FEATURE_ " "                                               \
+        _CGROUP_HIEARCHY_
diff --git a/src/basic/cgroup-util.c b/src/basic/cgroup-util.c
index 6948ed3931..f76b7f47e5 100644
--- a/src/basic/cgroup-util.c
+++ b/src/basic/cgroup-util.c
@@ -208,6 +208,12 @@ int cg_rmdir(const char *controller, const char *path) {
         if (r < 0 && errno != ENOENT)
                 return -errno;
 
+        if (streq(controller, SYSTEMD_CGROUP_CONTROLLER) && cg_hybrid_unified()) {
+                r = cg_rmdir(SYSTEMD_CGROUP_CONTROLLER_LEGACY, path);
+                if (r < 0)
+                        log_warning_errno(r, "Failed to remove compat systemd cgroup %s: %m", path);
+        }
+
         return 0;
 }
 
@@ -542,6 +548,13 @@ static const char *controller_to_dirname(const char *controller) {
          * just cuts off the name= prefixed used for named
          * hierarchies, if it is specified. */
 
+        if (streq(controller, SYSTEMD_CGROUP_CONTROLLER)) {
+                if (cg_hybrid_unified())
+                        controller = SYSTEMD_CGROUP_CONTROLLER_HYBRID;
+                else
+                        controller = SYSTEMD_CGROUP_CONTROLLER_LEGACY;
+        }
+
         e = startswith(controller, "name=");
         if (e)
                 return e;
@@ -594,7 +607,7 @@ static int join_path_unified(const char *path, const char *suffix, char **fs) {
 }
 
 int cg_get_path(const char *controller, const char *path, const char *suffix, char **fs) {
-        int unified, r;
+        int r;
 
         assert(fs);
 
@@ -623,11 +636,7 @@ int cg_get_path(const char *controller, const char *path, const char *suffix, ch
         if (!cg_controller_is_valid(controller))
                 return -EINVAL;
 
-        unified = cg_all_unified();
-        if (unified < 0)
-                return unified;
-
-        if (unified > 0)
+        if (cg_all_unified())
                 r = join_path_unified(path, suffix, fs);
         else
                 r = join_path_legacy(controller, path, suffix, fs);
@@ -639,7 +648,6 @@ int cg_get_path(const char *controller, const char *path, const char *suffix, ch
 }
 
 static int controller_is_accessible(const char *controller) {
-        int unified;
 
         assert(controller);
 
@@ -651,10 +659,7 @@ static int controller_is_accessible(const char *controller) {
         if (!cg_controller_is_valid(controller))
                 return -EINVAL;
 
-        unified = cg_all_unified();
-        if (unified < 0)
-                return unified;
-        if (unified > 0) {
+        if (cg_all_unified()) {
                 /* We don't support named hierarchies if we are using
                  * the unified hierarchy. */
 
@@ -708,7 +713,7 @@ static int trim_cb(const char *path, const struct stat *sb, int typeflag, struct
 
 int cg_trim(const char *controller, const char *path, bool delete_root) {
         _cleanup_free_ char *fs = NULL;
-        int r = 0;
+        int r = 0, q;
 
         assert(path);
 
@@ -731,6 +736,12 @@ int cg_trim(const char *controller, const char *path, bool delete_root) {
                         return -errno;
         }
 
+        if (streq(controller, SYSTEMD_CGROUP_CONTROLLER) && cg_hybrid_unified()) {
+                q = cg_trim(SYSTEMD_CGROUP_CONTROLLER_LEGACY, path, delete_root);
+                if (q < 0)
+                        log_warning_errno(q, "Failed to trim compat systemd cgroup %s: %m", path);
+        }
+
         return r;
 }
 
@@ -754,6 +765,12 @@ int cg_create(const char *controller, const char *path) {
                 return -errno;
         }
 
+        if (streq(controller, SYSTEMD_CGROUP_CONTROLLER) && cg_hybrid_unified()) {
+                r = cg_create(SYSTEMD_CGROUP_CONTROLLER_LEGACY, path);
+                if (r < 0)
+                        log_warning_errno(r, "Failed to create compat systemd cgroup %s: %m", path);
+        }
+
         return 1;
 }
 
@@ -791,7 +808,17 @@ int cg_attach(const char *controller, const char *path, pid_t pid) {
 
         xsprintf(c, PID_FMT "\n", pid);
 
-        return write_string_file(fs, c, 0);
+        r = write_string_file(fs, c, 0);
+        if (r < 0)
+                return r;
+
+        if (streq(controller, SYSTEMD_CGROUP_CONTROLLER) && cg_hybrid_unified()) {
+                r = cg_attach(SYSTEMD_CGROUP_CONTROLLER_LEGACY, path, pid);
+                if (r < 0)
+                        log_warning_errno(r, "Failed to attach %d to compat systemd cgroup %s: %m", pid, path);
+        }
+
+        return 0;
 }
 
 int cg_attach_fallback(const char *controller, const char *path, pid_t pid) {
@@ -840,7 +867,17 @@ int cg_set_group_access(
         if (r < 0)
                 return r;
 
-        return chmod_and_chown(fs, mode, uid, gid);
+        r = chmod_and_chown(fs, mode, uid, gid);
+        if (r < 0)
+                return r;
+
+        if (streq(controller, SYSTEMD_CGROUP_CONTROLLER) && cg_hybrid_unified()) {
+                r = cg_set_group_access(SYSTEMD_CGROUP_CONTROLLER_LEGACY, path, mode, uid, gid);
+                if (r < 0)
+                        log_warning_errno(r, "Failed to set group access on compat systemd cgroup %s: %m", path);
+        }
+
+        return 0;
 }
 
 int cg_set_task_access(
@@ -851,7 +888,7 @@ int cg_set_task_access(
                 gid_t gid) {
 
         _cleanup_free_ char *fs = NULL, *procs = NULL;
-        int r, unified;
+        int r;
 
         assert(path);
 
@@ -869,16 +906,18 @@ int cg_set_task_access(
         if (r < 0)
                 return r;
 
-        unified = cg_unified(controller);
-        if (unified < 0)
-                return unified;
-        if (unified)
-                return 0;
+        if (!cg_unified(controller)) {
+                /* Compatibility, Always keep values for "tasks" in sync with
+                 * "cgroup.procs" */
+                if (cg_get_path(controller, path, "tasks", &procs) >= 0)
+                        (void) chmod_and_chown(procs, mode, uid, gid);
+        }
 
-        /* Compatibility, Always keep values for "tasks" in sync with
-         * "cgroup.procs" */
-        if (cg_get_path(controller, path, "tasks", &procs) >= 0)
-                (void) chmod_and_chown(procs, mode, uid, gid);
+        if (streq(controller, SYSTEMD_CGROUP_CONTROLLER) && cg_hybrid_unified()) {
+                r = cg_set_task_access(SYSTEMD_CGROUP_CONTROLLER_LEGACY, path, mode, uid, gid);
+                if (r < 0)
+                        log_warning_errno(r, "Failed to set task access on compat systemd cgroup %s: %m", path);
+        }
 
         return 0;
 }
@@ -923,9 +962,9 @@ int cg_get_xattr(const char *controller, const char *path, const char *name, voi
 int cg_pid_get_path(const char *controller, pid_t pid, char **path) {
         _cleanup_fclose_ FILE *f = NULL;
         char line[LINE_MAX];
-        const char *fs;
+        const char *fs, *controller_str;
         size_t cs = 0;
-        int unified;
+        bool unified;
 
         assert(path);
         assert(pid >= 0);
@@ -937,10 +976,14 @@ int cg_pid_get_path(const char *controller, pid_t pid, char **path) {
                 controller = SYSTEMD_CGROUP_CONTROLLER;
 
         unified = cg_unified(controller);
-        if (unified < 0)
-                return unified;
-        if (unified == 0)
-                cs = strlen(controller);
+        if (!unified) {
+                if (streq(controller, SYSTEMD_CGROUP_CONTROLLER))
+                        controller_str = SYSTEMD_CGROUP_CONTROLLER_LEGACY;
+                else
+                        controller_str = controller;
+
+                cs = strlen(controller_str);
+        }
 
         fs = procfs_file_alloca(pid, "cgroup");
         f = fopen(fs, "re");
@@ -977,7 +1020,7 @@ int cg_pid_get_path(const char *controller, pid_t pid, char **path) {
 
                         *e = 0;
                         FOREACH_WORD_SEPARATOR(word, k, l, ",", state) {
-                                if (k == cs && memcmp(word, controller, cs) == 0) {
+                                if (k == cs && memcmp(word, controller_str, cs) == 0) {
                                         found = true;
                                         break;
                                 }
@@ -1001,14 +1044,11 @@ int cg_pid_get_path(const char *controller, pid_t pid, char **path) {
 int cg_install_release_agent(const char *controller, const char *agent) {
         _cleanup_free_ char *fs = NULL, *contents = NULL;
         const char *sc;
-        int r, unified;
+        int r;
 
         assert(agent);
 
-        unified = cg_unified(controller);
-        if (unified < 0)
-                return unified;
-        if (unified) /* doesn't apply to unified hierarchy */
+        if (cg_unified(controller)) /* doesn't apply to unified hierarchy */
                 return -EOPNOTSUPP;
 
         r = cg_get_path(controller, NULL, "release_agent", &fs);
@@ -1054,12 +1094,9 @@ int cg_install_release_agent(const char *controller, const char *agent) {
 
 int cg_uninstall_release_agent(const char *controller) {
         _cleanup_free_ char *fs = NULL;
-        int r, unified;
+        int r;
 
-        unified = cg_unified(controller);
-        if (unified < 0)
-                return unified;
-        if (unified) /* Doesn't apply to unified hierarchy */
+        if (cg_unified(controller)) /* Doesn't apply to unified hierarchy */
                 return -EOPNOTSUPP;
 
         r = cg_get_path(controller, NULL, "notify_on_release", &fs);
@@ -1104,7 +1141,7 @@ int cg_is_empty(const char *controller, const char *path) {
 }
 
 int cg_is_empty_recursive(const char *controller, const char *path) {
-        int unified, r;
+        int r;
 
         assert(path);
 
@@ -1112,11 +1149,7 @@ int cg_is_empty_recursive(const char *controller, const char *path) {
         if (controller && (isempty(path) || path_equal(path, "/")))
                 return false;
 
-        unified = cg_unified(controller);
-        if (unified < 0)
-                return unified;
-
-        if (unified > 0) {
+        if (cg_unified(controller)) {
                 _cleanup_free_ char *t = NULL;
 
                 /* On the unified hierarchy we can check empty state
@@ -1833,6 +1866,9 @@ bool cg_controller_is_valid(const char *p) {
         if (!p)
                 return false;
 
+        if (streq(p, SYSTEMD_CGROUP_CONTROLLER))
+                return true;
+
         s = startswith(p, "name=");
         if (s)
                 p = s;
@@ -1986,7 +2022,7 @@ int cg_get_keyed_attribute(const char *controller, const char *path, const char
 
 int cg_create_everywhere(CGroupMask supported, CGroupMask mask, const char *path) {
         CGroupController c;
-        int r, unified;
+        int r;
 
         /* This one will create a cgroup in our private tree, but also
          * duplicate it in the trees specified in mask, and remove it
@@ -1998,10 +2034,7 @@ int cg_create_everywhere(CGroupMask supported, CGroupMask mask, const char *path
                 return r;
 
         /* If we are in the unified hierarchy, we are done now */
-        unified = cg_all_unified();
-        if (unified < 0)
-                return unified;
-        if (unified > 0)
+        if (cg_all_unified())
                 return 0;
 
         /* Otherwise, do the same in the other hierarchies */
@@ -2022,16 +2055,13 @@ int cg_create_everywhere(CGroupMask supported, CGroupMask mask, const char *path
 
 int cg_attach_everywhere(CGroupMask supported, const char *path, pid_t pid, cg_migrate_callback_t path_callback, void *userdata) {
         CGroupController c;
-        int r, unified;
+        int r;
 
         r = cg_attach(SYSTEMD_CGROUP_CONTROLLER, path, pid);
         if (r < 0)
                 return r;
 
-        unified = cg_all_unified();
-        if (unified < 0)
-                return unified;
-        if (unified > 0)
+        if (cg_all_unified())
                 return 0;
 
         for (c = 0; c < _CGROUP_CONTROLLER_MAX; c++) {
@@ -2072,7 +2102,7 @@ int cg_attach_many_everywhere(CGroupMask supported, const char *path, Set* pids,
 
 int cg_migrate_everywhere(CGroupMask supported, const char *from, const char *to, cg_migrate_callback_t to_callback, void *userdata) {
         CGroupController c;
-        int r = 0, unified;
+        int r = 0;
 
         if (!path_equal(from, to))  {
                 r = cg_migrate_recursive(SYSTEMD_CGROUP_CONTROLLER, from, SYSTEMD_CGROUP_CONTROLLER, to, CGROUP_REMOVE);
@@ -2080,10 +2110,7 @@ int cg_migrate_everywhere(CGroupMask supported, const char *from, const char *to
                         return r;
         }
 
-        unified = cg_all_unified();
-        if (unified < 0)
-                return unified;
-        if (unified > 0)
+        if (cg_all_unified())
                 return r;
 
         for (c = 0; c < _CGROUP_CONTROLLER_MAX; c++) {
@@ -2107,16 +2134,13 @@ int cg_migrate_everywhere(CGroupMask supported, const char *from, const char *to
 
 int cg_trim_everywhere(CGroupMask supported, const char *path, bool delete_root) {
         CGroupController c;
-        int r, unified;
+        int r;
 
         r = cg_trim(SYSTEMD_CGROUP_CONTROLLER, path, delete_root);
         if (r < 0)
                 return r;
 
-        unified = cg_all_unified();
-        if (unified < 0)
-                return unified;
-        if (unified > 0)
+        if (cg_all_unified())
                 return r;
 
         for (c = 0; c < _CGROUP_CONTROLLER_MAX; c++) {
@@ -2133,16 +2157,13 @@ int cg_trim_everywhere(CGroupMask supported, const char *path, bool delete_root)
 
 int cg_mask_supported(CGroupMask *ret) {
         CGroupMask mask = 0;
-        int r, unified;
+        int r;
 
         /* Determines the mask of supported cgroup controllers. Only
          * includes controllers we can make sense of and that are
          * actually accessible. */
 
-        unified = cg_all_unified();
-        if (unified < 0)
-                return unified;
-        if (unified > 0) {
+        if (cg_all_unified()) {
                 _cleanup_free_ char *root = NULL, *controllers = NULL, *path = NULL;
                 const char *c;
 
@@ -2262,6 +2283,20 @@ int cg_kernel_controllers(Set *controllers) {
 
 static thread_local CGroupUnified unified_cache = CGROUP_UNIFIED_UNKNOWN;
 
+/* The hybrid mode was initially implemented in v232 and simply mounted
+ * cgroup v2 on /sys/fs/cgroup/systemd.  This unfortunately broke other
+ * tools (such as docker) which expected the v1 "name=systemd" hierarchy
+ * on /sys/fs/cgroup/systemd.  From v233 and on, the hybrid mode mountnbs
+ * v2 on /sys/fs/cgroup/unified and maintains "name=systemd" hierarchy
+ * on /sys/fs/cgroup/systemd for compatibility with other tools.
+ *
+ * To keep live upgrade working, we detect and support v232 layout.  When
+ * v232 layout is detected, to keep cgroup v2 process management but
+ * disable the compat dual layout, we return %true on
+ * cg_unified(SYSTEMD_CGROUP_CONTROLLER) and %false on cg_hybrid_unified().
+ */
+static thread_local bool unified_systemd_v232;
+
 static int cg_update_unified(void) {
 
         struct statfs fs;
@@ -2280,24 +2315,30 @@ static int cg_update_unified(void) {
         if (F_TYPE_EQUAL(fs.f_type, CGROUP2_SUPER_MAGIC))
                 unified_cache = CGROUP_UNIFIED_ALL;
         else if (F_TYPE_EQUAL(fs.f_type, TMPFS_MAGIC)) {
-                if (statfs("/sys/fs/cgroup/systemd/", &fs) < 0)
-                        return -errno;
-
-                unified_cache = F_TYPE_EQUAL(fs.f_type, CGROUP2_SUPER_MAGIC) ?
-                        CGROUP_UNIFIED_SYSTEMD : CGROUP_UNIFIED_NONE;
+                if (statfs("/sys/fs/cgroup/unified/", &fs) == 0 &&
+                    F_TYPE_EQUAL(fs.f_type, CGROUP2_SUPER_MAGIC)) {
+                        unified_cache = CGROUP_UNIFIED_SYSTEMD;
+                        unified_systemd_v232 = false;
+                } else if (statfs("/sys/fs/cgroup/systemd/", &fs) == 0 &&
+                           F_TYPE_EQUAL(fs.f_type, CGROUP2_SUPER_MAGIC)) {
+                        unified_cache = CGROUP_UNIFIED_SYSTEMD;
+                        unified_systemd_v232 = true;
+                } else {
+                        if (statfs("/sys/fs/cgroup/systemd/", &fs) < 0)
+                                return -errno;
+                        if (!F_TYPE_EQUAL(fs.f_type, CGROUP_SUPER_MAGIC))
+                                return -ENOMEDIUM;
+                        unified_cache = CGROUP_UNIFIED_NONE;
+                }
         } else
                 return -ENOMEDIUM;
 
         return 0;
 }
 
-int cg_unified(const char *controller) {
+bool cg_unified(const char *controller) {
 
-        int r;
-
-        r = cg_update_unified();
-        if (r < 0)
-                return r;
+        assert(cg_update_unified() >= 0);
 
         if (streq_ptr(controller, SYSTEMD_CGROUP_CONTROLLER))
                 return unified_cache >= CGROUP_UNIFIED_SYSTEMD;
@@ -2305,29 +2346,35 @@ int cg_unified(const char *controller) {
                 return unified_cache >= CGROUP_UNIFIED_ALL;
 }
 
-int cg_all_unified(void) {
+bool cg_all_unified(void) {
 
         return cg_unified(NULL);
 }
 
-void cg_unified_flush(void) {
+bool cg_hybrid_unified(void) {
+
+        assert(cg_update_unified() >= 0);
+
+        return unified_cache == CGROUP_UNIFIED_SYSTEMD && !unified_systemd_v232;
+}
+
+int cg_unified_flush(void) {
         unified_cache = CGROUP_UNIFIED_UNKNOWN;
+
+        return cg_update_unified();
 }
 
 int cg_enable_everywhere(CGroupMask supported, CGroupMask mask, const char *p) {
         _cleanup_free_ char *fs = NULL;
         CGroupController c;
-        int r, unified;
+        int r;
 
         assert(p);
 
         if (supported == 0)
                 return 0;
 
-        unified = cg_all_unified();
-        if (unified < 0)
-                return unified;
-        if (!unified) /* on the legacy hiearchy there's no joining of controllers defined */
+        if (!cg_all_unified()) /* on the legacy hiearchy there's no joining of controllers defined */
                 return 0;
 
         r = cg_get_path(SYSTEMD_CGROUP_CONTROLLER, p, "cgroup.subtree_control", &fs);
@@ -2359,63 +2406,69 @@ int cg_enable_everywhere(CGroupMask supported, CGroupMask mask, const char *p) {
 
 bool cg_is_unified_wanted(void) {
         static thread_local int wanted = -1;
-        int r, unified;
+        int r;
         bool b;
+        const bool is_default = DEFAULT_HIERARCHY == CGROUP_UNIFIED_ALL;
 
-        /* If the hierarchy is already mounted, then follow whatever
-         * was chosen for it. */
-        unified = cg_all_unified();
-        if (unified >= 0)
-                return unified;
-
-        /* Otherwise, let's see what the kernel command line has to
-         * say. Since checking that is expensive, let's cache the
-         * result. */
+        /* If we have a cached value, return that. */
         if (wanted >= 0)
                 return wanted;
 
+        /* If the hierarchy is already mounted, then follow whatever
+         * was chosen for it. */
+        if (cg_unified_flush() >= 0)
+                return (wanted = cg_all_unified());
+
+        /* Otherwise, let's see what the kernel command line has to say.
+         * Since checking is expensive, cache a non-error result. */
         r = proc_cmdline_get_bool("systemd.unified_cgroup_hierarchy", &b);
-        if (r < 0)
-                return false;
 
-        return (wanted = r > 0 ? b : false);
+        return (wanted = r > 0 ? b : is_default);
 }
 
 bool cg_is_legacy_wanted(void) {
-        return !cg_is_unified_wanted();
+        static thread_local int wanted = -1;
+
+        /* If we have a cached value, return that. */
+        if (wanted >= 0)
+                return wanted;
+
+        /* Check if we have cgroups2 already mounted. */
+        if (cg_unified_flush() >= 0 &&
+            unified_cache == CGROUP_UNIFIED_ALL)
+                return (wanted = false);
+
+        /* Otherwise, assume that at least partial legacy is wanted,
+         * since cgroups2 should already be mounted at this point. */
+        return (wanted = true);
 }
 
-bool cg_is_unified_systemd_controller_wanted(void) {
+bool cg_is_hybrid_wanted(void) {
         static thread_local int wanted = -1;
-        int r, unified;
+        int r;
         bool b;
+        const bool is_default = DEFAULT_HIERARCHY >= CGROUP_UNIFIED_SYSTEMD;
+        /* We default to true if the default is "hybrid", obviously,
+         * but also when the default is "unified", because if we get
+         * called, it means that unified hierarchy was not mounted. */
 
-        /* If the unified hierarchy is requested in full, no need to
-         * bother with this. */
-        if (cg_is_unified_wanted())
-                return 0;
+        /* If we have a cached value, return that. */
+        if (wanted >= 0)
+                return wanted;
 
         /* If the hierarchy is already mounted, then follow whatever
          * was chosen for it. */
-        unified = cg_unified(SYSTEMD_CGROUP_CONTROLLER);
-        if (unified >= 0)
-                return unified;
-
-        /* Otherwise, let's see what the kernel command line has to
-         * say. Since checking that is expensive, let's cache the
-         * result. */
-        if (wanted >= 0)
-                return wanted;
+        if (cg_unified_flush() >= 0 &&
+            unified_cache == CGROUP_UNIFIED_ALL)
+                return (wanted = false);
 
+        /* Otherwise, let's see what the kernel command line has to say.
+         * Since checking is expensive, cache a non-error result. */
         r = proc_cmdline_get_bool("systemd.legacy_systemd_cgroup_controller", &b);
-        if (r < 0)
-                return false;
-
-        return (wanted = r > 0 ? b : false);
-}
 
-bool cg_is_legacy_systemd_controller_wanted(void) {
-        return cg_is_legacy_wanted() && !cg_is_unified_systemd_controller_wanted();
+        /* The meaning of the kernel option is reversed wrt. to the return value
+         * of this function, hence the negation. */
+        return (wanted = r > 0 ? !b : is_default);
 }
 
 int cg_weight_parse(const char *s, uint64_t *ret) {
diff --git a/src/basic/cgroup-util.h b/src/basic/cgroup-util.h
index 0aa27c4cd7..3d14ec3d9d 100644
--- a/src/basic/cgroup-util.h
+++ b/src/basic/cgroup-util.h
@@ -240,14 +240,14 @@ int cg_kernel_controllers(Set *controllers);
 
 bool cg_ns_supported(void);
 
-int cg_all_unified(void);
-int cg_unified(const char *controller);
-void cg_unified_flush(void);
+bool cg_all_unified(void);
+bool cg_hybrid_unified(void);
+bool cg_unified(const char *controller);
+int cg_unified_flush(void);
 
 bool cg_is_unified_wanted(void);
 bool cg_is_legacy_wanted(void);
-bool cg_is_unified_systemd_controller_wanted(void);
-bool cg_is_legacy_systemd_controller_wanted(void);
+bool cg_is_hybrid_wanted(void);
 
 const char* cgroup_controller_to_string(CGroupController c) _const_;
 CGroupController cgroup_controller_from_string(const char *s) _pure_;
diff --git a/src/basic/def.h b/src/basic/def.h
index 10d776ec8e..200ea973c1 100644
--- a/src/basic/def.h
+++ b/src/basic/def.h
@@ -36,7 +36,9 @@
 /* The default value for the net.unix.max_dgram_qlen sysctl */
 #define DEFAULT_UNIX_MAX_DGRAM_QLEN 512UL
 
-#define SYSTEMD_CGROUP_CONTROLLER "name=systemd"
+#define SYSTEMD_CGROUP_CONTROLLER_LEGACY "name=systemd"
+#define SYSTEMD_CGROUP_CONTROLLER_HYBRID "name=unified"
+#define SYSTEMD_CGROUP_CONTROLLER "_systemd"
 
 #define SIGNALS_CRASH_HANDLER SIGSEGV,SIGILL,SIGFPE,SIGBUS,SIGQUIT,SIGABRT
 #define SIGNALS_IGNORE SIGPIPE
diff --git a/src/cgls/cgls.c b/src/cgls/cgls.c
index 5574c14555..40db82f9ae 100644
--- a/src/cgls/cgls.c
+++ b/src/cgls/cgls.c
@@ -158,7 +158,7 @@ static int parse_argv(int argc, char *argv[]) {
 
 static void show_cg_info(const char *controller, const char *path) {
 
-        if (cg_all_unified() <= 0 && controller && !streq(controller, SYSTEMD_CGROUP_CONTROLLER))
+        if (!cg_all_unified() && controller && !streq(controller, SYSTEMD_CGROUP_CONTROLLER))
                 printf("Controller %s; ", controller);
 
         printf("Control group %s:\n", isempty(path) ? "/" : path);
diff --git a/src/cgtop/cgtop.c b/src/cgtop/cgtop.c
index 50ac6a58b0..45c050c9c3 100644
--- a/src/cgtop/cgtop.c
+++ b/src/cgtop/cgtop.c
@@ -214,7 +214,7 @@ static int process(
                 uint64_t new_usage;
                 nsec_t timestamp;
 
-                if (cg_all_unified() > 0) {
+                if (cg_all_unified()) {
                         const char *keys[] = { "usage_usec", NULL };
                         _cleanup_free_ char *val = NULL;
 
@@ -274,7 +274,7 @@ static int process(
         } else if (streq(controller, "memory")) {
                 _cleanup_free_ char *p = NULL, *v = NULL;
 
-                if (cg_all_unified() <= 0)
+                if (!cg_all_unified())
                         r = cg_get_path(controller, path, "memory.usage_in_bytes", &p);
                 else
                         r = cg_get_path(controller, path, "memory.current", &p);
@@ -294,15 +294,14 @@ static int process(
                 if (g->memory > 0)
                         g->memory_valid = true;
 
-        } else if ((streq(controller, "io") && cg_all_unified() > 0) ||
-                   (streq(controller, "blkio") && cg_all_unified() <= 0)) {
+        } else if ((streq(controller, "io") && cg_all_unified()) ||
+                   (streq(controller, "blkio") && !cg_all_unified())) {
                 _cleanup_fclose_ FILE *f = NULL;
                 _cleanup_free_ char *p = NULL;
-                bool unified = cg_all_unified() > 0;
                 uint64_t wr = 0, rd = 0;
                 nsec_t timestamp;
 
-                r = cg_get_path(controller, path, unified ? "io.stat" : "blkio.io_service_bytes", &p);
+                r = cg_get_path(controller, path, cg_all_unified() ? "io.stat" : "blkio.io_service_bytes", &p);
                 if (r < 0)
                         return r;
 
@@ -325,7 +324,7 @@ static int process(
                         l += strcspn(l, WHITESPACE);
                         l += strspn(l, WHITESPACE);
 
-                        if (unified) {
+                        if (cg_all_unified()) {
                                 while (!isempty(l)) {
                                         if (sscanf(l, "rbytes=%" SCNu64, &k))
                                                 rd += k;
diff --git a/src/core/cgroup.c b/src/core/cgroup.c
index 5789e2aa82..fbb711782e 100644
--- a/src/core/cgroup.c
+++ b/src/core/cgroup.c
@@ -678,7 +678,7 @@ static void cgroup_context_apply(Unit *u, CGroupMask mask, ManagerState state) {
                 bool has_weight = cgroup_context_has_cpu_weight(c);
                 bool has_shares = cgroup_context_has_cpu_shares(c);
 
-                if (cg_all_unified() > 0) {
+                if (cg_all_unified()) {
                         uint64_t weight;
 
                         if (has_weight)
@@ -858,7 +858,7 @@ static void cgroup_context_apply(Unit *u, CGroupMask mask, ManagerState state) {
         }
 
         if ((mask & CGROUP_MASK_MEMORY) && !is_root) {
-                if (cg_all_unified() > 0) {
+                if (cg_all_unified()) {
                         uint64_t max;
                         uint64_t swap_max = CGROUP_LIMIT_MAX;
 
@@ -1033,7 +1033,7 @@ CGroupMask unit_get_own_mask(Unit *u) {
                 e = unit_get_exec_context(u);
                 if (!e ||
                     exec_context_maintains_privileges(e) ||
-                    cg_all_unified() > 0)
+                    cg_all_unified())
                         return _CGROUP_MASK_ALL;
         }
 
@@ -1260,10 +1260,7 @@ int unit_watch_cgroup(Unit *u) {
                 return 0;
 
         /* Only applies to the unified hierarchy */
-        r = cg_unified(SYSTEMD_CGROUP_CONTROLLER);
-        if (r < 0)
-                return log_unit_error_errno(u, r, "Failed detect whether the unified hierarchy is used: %m");
-        if (r == 0)
+        if (!cg_unified(SYSTEMD_CGROUP_CONTROLLER))
                 return 0;
 
         /* Don't watch the root slice, it's pointless. */
@@ -1683,7 +1680,7 @@ int unit_watch_all_pids(Unit *u) {
         if (!u->cgroup_path)
                 return -ENOENT;
 
-        if (cg_unified(SYSTEMD_CGROUP_CONTROLLER) > 0) /* On unified we can use proper notifications */
+        if (cg_unified(SYSTEMD_CGROUP_CONTROLLER)) /* On unified we can use proper notifications */
                 return 0;
 
         return unit_watch_pids_in_path(u, u->cgroup_path);
@@ -1756,7 +1753,7 @@ static int on_cgroup_inotify_event(sd_event_source *s, int fd, uint32_t revents,
 int manager_setup_cgroup(Manager *m) {
         _cleanup_free_ char *path = NULL;
         CGroupController c;
-        int r, all_unified, systemd_unified;
+        int r;
         char *e;
 
         assert(m);
@@ -1793,25 +1790,22 @@ int manager_setup_cgroup(Manager *m) {
         if (r < 0)
                 return log_error_errno(r, "Cannot find cgroup mount point: %m");
 
-        all_unified = cg_all_unified();
-        systemd_unified = cg_unified(SYSTEMD_CGROUP_CONTROLLER);
-
-        if (all_unified < 0 || systemd_unified < 0)
-                return log_error_errno(all_unified < 0 ? all_unified : systemd_unified,
-                                       "Couldn't determine if we are running in the unified hierarchy: %m");
+        r = cg_unified_flush();
+        if (r < 0)
+                return log_error_errno(r, "Couldn't determine if we are running in the unified hierarchy: %m");
 
-        if (all_unified > 0)
+        if (cg_all_unified())
                 log_debug("Unified cgroup hierarchy is located at %s.", path);
-        else if (systemd_unified > 0)
+        else if (cg_unified(SYSTEMD_CGROUP_CONTROLLER))
                 log_debug("Unified cgroup hierarchy is located at %s. Controllers are on legacy hierarchies.", path);
         else
-                log_debug("Using cgroup controller " SYSTEMD_CGROUP_CONTROLLER ". File system hierarchy is at %s.", path);
+                log_debug("Using cgroup controller " SYSTEMD_CGROUP_CONTROLLER_LEGACY ". File system hierarchy is at %s.", path);
 
         if (!m->test_run) {
                 const char *scope_path;
 
                 /* 3. Install agent */
-                if (systemd_unified) {
+                if (cg_unified(SYSTEMD_CGROUP_CONTROLLER)) {
 
                         /* In the unified hierarchy we can get
                          * cgroup empty notifications via inotify. */
@@ -1871,7 +1865,7 @@ int manager_setup_cgroup(Manager *m) {
                         return log_error_errno(errno, "Failed to open pin file: %m");
 
                 /* 6.  Always enable hierarchical support if it exists... */
-                if (!all_unified)
+                if (!cg_all_unified())
                         (void) cg_set_attribute("memory", "/", "memory.use_hierarchy", "1");
         }
 
@@ -1997,7 +1991,7 @@ int unit_get_memory_current(Unit *u, uint64_t *ret) {
         if ((u->cgroup_realized_mask & CGROUP_MASK_MEMORY) == 0)
                 return -ENODATA;
 
-        if (cg_all_unified() <= 0)
+        if (!cg_all_unified())
                 r = cg_get_attribute("memory", u->cgroup_path, "memory.usage_in_bytes", &v);
         else
                 r = cg_get_attribute("memory", u->cgroup_path, "memory.current", &v);
@@ -2042,7 +2036,7 @@ static int unit_get_cpu_usage_raw(Unit *u, nsec_t *ret) {
         if (!u->cgroup_path)
                 return -ENODATA;
 
-        if (cg_all_unified() > 0) {
+        if (cg_all_unified()) {
                 const char *keys[] = { "usage_usec", NULL };
                 _cleanup_free_ char *val = NULL;
                 uint64_t us;
diff --git a/src/core/manager.c b/src/core/manager.c
index d3f6efc91c..ea80585329 100644
--- a/src/core/manager.c
+++ b/src/core/manager.c
@@ -776,7 +776,7 @@ static int manager_setup_cgroups_agent(Manager *m) {
         if (!MANAGER_IS_SYSTEM(m))
                 return 0;
 
-        if (cg_unified(SYSTEMD_CGROUP_CONTROLLER) > 0) /* We don't need this anymore on the unified hierarchy */
+        if (cg_unified(SYSTEMD_CGROUP_CONTROLLER)) /* We don't need this anymore on the unified hierarchy */
                 return 0;
 
         if (m->cgroups_agent_fd < 0) {
diff --git a/src/core/mount-setup.c b/src/core/mount-setup.c
index 9c2bf3a0ef..7295efbf31 100644
--- a/src/core/mount-setup.c
+++ b/src/core/mount-setup.c
@@ -96,15 +96,15 @@ static const MountPoint mount_table[] = {
         { "tmpfs",       "/run",                      "tmpfs",      "mode=755",                MS_NOSUID|MS_NODEV|MS_STRICTATIME,
           NULL,          MNT_FATAL|MNT_IN_CONTAINER },
         { "cgroup",      "/sys/fs/cgroup",            "cgroup2",    NULL,                      MS_NOSUID|MS_NOEXEC|MS_NODEV,
-          cg_is_unified_wanted, MNT_FATAL|MNT_IN_CONTAINER },
+          cg_is_unified_wanted, MNT_IN_CONTAINER },
         { "tmpfs",       "/sys/fs/cgroup",            "tmpfs",      "mode=755",                MS_NOSUID|MS_NOEXEC|MS_NODEV|MS_STRICTATIME,
           cg_is_legacy_wanted, MNT_FATAL|MNT_IN_CONTAINER },
-        { "cgroup",      "/sys/fs/cgroup/systemd",    "cgroup2",    NULL,                      MS_NOSUID|MS_NOEXEC|MS_NODEV,
-          cg_is_unified_systemd_controller_wanted, MNT_IN_CONTAINER },
+        { "cgroup",      "/sys/fs/cgroup/unified",    "cgroup2",    NULL,                      MS_NOSUID|MS_NOEXEC|MS_NODEV,
+          cg_is_hybrid_wanted, MNT_IN_CONTAINER },
         { "cgroup",      "/sys/fs/cgroup/systemd",    "cgroup",     "none,name=systemd,xattr", MS_NOSUID|MS_NOEXEC|MS_NODEV,
-          cg_is_legacy_systemd_controller_wanted, MNT_IN_CONTAINER           },
+          cg_is_legacy_wanted, MNT_IN_CONTAINER     },
         { "cgroup",      "/sys/fs/cgroup/systemd",    "cgroup",     "none,name=systemd",       MS_NOSUID|MS_NOEXEC|MS_NODEV,
-          cg_is_legacy_systemd_controller_wanted, MNT_FATAL|MNT_IN_CONTAINER },
+          cg_is_legacy_wanted, MNT_FATAL|MNT_IN_CONTAINER },
         { "pstore",      "/sys/fs/pstore",            "pstore",     NULL,                      MS_NOSUID|MS_NOEXEC|MS_NODEV,
           NULL,          MNT_NONE                   },
 #ifdef ENABLE_EFI
diff --git a/src/core/scope.c b/src/core/scope.c
index 9540fb67d9..5e068a76d1 100644
--- a/src/core/scope.c
+++ b/src/core/scope.c
@@ -475,7 +475,7 @@ static void scope_sigchld_event(Unit *u, pid_t pid, int code, int status) {
 
         /* If the PID set is empty now, then let's finish this off
            (On unified we use proper notifications) */
-        if (cg_unified(SYSTEMD_CGROUP_CONTROLLER) <= 0 && set_isempty(u->pids))
+        if (!cg_unified(SYSTEMD_CGROUP_CONTROLLER) && set_isempty(u->pids))
                 scope_notify_cgroup_empty_event(u);
 }
 
diff --git a/src/core/service.c b/src/core/service.c
index 54074ff7bc..0c2eb18f38 100644
--- a/src/core/service.c
+++ b/src/core/service.c
@@ -2938,7 +2938,7 @@ static void service_sigchld_event(Unit *u, pid_t pid, int code, int status) {
 
         /* If the PID set is empty now, then let's finish this off
            (On unified we use proper notifications) */
-        if (cg_unified(SYSTEMD_CGROUP_CONTROLLER) <= 0 && set_isempty(u->pids))
+        if (!cg_unified(SYSTEMD_CGROUP_CONTROLLER) && set_isempty(u->pids))
                 service_notify_cgroup_empty_event(u);
 }
 
diff --git a/src/core/unit.c b/src/core/unit.c
index bb05d2abfb..685df6f00d 100644
--- a/src/core/unit.c
+++ b/src/core/unit.c
@@ -3897,8 +3897,8 @@ int unit_kill_context(
                          * there we get proper events. Hence rely on
                          * them.*/
 
-                        if  (cg_unified(SYSTEMD_CGROUP_CONTROLLER) > 0 ||
-                             (detect_container() == 0 && !unit_cgroup_delegate(u)))
+                        if (cg_unified(SYSTEMD_CGROUP_CONTROLLER) ||
+                            (detect_container() == 0 && !unit_cgroup_delegate(u)))
                                 wait_for_exit = true;
 
                         if (send_sighup) {
diff --git a/src/libsystemd/sd-bus/test-bus-creds.c b/src/libsystemd/sd-bus/test-bus-creds.c
index 6fdcfa4128..64bd76a576 100644
--- a/src/libsystemd/sd-bus/test-bus-creds.c
+++ b/src/libsystemd/sd-bus/test-bus-creds.c
@@ -31,7 +31,7 @@ int main(int argc, char *argv[]) {
         log_parse_environment();
         log_open();
 
-        if (cg_all_unified() == -ENOMEDIUM) {
+        if (cg_unified_flush() == -ENOMEDIUM) {
                 log_info("Skipping test: /sys/fs/cgroup/ not available");
                 return EXIT_TEST_SKIP;
         }
diff --git a/src/nspawn/nspawn-cgroup.c b/src/nspawn/nspawn-cgroup.c
index 5274767b96..4678a7e349 100644
--- a/src/nspawn/nspawn-cgroup.c
+++ b/src/nspawn/nspawn-cgroup.c
@@ -78,13 +78,9 @@ int sync_cgroup(pid_t pid, CGroupUnified unified_requested, uid_t arg_uid_shift)
         char tree[] = "/tmp/unifiedXXXXXX", pid_string[DECIMAL_STR_MAX(pid) + 1];
         bool undo_mount = false;
         const char *fn;
-        int unified, r;
-
-        unified = cg_unified(SYSTEMD_CGROUP_CONTROLLER);
-        if (unified < 0)
-                return log_error_errno(unified, "Failed to determine whether the unified hierarchy is used: %m");
+        int r;
 
-        if ((unified > 0) == (unified_requested >= CGROUP_UNIFIED_SYSTEMD))
+        if (cg_unified(SYSTEMD_CGROUP_CONTROLLER) == (unified_requested >= CGROUP_UNIFIED_SYSTEMD))
                 return 0;
 
         /* When the host uses the legacy cgroup setup, but the
@@ -100,7 +96,7 @@ int sync_cgroup(pid_t pid, CGroupUnified unified_requested, uid_t arg_uid_shift)
         if (!mkdtemp(tree))
                 return log_error_errno(errno, "Failed to generate temporary mount point for unified hierarchy: %m");
 
-        if (unified)
+        if (cg_unified(SYSTEMD_CGROUP_CONTROLLER))
                 r = mount_verbose(LOG_ERR, "cgroup", tree, "cgroup",
                                   MS_NOSUID|MS_NOEXEC|MS_NODEV, "none,name=systemd,xattr");
         else
@@ -142,7 +138,7 @@ finish:
 int create_subcgroup(pid_t pid, CGroupUnified unified_requested) {
         _cleanup_free_ char *cgroup = NULL;
         const char *child;
-        int unified, r;
+        int r;
         CGroupMask supported;
 
         /* In the unified hierarchy inner nodes may only contain
@@ -154,10 +150,7 @@ int create_subcgroup(pid_t pid, CGroupUnified unified_requested) {
         if (unified_requested == CGROUP_UNIFIED_NONE)
                 return 0;
 
-        unified = cg_unified(SYSTEMD_CGROUP_CONTROLLER);
-        if (unified < 0)
-                return log_error_errno(unified, "Failed to determine whether the unified hierarchy is used: %m");
-        if (unified == 0)
+        if (!cg_unified(SYSTEMD_CGROUP_CONTROLLER))
                 return 0;
 
         r = cg_mask_supported(&supported);
diff --git a/src/nspawn/nspawn-mount.c b/src/nspawn/nspawn-mount.c
index 4b2838b752..ed4f1f9db8 100644
--- a/src/nspawn/nspawn-mount.c
+++ b/src/nspawn/nspawn-mount.c
@@ -890,7 +890,7 @@ static int get_controllers(Set *subsystems) {
 
                 *e = 0;
 
-                if (STR_IN_SET(l, "", "name=systemd"))
+                if (STR_IN_SET(l, "", "name=systemd", "name=unified"))
                         continue;
 
                 p = strdup(l);
@@ -909,7 +909,6 @@ static int mount_legacy_cgroup_hierarchy(
                 const char *dest,
                 const char *controller,
                 const char *hierarchy,
-                CGroupUnified unified_requested,
                 bool read_only) {
 
         const char *to, *fstype, *opts;
@@ -927,14 +926,12 @@ static int mount_legacy_cgroup_hierarchy(
 
         /* The superblock mount options of the mount point need to be
          * identical to the hosts', and hence writable... */
-        if (streq(controller, SYSTEMD_CGROUP_CONTROLLER)) {
-                if (unified_requested >= CGROUP_UNIFIED_SYSTEMD) {
-                        fstype = "cgroup2";
-                        opts = NULL;
-                } else {
-                        fstype = "cgroup";
-                        opts = "none,name=systemd,xattr";
-                }
+        if (streq(controller, SYSTEMD_CGROUP_CONTROLLER_HYBRID)) {
+                fstype = "cgroup2";
+                opts = NULL;
+        } else if (streq(controller, SYSTEMD_CGROUP_CONTROLLER_LEGACY)) {
+                fstype = "cgroup";
+                opts = "none,name=systemd,xattr";
         } else {
                 fstype = "cgroup";
                 opts = controller;
@@ -994,7 +991,7 @@ static int mount_legacy_cgns_supported(
                         return r;
         }
 
-        if (cg_all_unified() > 0)
+        if (cg_all_unified())
                 goto skip_controllers;
 
         controllers = set_new(&string_hash_ops);
@@ -1012,7 +1009,7 @@ static int mount_legacy_cgns_supported(
                 if (!controller)
                         break;
 
-                r = mount_legacy_cgroup_hierarchy("", controller, controller, unified_requested, !userns);
+                r = mount_legacy_cgroup_hierarchy("", controller, controller, !userns);
                 if (r < 0)
                         return r;
 
@@ -1046,7 +1043,13 @@ static int mount_legacy_cgns_supported(
         }
 
 skip_controllers:
-        r = mount_legacy_cgroup_hierarchy("", SYSTEMD_CGROUP_CONTROLLER, "systemd", unified_requested, false);
+        if (unified_requested >= CGROUP_UNIFIED_SYSTEMD) {
+                r = mount_legacy_cgroup_hierarchy("", SYSTEMD_CGROUP_CONTROLLER_HYBRID, "unified", false);
+                if (r < 0)
+                        return r;
+        }
+
+        r = mount_legacy_cgroup_hierarchy("", SYSTEMD_CGROUP_CONTROLLER_LEGACY, "systemd", false);
         if (r < 0)
                 return r;
 
@@ -1091,7 +1094,7 @@ static int mount_legacy_cgns_unsupported(
                         return r;
         }
 
-        if (cg_all_unified() > 0)
+        if (cg_all_unified())
                 goto skip_controllers;
 
         controllers = set_new(&string_hash_ops);
@@ -1117,7 +1120,7 @@ static int mount_legacy_cgns_unsupported(
                 if (r == -EINVAL) {
                         /* Not a symbolic link, but directly a single cgroup hierarchy */
 
-                        r = mount_legacy_cgroup_hierarchy(dest, controller, controller, unified_requested, true);
+                        r = mount_legacy_cgroup_hierarchy(dest, controller, controller, true);
                         if (r < 0)
                                 return r;
 
@@ -1137,7 +1140,7 @@ static int mount_legacy_cgns_unsupported(
                                 continue;
                         }
 
-                        r = mount_legacy_cgroup_hierarchy(dest, combined, combined, unified_requested, true);
+                        r = mount_legacy_cgroup_hierarchy(dest, combined, combined, true);
                         if (r < 0)
                                 return r;
 
@@ -1150,7 +1153,13 @@ static int mount_legacy_cgns_unsupported(
         }
 
 skip_controllers:
-        r = mount_legacy_cgroup_hierarchy(dest, SYSTEMD_CGROUP_CONTROLLER, "systemd", unified_requested, false);
+        if (unified_requested >= CGROUP_UNIFIED_SYSTEMD) {
+                r = mount_legacy_cgroup_hierarchy(dest, SYSTEMD_CGROUP_CONTROLLER_HYBRID, "unified", false);
+                if (r < 0)
+                        return r;
+        }
+
+        r = mount_legacy_cgroup_hierarchy(dest, SYSTEMD_CGROUP_CONTROLLER_LEGACY, "systemd", false);
         if (r < 0)
                 return r;
 
@@ -1202,12 +1211,25 @@ int mount_cgroups(
         return mount_legacy_cgns_unsupported(dest, unified_requested, userns, uid_shift, uid_range, selinux_apifs_context);
 }
 
+static int mount_systemd_cgroup_writable_one(const char *systemd_own, const char *systemd_root)
+{
+        int r;
+
+        /* Make our own cgroup a (writable) bind mount */
+        r = mount_verbose(LOG_ERR, systemd_own, systemd_own,  NULL, MS_BIND, NULL);
+        if (r < 0)
+                return r;
+
+        /* And then remount the systemd cgroup root read-only */
+        return mount_verbose(LOG_ERR, NULL, systemd_root, NULL,
+                             MS_BIND|MS_REMOUNT|MS_NOSUID|MS_NOEXEC|MS_NODEV|MS_RDONLY, NULL);
+}
+
 int mount_systemd_cgroup_writable(
                 const char *dest,
                 CGroupUnified unified_requested) {
 
         _cleanup_free_ char *own_cgroup_path = NULL;
-        const char *systemd_root, *systemd_own;
         int r;
 
         assert(dest);
@@ -1220,22 +1242,19 @@ int mount_systemd_cgroup_writable(
         if (path_equal(own_cgroup_path, "/"))
                 return 0;
 
-        if (unified_requested >= CGROUP_UNIFIED_ALL) {
-                systemd_own = strjoina(dest, "/sys/fs/cgroup", own_cgroup_path);
-                systemd_root = prefix_roota(dest, "/sys/fs/cgroup");
-        } else {
-                systemd_own = strjoina(dest, "/sys/fs/cgroup/systemd", own_cgroup_path);
-                systemd_root = prefix_roota(dest, "/sys/fs/cgroup/systemd");
-        }
+        if (unified_requested >= CGROUP_UNIFIED_ALL)
+                return mount_systemd_cgroup_writable_one(strjoina(dest, "/sys/fs/cgroup", own_cgroup_path),
+                                                         prefix_roota(dest, "/sys/fs/cgroup"));
 
-        /* Make our own cgroup a (writable) bind mount */
-        r = mount_verbose(LOG_ERR, systemd_own, systemd_own,  NULL, MS_BIND, NULL);
-        if (r < 0)
-                return r;
+        if (unified_requested >= CGROUP_UNIFIED_SYSTEMD) {
+                r = mount_systemd_cgroup_writable_one(strjoina(dest, "/sys/fs/cgroup/unified", own_cgroup_path),
+                                                      prefix_roota(dest, "/sys/fs/cgroup/unified"));
+                if (r < 0)
+                        return r;
+        }
 
-        /* And then remount the systemd cgroup root read-only */
-        return mount_verbose(LOG_ERR, NULL, systemd_root, NULL,
-                             MS_BIND|MS_REMOUNT|MS_NOSUID|MS_NOEXEC|MS_NODEV|MS_RDONLY, NULL);
+        return mount_systemd_cgroup_writable_one(strjoina(dest, "/sys/fs/cgroup/systemd", own_cgroup_path),
+                                                 prefix_roota(dest, "/sys/fs/cgroup/systemd"));
 }
 
 int setup_volatile_state(
diff --git a/src/nspawn/nspawn.c b/src/nspawn/nspawn.c
index 751f26272b..42355115ff 100644
--- a/src/nspawn/nspawn.c
+++ b/src/nspawn/nspawn.c
@@ -316,7 +316,7 @@ static int custom_mount_check_all(void) {
 
 static int detect_unified_cgroup_hierarchy(const char *directory) {
         const char *e;
-        int r, all_unified, systemd_unified;
+        int r;
 
         /* Allow the user to control whether the unified hierarchy is used */
         e = getenv("UNIFIED_CGROUP_HIERARCHY");
@@ -332,15 +332,8 @@ static int detect_unified_cgroup_hierarchy(const char *directory) {
                 return 0;
         }
 
-        all_unified = cg_all_unified();
-        systemd_unified = cg_unified(SYSTEMD_CGROUP_CONTROLLER);
-
-        if (all_unified < 0 || systemd_unified < 0)
-                return log_error_errno(all_unified < 0 ? all_unified : systemd_unified,
-                                       "Failed to determine whether the unified cgroups hierarchy is used: %m");
-
         /* Otherwise inherit the default from the host system */
-        if (all_unified > 0) {
+        if (cg_all_unified()) {
                 /* Unified cgroup hierarchy support was added in 230. Unfortunately the detection
                  * routine only detects 231, so we'll have a false negative here for 230. */
                 r = systemd_installation_has_version(directory, 230);
@@ -350,9 +343,9 @@ static int detect_unified_cgroup_hierarchy(const char *directory) {
                         arg_unified_cgroup_hierarchy = CGROUP_UNIFIED_ALL;
                 else
                         arg_unified_cgroup_hierarchy = CGROUP_UNIFIED_NONE;
-        } else if (systemd_unified > 0) {
-                /* Mixed cgroup hierarchy support was added in 232 */
-                r = systemd_installation_has_version(directory, 232);
+        } else if (cg_unified(SYSTEMD_CGROUP_CONTROLLER)) {
+                /* Mixed cgroup hierarchy support was added in 233 */
+                r = systemd_installation_has_version(directory, 233);
                 if (r < 0)
                         return log_error_errno(r, "Failed to determine systemd version in container: %m");
                 if (r > 0)
@@ -2168,8 +2161,6 @@ static int inner_child(
         assert(directory);
         assert(kmsg_socket >= 0);
 
-        cg_unified_flush();
-
         if (arg_userns_mode != USER_NAMESPACE_NO) {
                 /* Tell the parent, that it now can write the UID map. */
                 (void) barrier_place(barrier); /* #1 */
@@ -2440,8 +2431,6 @@ static int outer_child(
         assert(notify_socket >= 0);
         assert(kmsg_socket >= 0);
 
-        cg_unified_flush();
-
         if (prctl(PR_SET_PDEATHSIG, SIGKILL) < 0)
                 return log_error_errno(errno, "PR_SET_PDEATHSIG failed: %m");
 
@@ -2486,10 +2475,6 @@ static int outer_child(
         if (r < 0)
                 return r;
 
-        r = detect_unified_cgroup_hierarchy(directory);
-        if (r < 0)
-                return r;
-
         if (arg_userns_mode != USER_NAMESPACE_NO) {
                 /* Let the parent know which UID shift we read from the image */
                 l = send(uid_shift_socket, &arg_uid_shift, sizeof(arg_uid_shift), MSG_NOSIGNAL);
@@ -3542,6 +3527,10 @@ int main(int argc, char *argv[]) {
         log_parse_environment();
         log_open();
 
+        r = cg_unified_flush();
+        if (r < 0)
+                return log_error_errno(r, "Failed to determine whether the unified cgroups hierarchy is used: %m");
+
         /* Make sure rename_process() in the stub init process can work */
         saved_argv = argv;
         saved_argc = argc;
@@ -3810,6 +3799,10 @@ int main(int argc, char *argv[]) {
         if (r < 0)
                 goto finish;
 
+        r = detect_unified_cgroup_hierarchy(arg_directory);
+        if (r < 0)
+                goto finish;
+
         interactive =
                 isatty(STDIN_FILENO) > 0 &&
                 isatty(STDOUT_FILENO) > 0;
diff --git a/src/test/test-cgroup-util.c b/src/test/test-cgroup-util.c
index c60fb631fa..30cd463722 100644
--- a/src/test/test-cgroup-util.c
+++ b/src/test/test-cgroup-util.c
@@ -18,11 +18,13 @@
 ***/
 
 #include "alloc-util.h"
+#include "build.h"
 #include "cgroup-util.h"
 #include "dirent-util.h"
 #include "fd-util.h"
 #include "format-util.h"
 #include "parse-util.h"
+#include "proc-cmdline.h"
 #include "process-util.h"
 #include "stat-util.h"
 #include "string-util.h"
@@ -332,7 +334,49 @@ static void test_fd_is_cgroup_fs(void) {
         fd = safe_close(fd);
 }
 
+static void test_is_wanted_print(bool header) {
+        _cleanup_free_ char *cmdline = NULL;
+
+        log_info("-- %s --", __func__);
+        assert_se(proc_cmdline(&cmdline) >= 0);
+        log_info("cmdline: %s", cmdline);
+        if (header) {
+
+                log_info(_CGROUP_HIEARCHY_);
+                (void) system("findmnt -n /sys/fs/cgroup");
+        }
+
+        log_info("is_unified_wanted() → %s", yes_no(cg_is_unified_wanted()));
+        log_info("is_hybrid_wanted() → %s", yes_no(cg_is_hybrid_wanted()));
+        log_info("is_legacy_wanted() → %s", yes_no(cg_is_legacy_wanted()));
+        log_info(" ");
+}
+
+static void test_is_wanted(void) {
+        assert_se(setenv("SYSTEMD_PROC_CMDLINE",
+                         "systemd.unified_cgroup_hierarchy", 1) >= 0);
+        test_is_wanted_print(false);
+
+        assert_se(setenv("SYSTEMD_PROC_CMDLINE",
+                         "systemd.unified_cgroup_hierarchy=0", 1) >= 0);
+        test_is_wanted_print(false);
+
+        assert_se(setenv("SYSTEMD_PROC_CMDLINE",
+                         "systemd.unified_cgroup_hierarchy=0 "
+                         "systemd.legacy_systemd_cgroup_controller", 1) >= 0);
+        test_is_wanted_print(false);
+
+        assert_se(setenv("SYSTEMD_PROC_CMDLINE",
+                         "systemd.unified_cgroup_hierarchy=0 "
+                         "systemd.legacy_systemd_cgroup_controller=0", 1) >= 0);
+        test_is_wanted_print(false);
+}
+
 int main(void) {
+        log_set_max_level(LOG_DEBUG);
+        log_parse_environment();
+        log_open();
+
         test_path_decode_unit();
         test_path_get_unit();
         test_path_get_user_unit();
@@ -349,6 +393,9 @@ int main(void) {
         TEST_REQ_RUNNING_SYSTEMD(test_mask_supported());
         TEST_REQ_RUNNING_SYSTEMD(test_is_cgroup_fs());
         TEST_REQ_RUNNING_SYSTEMD(test_fd_is_cgroup_fs());
+        test_is_wanted_print(true);
+        test_is_wanted_print(false); /* run twice to test caching */
+        test_is_wanted();
 
         return 0;
 }