Don't return an error on failure to create blkio controller

This patch enables cgroup controllers as much as possible by skipping
the creation of blkio controller when running with old kernels that
doesn't support multi-level directory for blkio controller.
Signed-off-by: NHu Tao <hutao@cn.fujitsu.com>
Signed-off-by: NEric Blake <eblake@redhat.com>

src/util/cgroup.c

@@ -527,9 +527,20 @@ static int virCgroupMakeGroup(virCgroupPtr parent, virCgroupPtr group,
         if (access(path, F_OK) != 0) {
             if (!create ||
                 mkdir(path, 0755) < 0) {
-                rc = -errno;
-                VIR_FREE(path);
-                break;
+                /* With a kernel that doesn't support multi-level directory
+                 * for blkio controller, libvirt will fail and disable all
+                 * other controllers even though they are available. So
+                 * treat blkio as unmounted if mkdir fails. */
+                if (i == VIR_CGROUP_CONTROLLER_BLKIO) {
+                    rc = 0;
+                    VIR_FREE(group->controllers[i].mountPoint);
+                    VIR_FREE(path);
+                    continue;
+                } else {
+                    rc = -errno;
+                    VIR_FREE(path);
+                    break;
+                }
             }
             if (group->controllers[VIR_CGROUP_CONTROLLER_CPUSET].mountPoint != NULL &&
                 (i == VIR_CGROUP_CONTROLLER_CPUSET ||