cgroups: Add ability to reuse existing cgroup yard directory

Currently we always create temporary directory where we restore
cgroups, but this won't work in case if mounting cgroups is forbidden
from inside of a container for some reason (as in OpenVZ kernel).

So one can pass --cgroup-yard option to specify an existing
directory where cgroups are living. By default we assume it
lays in /sys/fs/cgroup.
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Acked-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

Documentation/criu.txt

@@ -218,6 +218,9 @@ Restores previously checkpointed processes.
     Change the root cgroup the controller will be installed into. No controller
     means that root is the default for all controllers not specified.
 
+*--cgroup-yard* '<dir>'::
+   Use directory '<dir>' as cgroups yard instead of /sys/fs/cgroup.
+
 *--tcp-established*::
     Restore previously dumped established TCP connections. This implies that
     the network has been locked between *dump* and *restore* phases so other

cgroup.c

@@ -935,14 +935,10 @@ int prepare_task_cgroup(struct pstree_item *me)
 
 void fini_cgroup(void)
 {
-	if (!cg_yard)
-		return;
-
-	close_service_fd(CGROUP_YARD);
-	umount2(cg_yard, MNT_DETACH);
-	rmdir(cg_yard);
-	xfree(cg_yard);
-	cg_yard = NULL;
+	if (cg_yard) {
+		close_service_fd(CGROUP_YARD);
+		cg_yard = NULL;
+	}
 }
 
 static int restore_cgroup_prop(const CgroupPropEntry * cg_prop_entry_p,
@@ -1128,27 +1124,8 @@ static int prepare_cgroup_sfd(CgroupEntry *ce)
 
 	pr_info("Preparing cgroups yard\n");
 
-	off = sprintf(paux, ".criu.cgyard.XXXXXX");
-	if (mkdtemp(paux) == NULL) {
-		pr_perror("Can't make temp cgyard dir");
-		return -1;
-	}
-
-	cg_yard = xstrdup(paux);
-	if (!cg_yard) {
-		rmdir(paux);
-		return -1;
-	}
-
-	if (mount("none", cg_yard, "tmpfs", 0, NULL)) {
-		pr_perror("Can't mount tmpfs in cgyard");
-		goto err;
-	}
-
-	if (mount("none", cg_yard, NULL, MS_PRIVATE, NULL)) {
-		pr_perror("Can't make cgyard private");
-		goto err;
-	}
+	cg_yard = opts.cg_yard;
+	off = strlen(opts.cg_yard);
 
 	pr_debug("Opening %s as cg yard\n", cg_yard);
 	i = open(cg_yard, O_DIRECTORY);

crtools.c

@@ -56,6 +56,7 @@ void init_opts(void)
 	INIT_LIST_HEAD(&opts.inherit_fds);
 	INIT_LIST_HEAD(&opts.new_cgroup_roots);
 
+	opts.cg_yard = "/sys/fs/cgroup";
 	opts.cpu_cap = CPU_CAP_DEFAULT;
 	opts.manage_cgroups = false;
 	opts.ps_socket = -1;
@@ -207,6 +208,7 @@ int main(int argc, char *argv[], char *envp[])
 		{ "enable-fs",			required_argument,	0, 1065 },
 		{ "enable-external-sharing", 	no_argument, 		0, 1066 },
 		{ "enable-external-masters", 	no_argument, 		0, 1067 },
+		{ "cgroup-yard",		required_argument,	0, 1068 },
 		{ },
 	};
 
@@ -434,6 +436,9 @@ int main(int argc, char *argv[], char *envp[])
 		case 1067:
 			opts.enable_external_masters = true;
 			break;
+		case 1068:
+			opts.cg_yard = optarg;
+			break;
 		case 'M':
 			{
 				char *aux;
@@ -674,6 +679,7 @@ usage:
 "                        change the root cgroup the controller will be\n"
 "                        installed into. No controller means that root is the\n"
 "                        default for all controllers not specified.\n"
+"  --cgroup-yard DIR     use DIR as premounted cgroups external yard\n"
 "  --skip-mnt PATH       ignore this mountpoint when dumping the mount namespace.\n"
 "  --enable-fs FSNAMES   a comma separated list of filesystem names or \"all\".\n"
 "                        force criu to (try to) dump/restore these filesystem's\n"

include/cr_options.h

@@ -60,6 +60,7 @@ struct cr_options {
 	bool			force_irmap;
 	char			**exec_cmd;
 	bool			manage_cgroups;
+	char			*cg_yard;
 	char			*new_global_cg_root;
 	struct list_head	new_cgroup_roots;
 	bool			autodetect_ext_mounts;