Fix kmem accouting when use with cgroupsPath

Fixes: opencontainers#1347
Fixes: opencontainers#1083

The root cause of opencontainers#1083 is because we're joining an
existed cgroup whose kmem accouting is not initialized,
and it has child cgroup or tasks in it.

Fix it by checking if the cgroup is first time created,
and we should enable kmem accouting if the cgroup is
craeted by libcontainer with or without kmem limit
configed. Otherwise we'll get issue like opencontainers#1347

Signed-off-by: Qiang Huang <h.huangqiang@huawei.com>

libcontainer/cgroups/fs/memory.go

@@ -33,14 +33,18 @@ func (s *MemoryGroup) Apply(d *cgroupData) (err error) {
 	path, err := d.path("memory")
 	if err != nil && !cgroups.IsNotFound(err) {
 		return err
+	} else if path == "" {
+		return nil
 	}
 	if memoryAssigned(d.config) {
-		if path != "" {
+		if _, err := os.Stat(path); os.IsNotExist(err) {
 			if err := os.MkdirAll(path, 0755); err != nil {
 				return err
 			}
-		}
-		if d.config.KernelMemory != 0 {
+			// Only enable kernel memory accouting when this cgroup
+			// is created by libcontainer, otherwise we might get
+			// error when people use `cgroupsPath` to join an existed
+			// cgroup whose kernel memory is not initialized.
 			if err := EnableKernelMemoryAccounting(path); err != nil {
 				return err
 			}