cgroupv1/fs/cpuset/getMount: avoid parsing mountinfo

It is much faster to traverse up a directory tree, comparing the dev_t
field of stat(2) until we hit another device.

Signed-off-by: Kir Kolyshkin <[email protected]>

libcontainer/cgroups/fs/cpuset.go

@@ -7,8 +7,8 @@ import (
 	"io/ioutil"
 	"os"
 	"path/filepath"
+	"syscall"
 
-	"github.com/moby/sys/mountinfo"
 	"github.com/opencontainers/runc/libcontainer/cgroups"
 	"github.com/opencontainers/runc/libcontainer/cgroups/fscommon"
 	"github.com/opencontainers/runc/libcontainer/configs"
@@ -47,24 +47,30 @@ func (s *CpusetGroup) GetStats(path string, stats *cgroups.Stats) error {
 
 // Get the source mount point of directory passed in as argument.
 func getMount(dir string) (string, error) {
-	mi, err := mountinfo.GetMounts(mountinfo.ParentsFilter(dir))
+	dir, err := filepath.Abs(dir)
 	if err != nil {
 		return "", err
 	}
-	if len(mi) < 1 {
-		return "", errors.Errorf("Can't find mount point of %s", dir)
+	st, err := os.Lstat(dir)
+	if err != nil {
+		return "", err
 	}
-
-	// find the longest mount point
-	var idx, maxlen int
-	for i := range mi {
-		if len(mi[i].Mountpoint) > maxlen {
-			maxlen = len(mi[i].Mountpoint)
-			idx = i
+	if !st.IsDir() {
+		return "", &os.PathError{Op: "getMount", Path: dir, Err: syscall.ENOTDIR}
+	}
+	for dir != "/" {
+		parent := filepath.Dir(dir)
+		pst, err := os.Lstat(parent)
+		if err != nil {
+			return "", err
+		}
+		if st.Sys().(*syscall.Stat_t).Dev != pst.Sys().(*syscall.Stat_t).Dev {
+			return dir, nil
 		}
+		dir = parent
 	}
 
-	return mi[idx].Mountpoint, nil
+	return dir, nil
 }
 
 func (s *CpusetGroup) ApplyDir(dir string, cgroup *configs.Cgroup, pid int) error {
@@ -75,7 +81,7 @@ func (s *CpusetGroup) ApplyDir(dir string, cgroup *configs.Cgroup, pid int) erro
 	}
 	root, err := getMount(dir)
 	if err != nil {
-		return err
+		return errors.Wrap(err, "CpusetCgroup.Apply")
 	}
 	root = filepath.Dir(root)
 	// 'ensureParent' start with parent because we don't want to