mount-utils: don't reread mountinfo on newer kernels

1. Background.

Since the dawn of times mount-utils package tries to work around the bug
in the Linux kernel, which results in occasional incomplete read of
mountinfo entries (from either /proc/mounts or /proc/PID/mountinfo).
The workaround used is to read the whole file twice and compare the two
blobs. If they differ, try again.

The kernel bug is manifesting when mountinfo read is performed
concurrently with an unmount, and can easily be reproduced by running
lots of mounts and unmounts in parallel with the code reading mountinfo.
For one such reproducer, see https://github.com/kolyshkin/procfs-test.

On a Kubernetes node with lots of short-lived containers, mounts and
unmounts are quite frequent. This leads to the occasional bug, and
surely results in much more re-reads of mountinfo, because the
workaround assumes its content is more-or-less static.

The good news is, this bug was finally fixed by kernel commit
9f6c61f96f2d97, which made its way into Linux 5.8.

2. The issue.

The code still read every file at least twice, and up to 10 times. The
chance of re-reading is higher if there is a mount or unmount going on
at the same time. The result is higher system and kernel load, and
degraded performance.

3. The fix.

As the re-reading is not necessary for newer kernels, let's check the
kernel version and skip the workaround if running Linux >= 5.8.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

staging/src/k8s.io/mount-utils/go.mod

@@ -7,6 +7,7 @@ go 1.20
 require (
 	github.com/moby/sys/mountinfo v0.6.2
 	github.com/stretchr/testify v1.8.2
+	golang.org/x/sys v0.8.0
 	k8s.io/klog/v2 v2.100.1
 	k8s.io/utils v0.0.0-20230406110748-d93618cff8a2
 )
@@ -16,7 +17,6 @@ require (
 	github.com/go-logr/logr v1.2.4 // indirect
 	github.com/kr/pretty v0.3.0 // indirect
 	github.com/pmezard/go-difflib v1.0.0 // indirect
-	golang.org/x/sys v0.8.0 // indirect
 	gopkg.in/check.v1 v1.0.0-20201130134442-10cb98267c6c // indirect
 	gopkg.in/yaml.v3 v3.0.1 // indirect
 )

staging/src/k8s.io/mount-utils/mount_helper_unix.go

@@ -20,14 +20,17 @@ limitations under the License.
 package mount
 
 import (
+	"bytes"
 	"errors"
 	"fmt"
 	"io/fs"
 	"os"
 	"strconv"
 	"strings"
+	"sync"
 	"syscall"
 
+	"golang.org/x/sys/unix"
 	"k8s.io/klog/v2"
 	utilio "k8s.io/utils/io"
 )
@@ -91,7 +94,7 @@ type MountInfo struct { // nolint: golint
 
 // ParseMountInfo parses /proc/xxx/mountinfo.
 func ParseMountInfo(filename string) ([]MountInfo, error) {
-	content, err := utilio.ConsistentRead(filename, maxListTries)
+	content, err := readMountInfo(filename)
 	if err != nil {
 		return []MountInfo{}, err
 	}
@@ -198,3 +201,50 @@ func PathExists(path string) (bool, error) {
 	}
 	return false, err
 }
+
+// These variables are used solely by kernelHasMountinfoBug.
+var (
+	hasMountinfoBug       bool
+	checkMountinfoBugOnce sync.Once
+)
+
+// kernelHasMountinfoBug checks if the kernel bug that can lead to incomplete
+// mountinfo being read is fixed. It does so by checking the kernel version.
+//
+// The bug was fixed by the kernel commit 9f6c61f96f2d97 (since Linux 5.8).
+// Alas, there is no better way to check if the bug is fixed other than to
+// rely on the kernel version returned by uname.
+func kernelHasMountinfoBug() bool {
+	checkMountinfoBugOnce.Do(func() {
+		// Assume old kernel.
+		hasMountinfoBug = true
+
+		uname := unix.Utsname{}
+		err := unix.Uname(&uname)
+		if err != nil {
+			return
+		}
+
+		end := bytes.IndexByte(uname.Release[:], 0)
+		v := bytes.SplitN(uname.Release[:end], []byte{'.'}, 3)
+		if len(v) != 3 {
+			return
+		}
+		major, _ := strconv.Atoi(string(v[0]))
+		minor, _ := strconv.Atoi(string(v[1]))
+
+		if major > 5 || (major == 5 && minor >= 8) {
+			hasMountinfoBug = false
+		}
+	})
+
+	return hasMountinfoBug
+}
+
+func readMountInfo(path string) ([]byte, error) {
+	if kernelHasMountinfoBug() {
+		return utilio.ConsistentRead(path, maxListTries)
+	}
+
+	return os.ReadFile(path)
+}

staging/src/k8s.io/mount-utils/mount_linux.go

@@ -37,7 +37,6 @@ import (
 
 	"k8s.io/klog/v2"
 	utilexec "k8s.io/utils/exec"
-	utilio "k8s.io/utils/io"
 )
 
 const (
@@ -633,7 +632,7 @@ func (mounter *SafeFormatAndMount) GetDiskFormat(disk string) (string, error) {
 
 // ListProcMounts is shared with NsEnterMounter
 func ListProcMounts(mountFilePath string) ([]MountPoint, error) {
-	content, err := utilio.ConsistentRead(mountFilePath, maxListTries)
+	content, err := readMountInfo(mountFilePath)
 	if err != nil {
 		return nil, err
 	}