BACKPORT: devmapper: Log fstype and mount options during mount error

Upstream reference: moby#35732

Signed-off-by: Antonio Murdaca <runcom@redhat.com>

Author: runcom

daemon/graphdriver/devmapper/deviceset.go

@@ -20,10 +20,10 @@ import (
 	"time"
 
 	"github.com/Sirupsen/logrus"
-
 	"github.com/docker/docker/daemon/graphdriver"
 	"github.com/docker/docker/dockerversion"
 	"github.com/docker/docker/pkg/devicemapper"
+	"github.com/docker/docker/pkg/dmesg"
 	"github.com/docker/docker/pkg/idtools"
 	"github.com/docker/docker/pkg/loopback"
 	"github.com/docker/docker/pkg/mount"
@@ -1191,7 +1191,7 @@ func (devices *DeviceSet) growFS(info *devInfo) error {
 	options = joinMountOptions(options, devices.mountOptions)
 
 	if err := mount.Mount(info.DevName(), fsMountPoint, devices.BaseDeviceFilesystem, options); err != nil {
-		return fmt.Errorf("Error mounting '%s' on '%s': %s", info.DevName(), fsMountPoint, err)
+		return fmt.Errorf("Error mounting '%s' on '%s'. fstype=%s options=%s: %s\n%v", info.DevName(), fsMountPoint, devices.BaseDeviceFilesystem, options, err, string(dmesg.Dmesg(256)))
 	}
 
 	defer syscall.Unmount(fsMountPoint, syscall.MNT_DETACH)
@@ -2365,7 +2365,7 @@ func (devices *DeviceSet) MountDevice(hash, path, mountLabel string) error {
 	options = joinMountOptions(options, label.FormatMountLabel("", mountLabel))
 
 	if err := mount.Mount(info.DevName(), path, fstype, options); err != nil {
-		return fmt.Errorf("devmapper: Error mounting '%s' on '%s': %s", info.DevName(), path, err)
+		return fmt.Errorf("devmapper: Error mounting '%s' on '%s'. fstype=%s options=%s: %s\n%v", info.DevName(), path, fstype, options, err, string(dmesg.Dmesg(256)))
 	}
 
 	if fstype == "xfs" && devices.xfsNospaceRetries != "" {

pkg/dmesg/dmesg_linux.go

@@ -0,0 +1,19 @@
+// +build linux
+
+package dmesg
+
+import (
+	"syscall"
+	"unsafe"
+)
+
+// Dmesg returns last messages from the kernel log, up to size bytes
+func Dmesg(size int) []byte {
+	t := uintptr(3) // SYSLOG_ACTION_READ_ALL
+	b := make([]byte, size)
+	amt, _, err := syscall.Syscall(syscall.SYS_SYSLOG, t, uintptr(unsafe.Pointer(&b[0])), uintptr(len(b)))
+	if err != 0 {
+		return []byte{}
+	}
+	return b[:amt]
+}

pkg/dmesg/dmesg_linux_test.go

@@ -0,0 +1,9 @@
+package dmesg
+
+import (
+	"testing"
+)
+
+func TestDmesg(t *testing.T) {
+	t.Logf("dmesg output follows:\n%v", string(Dmesg(512)))
+}