devmapper: show dmesg if mount fails

If mount fails, the reason might be right there in the kernel log ring buffer.
Let's include it in the error message, it might be of great help.

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

daemon/graphdriver/devmapper/deviceset.go

@@ -21,6 +21,7 @@ import (
 	"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"
@@ -1199,7 +1200,7 @@ 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': %s\n%v", info.DevName(), fsMountPoint, err, string(dmesg.Dmesg(256)))
 	}
 
 	defer unix.Unmount(fsMountPoint, unix.MNT_DETACH)
@@ -2390,7 +2391,7 @@ 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': %s\n%v", info.DevName(), path, err, string(dmesg.Dmesg(256)))
 	}
 
 	if fstype == "xfs" && devices.xfsNospaceRetries != "" {

pkg/dmesg/dmesg_linux.go

@@ -0,0 +1,20 @@
+// +build linux
+
+package dmesg
+
+import (
+	"unsafe"
+
+	"golang.org/x/sys/unix"
+)
+
+// 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 := unix.Syscall(unix.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)))
+}