devmapper: show dmesg if mount fails

kolyshkin · kolyshkin · commit 46833ee1c353 · 2017-09-17T22:04:31.000-07:00
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 &lt;kolyshkin@gmail.com&gt;
diff --git a/daemon/graphdriver/devmapper/deviceset.go b/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 != "" {
diff --git a/pkg/dmesg/dmesg_linux.go b/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]
+}
diff --git a/pkg/dmesg/dmesg_linux_test.go b/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)))
+}
