devmapper: add useful comments

Upstream-commit: ad968ef3ef54f3161e8e1012f0ef20b8757ac0aa
Component: engine

components/engine/devmapper/deviceset_devmapper.go

@@ -88,6 +88,10 @@ func (devices *DeviceSetDM) hasImage(name string) bool {
 	return err == nil
 }
 
+// ensureImage creates a sparse file of <size> bytes at the path
+// <root>/devicemapper/<name>.
+// If the file already exists, it does nothing.
+// Either way it returns the full path.
 func (devices *DeviceSetDM) ensureImage(name string, size int64) (string, error) {
 	dirname := devices.loopbackDir()
 	filename := path.Join(dirname, name)

components/engine/devmapper/devmapper.go

@@ -18,6 +18,7 @@ package devmapper
 #define LOOP_CTL_GET_FREE       0x4C82
 #endif
 
+// FIXME: this could easily be rewritten in go
 char*			attach_loop_device(const char *filename, int *loop_fd_out)
 {
   struct loop_info64	loopinfo = {0};
@@ -441,6 +442,7 @@ func free(p *C.char) {
 	C.free(unsafe.Pointer(p))
 }
 
+// This is the programmatic example of "dmsetup create"
 func createPool(poolName string, dataFile *os.File, metadataFile *os.File) error {
 	task, err := createTask(DeviceCreate, poolName)
 	if task == nil {