White smoke is usually caused mainly by two things on a diesel. The first obvious one is water. But, if water level is not going down at all then the latter is not likely. However keep an eye on it as likely cause would be head gasket. Take of oil filler cap, is there loads of mayonnaise like deposits on it? If so water is definately in the oil.
The second is diesel, leaking injector seals being the most likely. But sticking injectors might be a cause also. I would put a good quality injector cleaner in the fuel in the mean time to see if it helps.