Cracked/warped head and/or head-gasket allowing coolant to enter the combustion chamber.
Does the white smoke smell slightly sweet? That would be a good indicator that the smoke is coolant being heated/vaporized in the engine. Check your spark plugs, they will probably be wet and/or have some rust on the threads and possible white, powdery residue on the end of the plug. Pressure test the radiator and check to see if the pressure leaks down rapidly or perform a compression test on the engine, the pressure may leak down and bubbles will form in the radiator.