It could be an oil consumption or an aftermarket computer calibration.Scan the engine & check for the latest GM calibrations.If necessary do an oil consumption test.Check out these systems.
DEFINITION: White smoke under load, idle or start up hot or cold.
Fuel System Inspection
If excessive smoke is present, inspect for a stuck fuel injector.
Inspect the engine coolant temperature (ECT) sensor. Use the scan tool in order to compare the ECT with the ambient air temperature on a cold engine. If the coolant temperature reading is more than 5 degrees more or less than the ambient air temperature on a cold engine, inspect for a high resistance in the coolant sensor circuit or the sensor itself.
Air Intake System Inspection
Engine Mechanical Inspection
Checked oil and coolant levels prior to contacting you. They are ok. Just returned from an 1800+ road trip and averaged 21.7 mpl so I feel my truck is running OK except for the white smoke and diesel odor when idiling. Will check other suggested items as a process of elimination