All diesels have blow by at idle. Usually more than people are used to seeing from a gas engine. The rings are designed to cope with very high cylinder pressures and for maximum durability. A softer ring (like is used in a gas engine) would seal better at low CP's and reduce blow by but would not last 300,000mi which is what is expected from a diesel. If it smokes when cold, I'd look at the glow plug system first. If even one GP is not fuctioning, that cylinder will smoke until warmed up. After that, I'd get some oil samples tested for excessive soot and carbon (from unburned fuel bypassing the rings under power) before a tear down. Watch the oil consumption, if it is more than 4qts per 5000mi you might consider a rebuild.