I've smoked whole potatoes many times. A little olive oil on the outside, a sprinkle of coarse salt and put them in the smoker till soft (I usually plan 2-3 hours for large potatoes when the smoker is at 225 or so. You get a good smoke flavor on the skin and save using your oven.
For mashed, I usually peel, chop into chunks, and place into a large pot of boiling salted water, cover and reduce heat to medium and simmer for about 20 minutes till tender. (small chunks take less time, large chunks take more time) Then take out of the water, drain, and mash with some sour cream and cream cheese, (8 oz of each for every 5 lbs of potatoes) then put in an aluminum or other open container and smoke for 2-3 hours, stirring the bottom up over the top avery 30-45 minutes to get the smoke flavor all the way through and not just on the surface.
If you are maintaining thin blue smoke, they'll be fine. The less you stir, the more the top gets a nice brown tint and a bit of a crust, just like in the oven.
I'm planting some large sweet potatoes whole on the side of my turkey tomorrow.