The surname “Milner”

My dad’s grandpa came here to America from Russia in the early 1900s. However, I’ve been trying to find out where the surname came from for several years now, and have had no success, as there aren’t many “Milner’s” in America or Europe and I have to assume my grandpa changed his original surname to Milner when he immigrated here … I want to find where it came from so bad so that I can maybe track our real original surname back to any relatives or ancestors we may still have in Russia and all throughout eastern Europe.