I have painted over a popcorn ceiling and it can be done. However, I would really consider removing the texture first. I found that as the texture becomes damp it will fall off the drywall. Spray it with a fine mist of water from a garden sprayer and then scrape off with a broad knife. If you are determined to just paint over it. Load up the roller heavy and make ONE pass. Any subsequent passes will likely peel the texture right off onto the roller. A real mess.