Drywall screws for subfloor
By "drywall screws", I think you are meaning 1 5/8" blued screws. I do not think that they would be a good choice. We usually shoot down plywood subflooring with 2 1/2" to 3" ring shank nails, however 2" to 2 1/2" decking screws work well, but just a lot more work. I would encourage you to use a little adhesive on top of the floor joists or strips of felt paper or Grace ice/water shield to avoid any squeaks later on.