We measured the viscosity of poly(methyl methacrylate) (PMMA) films supported by silica, where the carbonyl group on the side chains of the polymer interacts strongly with the hydroxyl groups of the surface. The result shows that the viscosity increases with decreasing film thickness at temperatures above 110 degrees C, but displays an opposite trend at lower temperatures. A three-layer model, consisting of a mobile top layer, a bulk-like middle layer and an immobile bottom layer was found to fit the 0 data well. A detailed breakdown of the layer contributions to the total mobility unveils that the mobility gain brought about by the top layer is balanced by the mobility loss by the bottom layer at 110 degrees C. When the temperature is lowered or raised, the balance is offset, in favor, of the top and bottom layer, respectively.