DC motor speed control using Pmos
The p channel mosfet you have chosen has an on resistance of about 0.4 ohms. If your motor current is (say) 4 amps, this will produce a power dissipation of 6.4 watts. It's likely that you should be choosing a mosfet with lower on-resistance or use a heat sink.

You also have the mosfet upside down in your schematic but I suspect this is just a diagrammatical error. 

Your motor peak current could be a lot higher than the 1 amp you state when driving a mechanical load. Also, if your pwm frequency is too high there could be significant extra load produced by your motor that you may not of accounted for.

You haven't specified the fly back diode and a poor choice here could mean more current. For instance, a 1N400x diode has a very poor reverse recovery time of about 30 us - in other words it takes this length of time to revert from forward conduction (fly back) to reverse blocking.