The upper part functions as a current mirror, and therefore as Vin increases so does the drain current of M3 and M4.
Now consider the lower part. As the drain current of M1 increases, that of M2 decreases, since they need to add up to ISS.
The current going through R1 and R2 is determined as the current drain current of M4 subtracting that of M2. Now that the former is increasing and the latter is decreasing, the current through the output node increases, hence Vout and Vx.
If Vx is raised, the drain current of M2 also rises, which is in opposite direction as the influence of Vin. So the feedback is negative.