Input Impedance of a MOSFET circuit

[CoolDude420]

Homework Statement

(From Razavi's Design of Analog CMOS Integrated Circuits - Chapter 6)

Homework Equations

The Attempt at a Solution

Not too sure about calculating the input impedance of this. Initially I thought that it would be the series combination of C1 and C2, since we are neglecting Cgs, so

But after I actually did it on paper, I got something like this,

Usually, I would try and use superposition for this but I don't see how that would work since Vx would dissapear. Honestly, I have no idea what to do now?

 

[CoolDude420]

EDIT: I just realized my mistake. When calculating input/output resistances, all independant sources have to be zeroed just like in Thevenin's Theorem, so that current source there shouldn't be there in my small-signal model.
From that it's just one KCL and job done.

 

[rude man]

So what is your answer?