Regulated Cascode current mirror
Look up how a cascoded mirror works, M5 and M2 are the mirror, M1 is the cascode.

The cascode is used to keep the Vds of M2 somewhat constant. M3 forms a local feedback loop to make the cascode's gate voltage (node X).

When Vo gets lower at some point M1 will have such a small Vds that it will enter its linear region.

M3 makes sure (through the feedback) that Vds of M2 is Vgs (of M3) so like a diode switched MOSFET, it must be in saturation.

M1 is not essential for the operation of the mirror itself because which transistors form the actual current mirror ? (I wrote that above).