Why negative feedback (common source degeneration) increases linearity range?
Suppose you have a nonlinear transfer function \$\frac{v_O}{v_{IN}} = a(v_{IN})\$ 

If you add negative feedback 1/m such that: 

\$ \frac{v_O}{v_{IN}}  = a(v_{IN})(v_{IN} - \frac{ v_O}{m})\$

so 

\$ \frac{v_O}{v_{IN}} = \frac {a(v_{IN})}{1+\frac{a(v_{IN})}{m}}\$

For \$\frac{a(v_{IN})}{m} \gg 1, \frac{v_O}{v_{IN}} \approx m \$  (linear gain of m)

For \$\frac{a(v_{IN})}{m} \ll 1, \frac{v_O}{v_{IN}} \approx a(v_{IN}) \$ (nonlinear with negligible feedback) 

As you can see, the closed loop gain must be reduced significantly from the open-loop gain: \$m \ll a(v_{IN}) \$ for there to be a large reduction in nonlinearity.