From: Matt Mackall <mpm@selenic.com>

Doing the algebra:

c = (a - b) * 3/2
a' = a - c = a - 3/2(a - b) = (2a - 3a + 3b)/2 = (3b - a)/2
a' >= 0
3b - a >= 0
3b >= a
b >= a/3
nr_free_pages() >= mempages/3

We can indeed get into trouble if we try to load a large kernel on a very
small box (ie kernel reserves more than 2/3 of usable memory).  Surprisingly I
haven't hit this, but here's a fix.