This is part of a series of patches intended to enable first-touch numa policies for UMA by default. It also reduces the cost of uma_zalloc/zfree by approximately 30% each in my tests.
UMA_ZONE_NUMA has confused people. It should be UMA_ZONE_FIRSTTOUCH and it should be the default policy. Some zones want to over-ride the default. So this change makes it so a zone with no policy specified gets FIRSTTOUCH and otherwise it gets the specified policy. This enables XDOMAIN on all NUMA builds. In my testing this is no slower and will increasingly become faster as more of the kernel behaves better WRT NUMA.