No performance cost, because these guarded buffers are allocated only when the
pools grow, which is a rare operation that occurs almost only at boot time. No
actual memory consumption either, because unmapped areas don't consume physical
memory, only virtual, and on 64bit arches we have plenty of that - eg 32TB on
amd64, far beyond what we will ever need -, so no problem with consuming VA.
i like this idea, but i would like to point out that HPE already
sell a machine with 48TiB ram and nvdimms are going to explode
the apparently memory in the coming years, so "32TiB" is very far
from "plenty". we have many challenges to get beyond 8TiB tho,
since we count pages in 'int' all over uvm.