From Bill Irwin

This allocates pgd's and pmd's using the slab and slab ctors.  It has a
benefit beyond preconstruction in that PAE pmd's are accounted via
/proc/slabinfo

Profiling of kernel builds by Martin Bligh shows a 30-40% drop in CPU load
due to pgd_alloc()'s page clearing activity.  But this was already a tiny
fraction of the overall CPU time.