[$] Multi-size THP creation, two different ways

Huge pages can increase the performance of many programs, but they can alsohave unfortunate performance impacts of their own. Over the last fewyears, multi-size transparent huge pages (mTHPs) have increasingly beenseen as a happy medium that bring the benefits of huge pages at a lower cost.The system cannot benefit from mTHPs, though, if it does not create them;two developers have independently posted patches to enable the creation ofmTHPs in the background.