[$] Reducing direct-map fragmentation with __GFP_UNMAPPED

The kernel's direct map makes all of a system's physical memory available
to the kernel within its address space — on 64-bit systems, at least. This
seemingly simple feature has proved to be hard to maintain, in the face of
the requirements faced by current systems, while keeping good performance.
The latest attempt to address this issue is this patch
set from Mike Rapoport adding more direct-map awareness to the kernel's
page allocator.