[$] Freezing out the page reference count

The pagestructure sits at the core of the kernel's memory-management subsystem(for now), and a key part of that structure is its reference count, storedin refcount. The page reference count tells the kernel how manyusers a given page has and when it can be freed. That count is not neededfor every page in the system, though. Matthew Wilcox has recently resurrectedan oldpatch set that expands the concept of a "frozen" page - one that lacks ameaningful reference count - to the immediate benefit of the slab allocatorbut in the service of a longer-term goal as well.