[$] Freezing out the page reference count

The page
structure sits at the core of the kernel's memory-management subsystem
(for now), and a key part of that structure is its reference count, stored
in refcount. The page reference count tells the kernel how many
users a given page has and when it can be freed. That count is not needed
for every page in the system, though. Matthew Wilcox has recently resurrected
an old
patch set that expands the concept of a 'frozen' page — one that lacks a
meaningful reference count — to the immediate benefit of the slab allocator
but in the service of a longer-term goal as well.