the page table page's wired count rather than its hold count to contain
the reference count. My rationale for this change is based on several
factors:
1. The machine-independent and pmap layers used the same hold count field
in subtly different ways. The machine-independent layer uses the hold
count to implement a form of ephemeral wiring that is used by pipes,
physio, etc. In other words, subsystems where we wish to temporarily
block a page from being swapped out while it is mapped into the kernel's
address space. Such pages are never removed from the page queues.
Instead, the page daemon recognizes a non-zero hold count to mean "hands
off this page." In contrast, page table pages are never in the page
queues; they are wired from birth to death. The hold count was being
used as a kind of reference count, specifically, the number of valid
page table entries within the page. Not surprisingly, these two
different uses imply different synchronization rules: in the machine-
independent layer access to the hold count requires the page queues
lock; whereas in the pmap layer the pmap lock is required. Thus,
continued use by the pmap layer of vm_page_unhold(), which asserts that
the page queues lock is held, made no sense.
2. _pmap_unwire_pte_hold() was too forgiving in its handling of the wired
count. An unexpected wired count on a page table page was ignored and
the underlying page leaked.
3. In a word, microoptimization. Using the wired count exclusively, rather
than a combination of the wired and hold counts, makes the code slightly
smaller and faster.
Reviewed by: tegge@
a58cafd11e