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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@
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