MFC revision 1.525   When support for 2MB/4MB pages was added in revision 1.148 an error was   made in pmap_protect(): The pmap's resident count should not be reduced   unless mappings are removed.
MFC revision 1.525
  When support for 2MB/4MB pages was added in revision 1.148 an error was
  made in pmap_protect(): The pmap's resident count should not be reduced
  unless mappings are removed.

  The errant change to the pmap's resident count could result in a later
  pmap_remove() failing to remove any mappings if the errant change has set
  the pmap's resident count to zero.

MFC revision 1.526
  Decouple the unrefing of a page table page from the removal of a pv entry.
  In other words, change pmap_remove_entry() such that it no longer unrefs
  the page table page.  Now, it only removes the pv entry.

MFC revision 1.527
  Eliminate unneeded diagnostic code.

MFC revision 1.528
  Eliminate unneeded diagnostic code.

  Eliminate an unused #include.  (Kernel stack allocation and deallocation
  long ago migrated to the machine-independent code.)

MFC revision 1.529
  Simplify the page table page reference counting by pmap_enter()'s change of
  mapping case.

  Eliminate a stale comment from pmap_enter().

MFC revision 1.530
  When pmap_allocpte() destroys a 2/4MB "superpage" mapping it does not
  reduce the pmap's resident count accordingly.  It should.

MFC revision 1.532
  Eliminate unnecessary TLB invalidations by pmap_enter().  Specifically,
  eliminate TLB invalidations when permissions are relaxed, such as when a
  read-only mapping is changed to a read/write mapping.  Additionally,
  eliminate TLB invalidations when bits that are ignored by the hardware,
  such as PG_W ("wired mapping"), are changed.

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