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MEMC enhancements for Linux 2.3
-------------------------------

The current interface:

  There is a cache of the MEMC settings held in tsk->tss.memcmap, which is
  kept up to date by the following functions (from the page tables):

  update_memc_all()                             hits:   2
        Updates all MEMC caches on all processes.  Update the real MEMC
        to reflect the `current' tasks page tables.

  update_memc_tsk(tsk)                          hits:   0
        Update the MEMC cache for the specified task.  If tsk is the
        `current' task, then update the real MEMC as well.

  update_memc_mm(mm)                            hits:   16
        Update the MEMC cache for any task which has a mm_struct
        corresponding to `mm'.  If the `current' tasks mm_struct
        includes this, then update the real MEMC as well.

  update_memc_addr(mm, addr, pte)               hits:   8
        Update the MEMC cache entry defined by the physical address
        in pte for any task which has a mm_struct corresponding to `mm'.
        If the `current' tasks mm_struct includes this, then update the
        real MEMC as well.

The proposed interface:

  Couple the MEMC cache into the mm_struct so that we only have to
  keep one copy per mm_struct.  This also allows us to reduce the
  number of loops through all existing tasks on each MEMC change.

  memc_clear(mm, physaddr)                      hits:   6
        Clear the MEMC mapping associated with the physical address
        `physaddr'.  If the `current' tasks mm_struct is `mm', then
        update the real MEMC as well.   (should equate to a possible
        two writes and zero reads).

  memc_update_addr(mm, pte, logaddr)            hits:   10
        Change the MEMC mapping for the physical address specified
        in `pte' to point to the logical address `logaddr', with the
        protection specified in `pte'.  If the `current' tasks mm_struct
        is `mm', then update the real MEMC as well.  (should again equate
        to a possible two writes and zero reads).

  memc_update_mm(mm)                            hits:   7
        Rebuild the MEMC mappings for the specified `mm' in the same way
        that update_memc_mm used to.  If the `current' tasks mm_struct
        is `mm', update the real MEMC as well.

  memc_update_all()                             hits:   2
        Rebuild the MEMC mappings for all mm_structs, including the real
        MEMC.

The hit numbers are approximate usage of each function in the 2.2.7
memory management (mm) code, and there are other uses outside this area.