memory access a mapping was caused by. This is passed through from uvm_fault()
and udv_fault(), and in most other cases is 0.
The pmap module may use this to preset R/M information. On MMUs which require
R/M emulation, the implementation may preset the bits and avoid taking another
fault. On MMUs which keep R/M information in hardware, the implementation may
preset its cached bits to speed up the next call to pmap_is_modified() or
pmap_is_referenced().
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