structure. This makes it possible to pre-allocate PTEs for the kernel,
which is necessary for a reliable implementation of pmap_kenter(). This
also avoids wasting space (about 48 bytes per page) for kernel mappings
and user mappings of memory-mapped devices.
This also fixes a bug with the previous version where the implementation
required the pv_entry structure to be physically contiguous but did not
enforce this (the structure size was not a power of two). This meant
that the pv_entry free list was quickly corrupted as soon as the system
was even mildly loaded.
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