is never 0, so one cannot test for a NULL pointer after a physical
address is translated into a virtual pointer with said macro. Instead,
keep the physical address around and test it against 0. Note that
this obviously implies that a PTE can never be allocated at physical
address 0. This isn't exactly guaranteed, but hasn't been a problem
so far. We test the physical address against 0 for as long as the ia64
port exists...
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