vm_map_wire() works by calling vm_fault(VM_FAULT_WIRE) on each page in
the rage. (For largepage mappings, it calls vm_fault() once per large
page.)
A pager's populate method may return more than one page to be mapped.
If VM_FAULT_WIRE is also specified, we'd wire each page in the run, not
just the fault page. Consider an object with two pages mapped in a
vm_map_entry, and suppose vm_map_wire() is called on the entry. Then,
the first vm_fault() would allocate and wire both pages, and the second
would encounter a valid page upon lookup and wire it again in the
regular fault handler. So the second page is wired twice and will be
leaked when the object is destroyed.
Fix the problem by modify vm_fault_populate() to wire only the fault
page. Similarly, wire only the PTE for the mapping of the fault page.
This leak can be triggered by loading and unloading a kernel module,
since OBJT_PHYS objects now use the populate method, and since the
kernel linker explicitly wires KLD sections/segments.
PR: 260347