It is likely that the backing object has the page ready to use, which can be handled with a read lock only (or even without locks, but I did not go for it right now).
This largely eliminates vm object contention when doing buildkernel:
before:
```
103744637 (rw:vm object)
6091096 (spin mutex:turnstile chain)
4682478 (rw:pmap pv list)
4658491 (sleep mutex:process lock)
3075988 (sx:vm map (user))
1431320 (sleep mutex:pmap)
```
after:
```
22508492 (rw:pmap pv list)
5903096 (sleep mutex:process lock)
2476543 (sx:vm map (user))
1119649 (sleep mutex:pmap)
665686 (sleep mutex:vm page free queue)
588529 (sleep mutex:vm active pagequeue)
455094 (sleep mutex:umtxql)
285416 (rw:vm object)
```