Do you allow your processes to own unmapped pages?

[OSwhatever]

This scenario is mostly in questions for micro kernels. For user processes to be able cache various memory, could be a disk cache, the ability to serve other processes shared libraries or DMA usage, you often don't need the contents to be mapped in process virtual memory at all times. Basically the process can own unmapped physical pages, but should still be able to grant physical pages to other processes. Is this a technique you are using or do you force the pages to be mapped in order for a process to be able to own a physical page?