- Special memory I/O pages cannot be freed into the general page pool, such as the physical address used for I/O APIC. If it happens, then another program would allocate the memory I/O page and then write to that physical address as if it's RAM.
- If two address spaces share a page (mmap w/ MAP_SHARED, or shmat), then pages need to be reference counted.
- If two processes share the same page table (clone system call), then page tables need to be reference counted.
Since he's too engaged in debugging, I'm writing these down for him in case he forgets. I said to him, maybe the reason why people haven't made ground-breaking research into redesigning operating system architecture because the moment you walk in, you get lost in the woods, and you don't see the forest anymore.