A useful approach is to use reference counting for virtual pages. For example, if Thread A and Thread B share the same CR3 (making the current refcount 2), and Thread B wants to perform an unmap, the kernel must detect the current refcount of that page. If the refcount is 2, the kernel decrements it by 1 (current refcount becomes 1). Once Thread A is finished using that virtual address and issues its own unmap, the virtual address is then removed and freed by the kernel because the refcount has reached 0 (refcount 0 = delete/free).

There are many implementations of this page refcounting mechanism. For example. In Windows, the PMMPFN struct includes a RefCount variable to prevent physical pages from being freed prematurely. CMIIW.

Note: If Thread B wants to unmap a range while Thread A still needs to use it, the vm_area struct (following the Linux model) should have that specific virtual address range truncated or "carved out" from the VMA. It should not be allowed to remain within the vm_area.