It does happen, and not just for two different processes. For example, a common trick to implement an efficient circular list for objects of varying size is to

It does happen, and not just for two different processes. For example, a common trick to implement an efficient circular list for objects of varying size is to map the same physical addresses twice in adjacent locations. Then you can just use a pointer to the start of each object and have no special handling for the case where an object straddles the end of the allocated area.