7.2 The unified buffer cache--vm_object_t

FreeBSD implements the idea of a generic “VM object”. VM objects can be associated with backing store of various types--unbacked, swap-backed, physical device-backed, or file-backed storage. Since the filesystem uses the same VM objects to manage in-core data relating to files, the result is a unified buffer cache.

VM objects can be shadowed. That is, they can be stacked on top of each other. For example, you might have a swap-backed VM object stacked on top of a file-backed VM object in order to implement a MAP_PRIVATE mmap()ing. This stacking is also used to implement various sharing properties, including copy-on-write, for forked address spaces.

It should be noted that a vm_page_t can only be associated with one VM object at a time. The VM object shadowing implements the perceived sharing of the same page across multiple instances.