“Linux layers the machine independent/dependent layer in an
unusual manner in comparison to other operating systems [CP99].
Other operating systems have objects that manage the underlying
physical pages, such as the pmap object in BSD. Linux instead
maintains the concept of a three-level page table in the
architecture-independent code even if the underlying architecture
does not support it. Although this is conceptually easy to
understand, it also means that the distinction between different
types of pages is very blurry, and page types are identified by
their flags or what lists they exist on rather than the objects
they belong to…”