From: Hugh Dickins <hugh.dickins@xxxxxxxxxxxxx>
KSM will need to identify its kernel merged pages unambiguously,
and /proc/kpageflags will probably like to do so too.
Since KSM will only be substituting anonymous pages, statistics are
best preserved by making a PageKsm page a special PageAnon page:
one with no anon_vma.
But KSM then needs its own page_add_ksm_rmap() - keep it in ksm.h near
PageKsm; and do_wp_page() must COW them, unlike singly mapped PageAnons.