From: Hugh Dickins <hugh@veritas.com>

Tracking anonymous pages by anon_vma,pgoff or mm,address needs a
pointer,offset pair in struct page: mapping,index the natural choice.  But
swapcache uses those for &swapper_space,swp_entry_t.

It's trivial to separate swapcache from pagecache with radix tree; most of
swapper_space is actually unused, just a fiction to pretend swap like file;
and page->private is a good place to keep swp_entry_t, now that swap never
uses bufferheads.

Define PG_anon bit, page_add_rmap SetPageAnon and put an oopsable address in
page->mapping to test that we're not confused by it.  Define
page_mapping(page) macro to give NULL when PageAnon, whatever may be in
page->mapping.  Define PG_swapcache bit, deduce swapper_space from that in
the few places we need it.

add_to_swap_cache now distinct from add_to_page_cache.  Separating the caches
somewhat simplifies the tmpfs swizzling in swap_state.c, now the page can
briefly be in both caches.

The rmap method remains pte chains, no change to that yet.  But one small
functional difference: the use of PageAnon implies that a page truncated
while still mapped will no longer be found and freed (swapped out) by
try_to_unmap, will only be freed by exit or munmap.  But normally pages are
unmapped by vmtruncate: this should only affect nonlinear mappings, and a
later patch not in this batch will fix that.