With page_map_lock out of the way, there's no need for page_referenced and
try_to_unmap to use trylocks - provided we switch anon_vma->lock and
mm->page_table_lock around in anon_vma_prepare.  Though I suppose it's
possible that we'll find that vmscan makes better progress with trylocks than
spinning - we're free to choose trylocks again if so.

Try to update the mm lock ordering documentation in filemap.c.  But I still
find it confusing, and I've no idea of where to stop.  So add an mm lock
ordering list I can understand to rmap.c.
Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>