Add a pointer to the ksm page into struct stable_node, holding a reference
to the page while the node exists.  Put a pointer to the stable_node into
the ksm page's ->mapping.

Then we don't need get_ksm_page() while traversing the stable tree: the
page to compare against is sure to be present and correct, even if it's no
longer visible through any of its existing rmap_items.

And we can handle the forked ksm page case more efficiently: no need to
memcmp our way through the tree to find its match.
Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Cc: Izik Eidus <ieidus@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>