mm: fix swapin race condition

mm: fix swapin race condition

commit 4969c1192d15afa3389e7ae3302096ff684ba655 upstream.

The pte_same check is reliable only if the swap entry remains pinned (by
the page lock on swapcache).  We've also to ensure the swapcache isn't
removed before we take the lock as try_to_free_swap won't care about the
page pin.

One of the possible impacts of this patch is that a KSM-shared page can
point to the anon_vma of another process, which could exit before the page
is freed.

This can leave a page with a pointer to a recycled anon_vma object, or
worse, a pointer to something that is no longer an anon_vma.

[Backport to 2.6.35.5 (anon_vma instead of anon_vma->root in ksm.h) by Hugh]

[riel@redhat.com: changelog help]
Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Acked-by: Hugh Dickins <hughd@google.com>
Reviewed-by: Rik van Riel <riel@redhat.com>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>