rmap: move exclusively owned pages to own anon_vma in do_wp_page()

When the parent process breaks the COW on a page, both the original which
is mapped at child and the new page which is mapped parent end up in that
same anon_vma.  Generally this won't be a problem, but for some workloads
it could preserve the O(N) rmap scanning complexity.

A simple fix is to ensure that, when a page which is mapped child gets
reused in do_wp_page, because we already are the exclusive owner, the page
gets moved to our own exclusive child's anon_vma.

Signed-off-by: Rik van Riel <riel@redhat.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Larry Woodman <lwoodman@redhat.com>
Cc: Lee Schermerhorn <Lee.Schermerhorn@hp.com>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 1 insertions, 0 deletions

diff --git a/include/linux/rmap.h b/include/linux/rmap.h
index 62da2001d55c..72be23b1480a 100644
--- a/include/linux/rmap.h
+++ b/include/linux/rmap.h
@@ -125,6 +125,7 @@ static inline void anon_vma_merge(struct vm_area_struct *vma,
 /*
  * rmap interfaces called when adding or removing pte of page
  */
+void page_move_anon_rmap(struct page *, struct vm_area_struct *, unsigned long);
 void page_add_anon_rmap(struct page *, struct vm_area_struct *, unsigned long);
 void page_add_new_anon_rmap(struct page *, struct vm_area_struct *, unsigned long);
 void page_add_file_rmap(struct page *);