[PATCH -mm 4/6] always use anon_vma root pointer

[Rik van Riel <riel@redhat.com>]

From: Andrea Arcangeli <aarcange@redhat.com>
Subject: always use anon_vma root pointer

Always use anon_vma->root pointer instead of anon_vma_chain.prev.

Also optimize the map-paths, if a mapping is already established no need to
overwrite it with root anon-vma list, we can keep the more finegrined anon-vma
and skip the overwrite: see the PageAnon check in !exclusive case. This is also
the optimization that hidden the ksm bug as this tends to make
ksm_might_need_to_copy skip the copy, but only the proper fix to
ksm_might_need_to_copy guarantees not triggering the ksm bug unless ksm is in
use. this is an optimization only...

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Rik van Riel <riel@redhat.com>

diff --git a/mm/rmap.c b/mm/rmap.c
index 006f223..2d9504d 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -776,15 +776,13 @@ static void __page_set_anon_rmap(struct page *page,
 	 * If the page isn't exclusively mapped into this vma,
 	 * we must use the _oldest_ possible anon_vma for the
 	 * page mapping!
-	 *
-	 * So take the last AVC chain entry in the vma, which is
-	 * the deepest ancestor, and use the anon_vma from that.
 	 */
 	if (!exclusive) {
-		struct anon_vma_chain *avc;
-		avc = list_entry(vma->anon_vma_chain.prev, struct anon_vma_chain, same_vma);
-		anon_vma = avc->anon_vma;
-	}
+		if (PageAnon(page))
+			return;
+		anon_vma = anon_vma->root;
+	} else
+		BUG_ON(PageAnon(page));
 
 	anon_vma = (void *) anon_vma + PAGE_MAPPING_ANON;
 	page->mapping = (struct address_space *) anon_vma;