Subject: root anon vma use root

From: Andrea Arcangeli <aarcange@redhat.com>

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>
---

diff --git a/mm/rmap.c b/mm/rmap.c
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -208,7 +208,7 @@ int anon_vma_clone(struct vm_area_struct
  */
 int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma)
 {
-	struct anon_vma_chain *avc, *root_avc;
+	struct anon_vma_chain *avc;
 	struct anon_vma *anon_vma;
 
 	/* Don't bother if the parent process has no anon_vma here. */
@@ -230,14 +230,7 @@ int anon_vma_fork(struct vm_area_struct 
 	if (!avc)
 		goto out_error_free_anon_vma;
 
-	/*
-	 * Get the root anon_vma on the list by depending on the ordering
-	 * of the same_vma list setup by previous invocations of anon_vma_fork.
-	 * The root anon_vma will always be referenced by the last item
-	 * in the anon_vma_chain list.
-	 */
-	root_avc = list_entry(vma->anon_vma_chain.prev, struct anon_vma_chain, same_vma);
-	anon_vma->root = root_avc->anon_vma;
+	anon_vma->root = pvma->anon_vma->root;
 	/*
 	 * With KSM refcounts, an anon_vma can stay around longer than the
 	 * process it belongs to.  The root anon_vma needs to be pinned
@@ -768,15 +761,13 @@ static void __page_set_anon_rmap(struct 
 	 * 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;