rmap: 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...

[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Rik van Riel <riel@redhat.com>
[kamezawa.hiroyu@jp.fujitsu.com: fix false positive BUG_ON in __page_set_anon_rmap]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 12 insertions, 6 deletions

diff --git a/mm/rmap.c b/mm/rmap.c
index dce74a9efdd6..f5d6799b8a74 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -767,14 +767,20 @@ 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 {
+                /*
+                 * In this case, swapped-out-but-not-discarded swap-cache
+                 * is remapped. So, no need to update page->mapping here.
+                 * We convice anon_vma poitned by page->mapping is not obsolete
+                 * because vma->anon_vma is necessary to be a family of it.
+                 */
+                if (PageAnon(page))
+                        return;
         }
 
         anon_vma = (void *) anon_vma + PAGE_MAPPING_ANON;