mm: use refcounts for page_lock_anon_vma()

Convert page_lock_anon_vma() over to use refcounts.  This is done to
prepare for the conversion of anon_vma from spinlock to mutex.

Sadly this inceases the cost of page_lock_anon_vma() from one to two
atomics, a follow up patch addresses this, lets keep that simple for now.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Acked-by: Hugh Dickins <hughd@google.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: David Miller <davem@davemloft.net>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Russell King <rmk@arm.linux.org.uk>
Cc: Paul Mundt <lethal@linux-sh.org>
Cc: Jeff Dike <jdike@addtoit.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: Nick Piggin <npiggin@kernel.dk>
Cc: Namhyung Kim <namhyung@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 31 insertions, 28 deletions

diff --git a/mm/migrate.c b/mm/migrate.c
index 34132f8e9109..e4a5c912983d 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -721,15 +721,11 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
                  * Only page_lock_anon_vma() understands the subtleties of
                  * getting a hold on an anon_vma from outside one of its mms.
                  */
-                anon_vma = page_lock_anon_vma(page);
+                anon_vma = page_get_anon_vma(page);
                 if (anon_vma) {
                         /*
-                         * Take a reference count on the anon_vma if the
-                         * page is mapped so that it is guaranteed to
-                         * exist when the page is remapped later
+                         * Anon page
                          */
-                        get_anon_vma(anon_vma);
-                        page_unlock_anon_vma(anon_vma);
                 } else if (PageSwapCache(page)) {
                         /*
                          * We cannot be sure that the anon_vma of an unmapped
@@ -857,13 +853,8 @@ static int unmap_and_move_huge_page(new_page_t get_new_page,
                 lock_page(hpage);
         }
 
-        if (PageAnon(hpage)) {
-                anon_vma = page_lock_anon_vma(hpage);
-                if (anon_vma) {
-                        get_anon_vma(anon_vma);
-                        page_unlock_anon_vma(anon_vma);
-                }
-        }
+        if (PageAnon(hpage))
+                anon_vma = page_get_anon_vma(hpage);
 
         try_to_unmap(hpage, TTU_MIGRATION|TTU_IGNORE_MLOCK|TTU_IGNORE_ACCESS);
 
diff --git a/mm/rmap.c b/mm/rmap.c
index cc140811af56..d271845d7d15 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -337,9 +337,9 @@ void __init anon_vma_init(void)
  * that the anon_vma pointer from page->mapping is valid if there is a
  * mapcount, we can dereference the anon_vma after observing those.
  */
-struct anon_vma *page_lock_anon_vma(struct page *page)
+struct anon_vma *page_get_anon_vma(struct page *page)
 {
-        struct anon_vma *anon_vma, *root_anon_vma;
+        struct anon_vma *anon_vma = NULL;
         unsigned long anon_mapping;
 
         rcu_read_lock();
@@ -350,30 +350,42 @@ struct anon_vma *page_lock_anon_vma(struct page *page)
                 goto out;
 
         anon_vma = (struct anon_vma *) (anon_mapping - PAGE_MAPPING_ANON);
-        root_anon_vma = ACCESS_ONCE(anon_vma->root);
-        spin_lock(&root_anon_vma->lock);
+        if (!atomic_inc_not_zero(&anon_vma->refcount)) {
+                anon_vma = NULL;
+                goto out;
+        }
 
         /*
          * If this page is still mapped, then its anon_vma cannot have been
-         * freed.  But if it has been unmapped, we have no security against
-         * the anon_vma structure being freed and reused (for another anon_vma:
-         * SLAB_DESTROY_BY_RCU guarantees that - so the spin_lock above cannot
-         * corrupt): with anon_vma_prepare() or anon_vma_fork() redirecting
-         * anon_vma->root before page_unlock_anon_vma() is called to unlock.
+         * freed.  But if it has been unmapped, we have no security against the
+         * anon_vma structure being freed and reused (for another anon_vma:
+         * SLAB_DESTROY_BY_RCU guarantees that - so the atomic_inc_not_zero()
+         * above cannot corrupt).
          */
-        if (page_mapped(page))
-                return anon_vma;
-
-        spin_unlock(&root_anon_vma->lock);
+        if (!page_mapped(page)) {
+                put_anon_vma(anon_vma);
+                anon_vma = NULL;
+        }
 out:
         rcu_read_unlock();
-        return NULL;
+
+        return anon_vma;
+}
+
+struct anon_vma *page_lock_anon_vma(struct page *page)
+{
+        struct anon_vma *anon_vma = page_get_anon_vma(page);
+
+        if (anon_vma)
+                anon_vma_lock(anon_vma);
+
+        return anon_vma;
 }
 
 void page_unlock_anon_vma(struct anon_vma *anon_vma)
 {
         anon_vma_unlock(anon_vma);
-        rcu_read_unlock();
+        put_anon_vma(anon_vma);
 }
 
 /*