mm: fix hugepage migration

2.6.37 added an unmap_and_move_huge_page() for memory failure recovery,
but its anon_vma handling was still based around the 2.6.35 conventions.
Update it to use page_lock_anon_vma, get_anon_vma, page_unlock_anon_vma,
drop_anon_vma in the same way as we're now changing unmap_and_move().

I don't particularly like to propose this for stable when I've not seen
its problems in practice nor tested the solution: but it's clearly out of
synch at present.

Signed-off-by: Hugh Dickins <[e-mail]>
Cc: Mel Gorman <[e-mail]>
Cc: Rik van Riel <[e-mail]>
Cc: Naoya Horiguchi <[e-mail]>
Cc: "Jun'ichi Nomura" <[e-mail]>
Cc: Andi Kleen <[e-mail]>
Cc: <[e-mail]> [2.6.37, 2.6.36]
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/migrate.c

···
 	int rc = 0;
 	int *result = NULL;
 	struct page *new_hpage = get_new_page(hpage, private, &result);
-	int rcu_locked = 0;
 	struct anon_vma *anon_vma = NULL;
 
 	if (!new_hpage)
···
 	}
 
 	if (PageAnon(hpage)) {
-		rcu_read_lock();
-		rcu_locked = 1;
-
-		if (page_mapped(hpage)) {
-			anon_vma = page_anon_vma(hpage);
-			atomic_inc(&anon_vma->external_refcount);
+		anon_vma = page_lock_anon_vma(hpage);
+		if (anon_vma) {
+			get_anon_vma(anon_vma);
+			page_unlock_anon_vma(anon_vma);
 		}
 	}
 
···
 	if (rc)
 		remove_migration_ptes(hpage, hpage);
 
-	if (anon_vma && atomic_dec_and_lock(&anon_vma->external_refcount,
-					    &anon_vma->lock)) {
-		int empty = list_empty(&anon_vma->head);
-		spin_unlock(&anon_vma->lock);
-		if (empty)
-			anon_vma_free(anon_vma);
-	}
-
-	if (rcu_locked)
-		rcu_read_unlock();
+	if (anon_vma)
+		drop_anon_vma(anon_vma);
 out:
 	unlock_page(hpage);