mm/zswap: use postorder iteration when destroying rbtree

Signed-off-by: Cody P Schafer <cody@linux.vnet.ibm.com>
Reviewed-by: Seth Jennings <sjenning@linux.vnet.ibm.com>
Cc: David Woodhouse <David.Woodhouse@intel.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Michel Lespinasse <walken@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/zswap.c

@@ -790,26 +790,14 @@ static void zswap_frontswap_invalidate_page(unsigned type, pgoff_t offset)
 static void zswap_frontswap_invalidate_area(unsigned type)
 {
 	struct zswap_tree *tree = zswap_trees[type];
-	struct rb_node *node;
-	struct zswap_entry *entry;
+	struct zswap_entry *entry, *n;
 
 	if (!tree)
 		return;
 
 	/* walk the tree and free everything */
 	spin_lock(&tree->lock);
-	/*
-	 * TODO: Even though this code should not be executed because
-	 * the try_to_unuse() in swapoff should have emptied the tree,
-	 * it is very wasteful to rebalance the tree after every
-	 * removal when we are freeing the whole tree.
-	 *
-	 * If post-order traversal code is ever added to the rbtree
-	 * implementation, it should be used here.
-	 */
-	while ((node = rb_first(&tree->rbroot))) {
-		entry = rb_entry(node, struct zswap_entry, rbnode);
-		rb_erase(&entry->rbnode, &tree->rbroot);
+	rbtree_postorder_for_each_entry_safe(entry, n, &tree->rbroot, rbnode) {
 		zbud_free(tree->pool, entry->handle);
 		zswap_entry_cache_free(entry);
 		atomic_dec(&zswap_stored_pages);