mm: zswap: remove unnecessary trees cleanups in zswap_swapoff()

During swapoff, try_to_unuse() makes sure that zswap_invalidate() is
called for all swap entries before zswap_swapoff() is called.  This means
that all zswap entries should already be removed from the tree.  Simplify
zswap_swapoff() by removing the trees cleanup code, and leave an assertion
in its place.

Link: https://lkml.kernel.org/r/20240124045113.415378-3-yosryahmed@google.comSigned-off-by: Yosry Ahmed <yosryahmed@google.com>
Reviewed-by: Chengming Zhou <zhouchengming@bytedance.com>
Cc: Chris Li <chrisl@kernel.org>
Cc: "Huang, Ying" <ying.huang@intel.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Nhat Pham <nphamcs@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/zswap.c

@@ -1808,19 +1808,9 @@ void zswap_swapoff(int type)
 	if (!trees)
 		return;
 
-	for (i = 0; i < nr_zswap_trees[type]; i++) {
-		struct zswap_tree *tree = trees + i;
-		struct zswap_entry *entry, *n;
-
-		/* walk the tree and free everything */
-		spin_lock(&tree->lock);
-		rbtree_postorder_for_each_entry_safe(entry, n,
-						     &tree->rbroot,
-						     rbnode)
-			zswap_free_entry(entry);
-		tree->rbroot = RB_ROOT;
-		spin_unlock(&tree->lock);
-	}
+	/* try_to_unuse() invalidated all the entries already */
+	for (i = 0; i < nr_zswap_trees[type]; i++)
+		WARN_ON_ONCE(!RB_EMPTY_ROOT(&trees[i].rbroot));
 
 	kvfree(trees);
 	nr_zswap_trees[type] = 0;