FAILED: patch "[PATCH] mm/z3fold.c: fix z3fold_destroy_pool() ordering" failed to apply to 4.14-stable tree

The patch below does not apply to the 4.14-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 6051d3bd3b91e96c59e62b8be2dba1cc2b19ee40 Mon Sep 17 00:00:00 2001
From: Henry Burns <[email protected]>
Date: Tue, 13 Aug 2019 15:37:21 -0700
Subject: [PATCH] mm/z3fold.c: fix z3fold_destroy_pool() ordering

The constraint from the zpool use of z3fold_destroy_pool() is there are no outstanding handles to memory (so no active allocations), but it is possible for there to be outstanding work on either of the two wqs in the pool.

If there is work queued on pool->compact_workqueue when it is called, z3fold_destroy_pool() will do:

z3fold_destroy_pool()

     destroy_workqueue(pool->release_wq)
     destroy_workqueue(pool->compact_wq)
       drain_workqueue(pool->compact_wq)
         do_compact_page(zhdr)
           kref_put(&zhdr->refcount)
             __release_z3fold_page(zhdr, ...)
               queue_work_on(pool->release_wq, &pool->work) *BOOM*

So compact_wq needs to be destroyed before release_wq.

index 1a029a7432ee..43de92f52961 100644
@@ -818,8 +818,15 @@ static void z3fold_destroy_pool(struct z3fold_pool *pool)
 {
 	kmem_cache_destroy(pool->c_handle);
 	z3fold_unregister_migration(pool);
-	destroy_workqueue(pool->release_wq);
+
+	/*
+	 * We need to destroy pool->compact_wq before pool->release_wq,
+	 * as any pending work on pool->compact_wq will call
+	 * queue_work(pool->release_wq, &pool->work).
+	 */
+
 	destroy_workqueue(pool->compact_wq);
+	destroy_workqueue(pool->release_wq);
 	kfree(pool);
 }