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.

Link: http://lkml.kernel.org/r/[email protected]
Fixes: 5d03a6613957 ("mm/z3fold.c: use kref to prevent page free/compact race")
Signed-off-by: Henry Burns <[email protected]>
Reviewed-by: Shakeel Butt <[email protected]>
Reviewed-by: Jonathan Adams <[email protected]>
Cc: Vitaly Vul <[email protected]>
Cc: Vitaly Wool <[email protected]>
Cc: David Howells <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Al Viro <[email protected]
Cc: Henry Burns <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

1 files changed, 8 insertions, 1 deletions

diff --git a/mm/z3fold.c b/mm/z3fold.c
index 1a029a7432ee..43de92f52961 100644
--- a/mm/z3fold.c
+++ b/mm/z3fold.c
@@ -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);
 }