[PATCH] mm/z3fold.c: Fix z3fold_destroy_pool() race condition

[Henry Burns <henryburns@google.com>]

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.

Calling z3fold_deregister_migration() before the workqueues are drained
means that there can be allocated pages referencing a freed inode,
causing any thread in compaction to be able to trip over the bad
pointer in PageMovable().

Fixes: 1f862989b04a ("mm/z3fold.c: support page migration")

Signed-off-by: Henry Burns <henryburns@google.com>
Cc: <stable@vger.kernel.org>
---
 mm/z3fold.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/mm/z3fold.c b/mm/z3fold.c
index 43de92f52961..ed19d98c9dcd 100644
--- a/mm/z3fold.c
+++ b/mm/z3fold.c
@@ -817,16 +817,19 @@ static struct z3fold_pool *z3fold_create_pool(const char *name, gfp_t gfp,
 static void z3fold_destroy_pool(struct z3fold_pool *pool)
 {
 	kmem_cache_destroy(pool->c_handle);
-	z3fold_unregister_migration(pool);
 
 	/*
 	 * 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).
+	 *
+	 * There are still outstanding pages until both workqueues are drained,
+	 * so we cannot unregister migration until then.
 	 */
 
 	destroy_workqueue(pool->compact_wq);
 	destroy_workqueue(pool->release_wq);
+	z3fold_unregister_migration(pool);
 	kfree(pool);
 }
 
-- 
2.22.0.709.g102302147b-goog