[PATCH 5.2 008/135] mm/z3fold.c: fix z3fold_destroy_pool() race condition

[Sasha Levin]

From: Henry Burns <henryburns@xxxxxxxxxx>

commit b997052bc3ac444a0bceab1093aff7ae71ed419e upstream.

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().

Link: http://lkml.kernel.org/r/20190726224810.79660-2-henryburns@xxxxxxxxxx
Fixes: 1f862989b04a ("mm/z3fold.c: support page migration")
Signed-off-by: Henry Burns <henryburns@xxxxxxxxxx>
Reviewed-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
Reviewed-by: Jonathan Adams <jwadams@xxxxxxxxxx>
Cc: Vitaly Vul <vitaly.vul@xxxxxxxx>
Cc: Vitaly Wool <vitalywool@xxxxxxxxx>
Cc: David Howells <dhowells@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Henry Burns <henrywolfeburns@xxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 mm/z3fold.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/mm/z3fold.c b/mm/z3fold.c
index d06d7f9560028..c4debbe683eba 100644
--- a/mm/z3fold.c
+++ b/mm/z3fold.c
@@ -819,16 +819,19 @@ 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);
 }
 
-- 
2.20.1