mm/z3fold.c: fix z3fold_destroy_pool() race condition

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@google.com
Fixes: fff1c7b8ec16 ("mm/z3fold.c: support page migration")
Signed-off-by: Henry Burns <henryburns@google.com>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
Reviewed-by: Jonathan Adams <jwadams@google.com>
Cc: Vitaly Vul <vitaly.vul@sony.com>
Cc: Vitaly Wool <vitalywool@gmail.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Henry Burns <henrywolfeburns@gmail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/z3fold.c b/mm/z3fold.c
index 43de92f529617fd18c143002b60a50fbccaa4df2..ed19d98c9dcd1641c5e66d27d586e93f43fadce1 100644 (file)
--- a/mm/z3fold.c
+++ b/mm/z3fold.c
@@ -817,16 +817,19 @@ out:
 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);
 }