[PATCH 1/2] mm,vmscan: Make unregister_shrinker() no-op if register_shrinker() failed.

[Michal Hocko]

From: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>

Since allowing register_shrinker() callers to call unregister_shrinker()
when register_shrinker() failed can simplify error recovery path, this
patch makes unregister_shrinker() no-op when register_shrinker() failed.
Let's also make sure that double unregister_shrinker doesn't blow up as
well and NULL nr_deferred on successful de-registration to make the
clean up even simpler and prevent from potential memory corruptions.

[akaraliou.dev@xxxxxxxxx: set nr_deferred = NULL to handle double
 unregister]
Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
Reported-by: syzbot <syzkaller@xxxxxxxxxxxxxxxx>
Cc: Glauber Costa <glauber@xxxxxxxxxxxx>
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>

Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
---
 mm/vmscan.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 80dea50f421b..7a5801040fd4 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -281,10 +281,13 @@ EXPORT_SYMBOL(register_shrinker);
  */
 void unregister_shrinker(struct shrinker *shrinker)
 {
+	if (!shrinker->nr_deferred)
+		return;
 	down_write(&shrinker_rwsem);
 	list_del(&shrinker->list);
 	up_write(&shrinker_rwsem);
 	kfree(shrinker->nr_deferred);
+	shrinker->nr_deferred = NULL;
 }
 EXPORT_SYMBOL(unregister_shrinker);
 
-- 
2.15.1