linux-kernel - [PATCH 1/2] mm,vmscan: Make unregister_shrinker() no-op if register

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Date:   Tue, 19 Dec 2017 14:28:43 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        Aliaksei Karaliou <akaraliou.dev@...il.com>,
        <linux-mm@...ck.org>, <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Glauber Costa <glauber@...lladb.com>,
        Michal Hocko <mhocko@...e.com>
Subject: [PATCH 1/2] mm,vmscan: Make unregister_shrinker() no-op if register_shrinker() failed.

From: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>

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@...il.com: set nr_deferred = NULL to handle double
 unregister]
Signed-off-by: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Reported-by: syzbot <syzkaller@...glegroups.com>
Cc: Glauber Costa <glauber@...lladb.com>
Cc: Al Viro <viro@...iv.linux.org.uk>

Signed-off-by: Michal Hocko <mhocko@...e.com>
---
 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