mm: shrinkers: fix double kfree on shrinker name

syzbot is reporting double kfree() at free_prealloced_shrinker() [1], for
destroy_unused_super() calls free_prealloced_shrinker() even if
prealloc_shrinker() returned an error.  Explicitly clear shrinker name
when prealloc_shrinker() called kfree().

[roman.gushchin@linux.dev: zero shrinker->name in all cases where shrinker->name is freed]
  Link: https://lkml.kernel.org/r/YtgteTnQTgyuKUSY@castle
Link: https://syzkaller.appspot.com/bug?extid=8b481578352d4637f510 [1]
Link: https://lkml.kernel.org/r/ffa62ece-6a42-2644-16cf-0d33ef32c676@I-love.SAKURA.ne.jp
Fixes: e33c267a ("mm: shrinkers: provide shrinkers with names")
Reported-by: syzbot <syzbot+8b481578352d4637f510@syzkaller.appspotmail.com>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Acked-by: Roman Gushchin <roman.gushchin@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/shrinker_debug.c

@@ -251,6 +251,7 @@ void shrinker_debugfs_remove(struct shrinker *shrinker)
 	lockdep_assert_held(&shrinker_rwsem);
 
 	kfree_const(shrinker->name);
+	shrinker->name = NULL;
 
 	if (!shrinker->debugfs_entry)
 		return;

mm/vmscan.c

@@ -644,8 +644,10 @@ int prealloc_shrinker(struct shrinker *shrinker, const char *fmt, ...)
 		return -ENOMEM;
 
 	err = __prealloc_shrinker(shrinker);
-	if (err)
+	if (err) {
 		kfree_const(shrinker->name);
+		shrinker->name = NULL;
+	}
 
 	return err;
 }
@@ -660,6 +662,7 @@ void free_prealloced_shrinker(struct shrinker *shrinker)
 {
 #ifdef CONFIG_SHRINKER_DEBUG
 	kfree_const(shrinker->name);
+	shrinker->name = NULL;
 #endif
 	if (shrinker->flags & SHRINKER_MEMCG_AWARE) {
 		down_write(&shrinker_rwsem);
@@ -704,8 +707,10 @@ int register_shrinker(struct shrinker *shrinker, const char *fmt, ...)
 		return -ENOMEM;
 
 	err = __register_shrinker(shrinker);
-	if (err)
+	if (err) {
 		kfree_const(shrinker->name);
+		shrinker->name = NULL;
+	}
 	return err;
 }
 #else