If 'enable' parameter of the 'mtier' DAMON sample module is set at boot
time via the kernel command line, memory allocation is tried before the
slab is initialized.  As a result kernel NULL pointer dereference BUG
can happen.  Fix it by checking the initialization status.

Fixes: 82a08bde3cf7 ("samples/damon: implement a DAMON module for memory tiering")
Cc: stable@vger.kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
---
 samples/damon/mtier.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/samples/damon/mtier.c b/samples/damon/mtier.c
index 97892ade7f31..20c3102242ec 100644
--- a/samples/damon/mtier.c
+++ b/samples/damon/mtier.c
@@ -157,6 +157,8 @@ static void damon_sample_mtier_stop(void)
 	damon_destroy_ctx(ctxs[1]);
 }
 
+static bool init_called;
+
 static int damon_sample_mtier_enable_store(
 		const char *val, const struct kernel_param *kp)
 {
@@ -170,6 +172,9 @@ static int damon_sample_mtier_enable_store(
 	if (enable == enabled)
 		return 0;
 
+	if (!init_called)
+		return 0;
+
 	if (enable) {
 		err = damon_sample_mtier_start();
 		if (err)
@@ -182,6 +187,14 @@ static int damon_sample_mtier_enable_store(
 
 static int __init damon_sample_mtier_init(void)
 {
+	int err = 0;
+
+	init_called = true;
+	if (enable) {
+		err = damon_sample_mtier_start();
+		if (err)
+			enable = false;
+	}
 	return 0;
 }
 
-- 
2.39.5