[PATCH] mm/damon/reclaim: schedule 'damon_reclaim_timer' only after 'system_wq' is initialized

[SeongJae Park <sj@kernel.org>]

Commit 059342d1dd4e ("mm/damon/reclaim: fix the timer always stays
active") made DAMON_RECLAIM's 'enabled' parameter store callback,
'enabled_store()', to schedule 'damon_reclaim_timer'.  The scheduling
uses 'system_wq', which is initialized in 'workqueue_init_early()'.  As
kernel parameters parsing function ('parse_args()') is called before
'workqueue_init_early()', 'enabled_store()' can be executed before
'workqueue_init_early()' and end up accessing the uninitialized
'system_wq'.  As a result, the booting hang[1].  This commit fixes the
issue by checking if the initialization is done before scheduling the
timer.

[1] https://lkml.kernel.org/20220604192222.1488-1-sj@kernel.org/

Fixes: 059342d1dd4e ("mm/damon/reclaim: fix the timer always stays active")
Reported-by: Greg White <gwhite@kupulau.com>
Signed-off-by: SeongJae Park <sj@kernel.org>
---
 mm/damon/reclaim.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/mm/damon/reclaim.c b/mm/damon/reclaim.c
index 8efbfb24f3a1..4b07c29effe9 100644
--- a/mm/damon/reclaim.c
+++ b/mm/damon/reclaim.c
@@ -374,6 +374,8 @@ static void damon_reclaim_timer_fn(struct work_struct *work)
 }
 static DECLARE_DELAYED_WORK(damon_reclaim_timer, damon_reclaim_timer_fn);
 
+static bool damon_reclaim_initialized;
+
 static int enabled_store(const char *val,
 		const struct kernel_param *kp)
 {
@@ -382,6 +384,10 @@ static int enabled_store(const char *val,
 	if (rc < 0)
 		return rc;
 
+	/* system_wq might not initialized yet */
+	if (!damon_reclaim_initialized)
+		return rc;
+
 	if (enabled)
 		schedule_delayed_work(&damon_reclaim_timer, 0);
 
@@ -449,6 +455,8 @@ static int __init damon_reclaim_init(void)
 	damon_add_target(ctx, target);
 
 	schedule_delayed_work(&damon_reclaim_timer, 0);
+
+	damon_reclaim_initialized = true;
 	return 0;
 }
 
-- 
2.25.1