mm/damon/reclaim: fix the timer always stays active

The timer stays active even if the reclaim mechanism is never enabled.  It
is unnecessary overhead can be completely avoided by using
module_param_cb() for enabled flag.

Link: https://lkml.kernel.org/r/20220421125910.1052459-1-tuhailong@gmail.com
Signed-off-by: Hailong Tu <tuhailong@gmail.com>
Reviewed-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/damon/reclaim.c

@@ -28,7 +28,6 @@
  * this.
  */
 static bool enabled __read_mostly;
-module_param(enabled, bool, 0600);
 
 /*
  * Time threshold for cold memory regions identification in microseconds.
@@ -358,11 +357,35 @@ static void damon_reclaim_timer_fn(struct work_struct *work)
 			enabled = last_enabled;
 	}
 
-	schedule_delayed_work(&damon_reclaim_timer,
+	if (enabled)
+		schedule_delayed_work(&damon_reclaim_timer,
 			msecs_to_jiffies(ENABLE_CHECK_INTERVAL_MS));
 }
 static DECLARE_DELAYED_WORK(damon_reclaim_timer, damon_reclaim_timer_fn);
 
+static int enabled_store(const char *val,
+		const struct kernel_param *kp)
+{
+	int rc = param_set_bool(val, kp);
+
+	if (rc < 0)
+		return rc;
+
+	if (enabled)
+		schedule_delayed_work(&damon_reclaim_timer, 0);
+
+	return 0;
+}
+
+static const struct kernel_param_ops enabled_param_ops = {
+	.set = enabled_store,
+	.get = param_get_bool,
+};
+
+module_param_cb(enabled, &enabled_param_ops, &enabled, 0600);
+MODULE_PARM_DESC(enabled,
+	"Enable or disable DAMON_RECLAIM (default: disabled)");
+
 static int damon_reclaim_after_aggregation(struct damon_ctx *c)
 {
 	struct damos *s;