[PATCH] mm/damon/stat: set last_refresh_jiffies to jiffies at startup

[Quanmin Yan]

In DAMON_STAT's damon_stat_damon_call_fn(), time_before_eq() is used to
avoid unnecessarily frequent stat update.

On 32-bit systems, the kernel initializes jiffies to "-5 minutes" to make
jiffies wrap bugs appear earlier. However, this causes time_before_eq()
in DAMON_STAT to unexpectedly return true during the first 5 minutes
after boot on 32-bit systems (see [1] for more explanation, which fixes
another jiffies-related issue in DAMON). As a result, DAMON_STAT does not
update any monitoring results during that period, which can be more
confusing when DAMON_STAT_ENABLED_DEFAULT is enabled.

Fix it by setting last_refresh_jiffies to jiffies at startup.

[1] https://lkml.kernel.org/r/20250822025057.1740854-1-ekffu200098@xxxxxxxxx

Fixes: fabdd1e911da ("mm/damon/stat: calculate and expose estimated memory bandwidth")
Signed-off-by: Quanmin Yan <yanquanmin1@xxxxxxxxxx>
---
 mm/damon/stat.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/damon/stat.c b/mm/damon/stat.c
index 6c4503d2aee3..6dc3e18de910 100644
--- a/mm/damon/stat.c
+++ b/mm/damon/stat.c
@@ -132,6 +132,9 @@ static int damon_stat_damon_call_fn(void *data)
 	struct damon_ctx *c = data;
 	static unsigned long last_refresh_jiffies;
 
+	if (unlikely(!last_refresh_jiffies))
+		last_refresh_jiffies = jiffies;
+
 	/* avoid unnecessarily frequent stat update */
 	if (time_before_eq(jiffies, last_refresh_jiffies +
 				msecs_to_jiffies(5 * MSEC_PER_SEC)))
-- 
2.43.0