[PATCH v2 2/2] mm/damon/sysfs: change next_update_jiffies to a global variable

[Quanmin Yan <yanquanmin1@huawei.com>]

In DAMON’s damon_sysfs_repeat_call_fn(), time_before() is used to compare
the current jiffies with next_update_jiffies to determine whether to
update the sysfs files at this moment.

On 32-bit systems, the kernel initializes jiffies to "-5 minutes" to make
jiffies wrap bugs appear earlier. However, this causes time_before() in
damon_sysfs_repeat_call_fn() 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 before). As a result, DAMON
does not update sysfs files during that period.

There is also an issue unrelated to the system’s word size[2]: if the
user stops DAMON just after next_update_jiffies is updated and restarts
it after 'refresh_ms' or a longer delay, next_update_jiffies will retain
an older value, causing time_before() to return false and the update to
happen earlier than expected.

Fix these issues by making next_update_jiffies a global variable and
initializing it each time DAMON is started.

[1] https://lkml.kernel.org/r/20250822025057.1740854-1-ekffu200098@gmail.com
[2] https://lore.kernel.org/all/20251029013038.66625-1-sj@kernel.org/

Fixes: d809a7c64ba8 ("mm/damon/sysfs: implement refresh_ms file internal work")
Suggested-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Quanmin Yan <yanquanmin1@huawei.com>
---
 mm/damon/sysfs.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/mm/damon/sysfs.c b/mm/damon/sysfs.c
index 43ee9ce4dd84..e2bd2d7becdd 100644
--- a/mm/damon/sysfs.c
+++ b/mm/damon/sysfs.c
@@ -1601,16 +1601,17 @@ static struct damon_ctx *damon_sysfs_build_ctx(
 	return ctx;
 }
 
+static unsigned long damon_sysfs_next_update_jiffies;
+
 static int damon_sysfs_repeat_call_fn(void *data)
 {
 	struct damon_sysfs_kdamond *sysfs_kdamond = data;
-	static unsigned long next_update_jiffies;
 
 	if (!sysfs_kdamond->refresh_ms)
 		return 0;
-	if (time_before(jiffies, next_update_jiffies))
+	if (time_before(jiffies, damon_sysfs_next_update_jiffies))
 		return 0;
-	next_update_jiffies = jiffies +
+	damon_sysfs_next_update_jiffies = jiffies +
 		msecs_to_jiffies(sysfs_kdamond->refresh_ms);
 
 	if (!mutex_trylock(&damon_sysfs_lock))
@@ -1656,6 +1657,9 @@ static int damon_sysfs_turn_damon_on(struct damon_sysfs_kdamond *kdamond)
 	}
 	kdamond->damon_ctx = ctx;
 
+	damon_sysfs_next_update_jiffies =
+		jiffies + msecs_to_jiffies(kdamond->refresh_ms);
+
 	repeat_call_control->fn = damon_sysfs_repeat_call_fn;
 	repeat_call_control->data = kdamond;
 	repeat_call_control->repeat = true;
-- 
2.43.0