[RFC PATCH v1.1 3/3] mm/damon/sysfs: set next refresh jiffies per sysfs context

[SJ Park <sj@kernel.org>]

When 'refresh_ms' is set, DAMON sysfs interface periodically updates
auto-tuned parameters and DAMOS stats.  The timestamp for the next
refresh is initialized when a DAMON context starts, and updated in its
damon_call() callback function.  That is, each DAMON context updates it.
However, the timestamp is a global variable that is shared with all the
contexts.  When there are multiple DAMON contexts having different
refresh_ms, the update frequency will be changed, depending on the order
of the contexts.  When there are multiple kdamonds, it will be even more
chaotic.  Fix the problem by having the timestamp per each context.

The user impact is not very critical.  It does not leak, corrupt or
crash.  The update will not be faster or slower than the lowest and
largest refresh_ms values of the contexts, respectively.  The user can
also manually ask the updates on demand using kdamond state commands.
That said, clearly this is a bug and can easily be reproduced.

Fixes: 9fd7bb5083d1 ("mm/damon/sysfs: change next_update_jiffies to a global variable")
Cc: <stable@vger.kernel.org> # 6.18.x
Signed-off-by: SJ Park <sj@kernel.org>
---
 mm/damon/sysfs.c | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/mm/damon/sysfs.c b/mm/damon/sysfs.c
index e3858ffab4b22..f05b256c90ee7 100644
--- a/mm/damon/sysfs.c
+++ b/mm/damon/sysfs.c
@@ -1789,6 +1789,7 @@ struct damon_sysfs_kdamond {
 	struct damon_sysfs_contexts *contexts;
 	struct damon_ctx *damon_ctx;
 	unsigned int refresh_ms;
+	unsigned long next_refresh_jiffies;
 };
 
 static struct damon_sysfs_kdamond *damon_sysfs_kdamond_alloc(void)
@@ -2215,17 +2216,15 @@ 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;
 
 	if (!sysfs_kdamond->refresh_ms)
 		return 0;
-	if (time_before(jiffies, damon_sysfs_next_update_jiffies))
+	if (time_before(jiffies, sysfs_kdamond->next_refresh_jiffies))
 		return 0;
-	damon_sysfs_next_update_jiffies = jiffies +
+	sysfs_kdamond->next_refresh_jiffies = jiffies +
 		msecs_to_jiffies(sysfs_kdamond->refresh_ms);
 
 	if (!mutex_trylock(&damon_sysfs_lock))
@@ -2273,8 +2272,8 @@ 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);
+	kdamond->next_refresh_jiffies = jiffies +
+		msecs_to_jiffies(kdamond->refresh_ms);
 
 	repeat_call_control->fn = damon_sysfs_repeat_call_fn;
 	repeat_call_control->data = kdamond;
-- 
2.47.3