DAMON's logics to determine if this is the time to apply damos schemes
assumes next_apply_sis is always set larger than current
passed_sample_intervals.  And therefore assume continuously incrementing
passed_sample_intervals will make it reaches to the next_apply_sis in
future.  The logic hence does apply the scheme and update next_apply_sis
only if passed_sample_intervals is same to next_apply_sis.

If Schemes apply interval is set as zero, however, next_apply_sis is set
same to current passed_sample_intervals, respectively.  And
passed_sample_intervals is incremented before doing the next_apply_sis
check.  Hence, next_apply_sis becomes larger than next_apply_sis, and
the logic says it is not the time to apply schemes and update
next_apply_sis.  In other words, DAMON stops applying schemes until
passed_sample_intervals overflows.

Based on the documents and the common sense, a reasonable behavior for
such inputs would be applying the schemes for every sampling interval.
Handle the case by removing the assumption.

Fixes: 42f994b71404 ("mm/damon/core: implement scheme-specific apply interval")
Cc: <stable@vger.kernel.org> # 6.7.x
Signed-off-by: SeongJae Park <sj@kernel.org>
---
 mm/damon/core.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index 931526fb2d2e..511c3f61ab44 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -1412,7 +1412,7 @@ static void damon_do_apply_schemes(struct damon_ctx *c,
 	damon_for_each_scheme(s, c) {
 		struct damos_quota *quota = &s->quota;
 
-		if (c->passed_sample_intervals != s->next_apply_sis)
+		if (c->passed_sample_intervals < s->next_apply_sis)
 			continue;
 
 		if (!s->wmarks.activated)
@@ -1636,7 +1636,7 @@ static void kdamond_apply_schemes(struct damon_ctx *c)
 	bool has_schemes_to_apply = false;
 
 	damon_for_each_scheme(s, c) {
-		if (c->passed_sample_intervals != s->next_apply_sis)
+		if (c->passed_sample_intervals < s->next_apply_sis)
 			continue;
 
 		if (!s->wmarks.activated)
@@ -1656,9 +1656,9 @@ static void kdamond_apply_schemes(struct damon_ctx *c)
 	}
 
 	damon_for_each_scheme(s, c) {
-		if (c->passed_sample_intervals != s->next_apply_sis)
+		if (c->passed_sample_intervals < s->next_apply_sis)
 			continue;
-		s->next_apply_sis +=
+		s->next_apply_sis = c->passed_sample_intervals +
 			(s->apply_interval_us ? s->apply_interval_us :
 			 c->attrs.aggr_interval) / sample_interval;
 	}
-- 
2.39.5