[PATCH] mm/damon/ops-common: fix age_in_sec overflow on 32-bit

[Jaeyeon Lee <jaeyeon.lee.dev@gmail.com>]

damon_hot_score() calculates the age of a region by multiplying r->age
and the aggregation interval.  On 32-bit architectures, unsigned long
is 32 bits, so the multiplication overflows once the age exceeds about
4295 seconds.  Then old regions are scored as if they were young.

Fix it by doing the calculation in 64-bit arithmetic using div_u64().
Also make age_in_sec u64, so the result is not truncated back to 32
bits.

The issue was discovered [1] by Sashiko.

[1] https://lore.kernel.org/20260623011652.1354-1-sj@kernel.org

Fixes: 198f0f4c58b9 ("mm/damon/vaddr,paddr: support pageout prioritization")
Assisted-by: LLM
Signed-off-by: Jaeyeon Lee <jaeyeon.lee.dev@gmail.com>
---
 mm/damon/ops-common.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/mm/damon/ops-common.c b/mm/damon/ops-common.c
index 77366f42b3e5..7a7429359dc8 100644
--- a/mm/damon/ops-common.c
+++ b/mm/damon/ops-common.c
@@ -164,7 +164,7 @@ int damon_hot_score(struct damon_ctx *c, struct damon_region *r,
 			struct damos *s)
 {
 	int freq_subscore;
-	unsigned int age_in_sec;
+	u64 age_in_sec;
 	int age_in_log, age_subscore;
 	unsigned int freq_weight = s->quota.weight_nr_accesses;
 	unsigned int age_weight = s->quota.weight_age;
@@ -174,7 +174,8 @@ int damon_hot_score(struct damon_ctx *c, struct damon_region *r,
 			DAMON_MAX_SUBSCORE,
 			damon_nr_samples_per_aggr(&c->attrs));
 
-	age_in_sec = (unsigned long)r->age * c->attrs.aggr_interval / 1000000;
+	age_in_sec = div_u64((u64)r->age * c->attrs.aggr_interval,
+			     USEC_PER_SEC);
 	if (age_in_sec)
 		age_in_log = min_t(int, ilog2(age_in_sec) + 1,
 				DAMON_MAX_AGE_IN_LOG);
-- 
2.43.0