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

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

[Jaeyeon Lee]

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

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

[SJ Park]

Hi Jaeyeon,

On Wed, 30 Sep 2026 13:58:12 +0200 Jaeyeon Lee <jaeyeon.lee.dev@gmail.com> wrote:

> 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.

Looks reasonable to me.  Thank you for this patch.  I'm thinking about if there
is a simpler and more complete way to fix whole possible 32bit overflow issues,
but that shouldn't be a blocker of this patch.

> 
> 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>

Reviewed-by: SJ Park <sj@kernel.org>

This patch is applied to damon/next [1] tree.  If this patch is not added to
mm.git in short term (~1 week?), I will ask mm.git maintainer (Andrew Morton)
to pick this.  So, no action from your side is needed for now.  If it seems I
also forgot doing that or you cannot wait for my action, please feel free to
ping me or Andrew.

[1] https://origin.kernel.org/doc/html/latest/mm/damon/maintainer-profile.html#scm-trees


Thanks,
SJ

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