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

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Patch series "mm/damon: fix a minor bug, clean code, and add a test".

Patch 1 from Jaeyeon Lee fixes a minor overflow issue in DAMOS.  Patch 2
from Gregory Price (Meta) cleans up the code for handling of hugetlb
pages.  Patch 3 from Liew Rui Yan adds a kunit test for a recently fixed
bug.


This patch (of 3):

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.

Link: https://lore.kernel.org/20261006065209.41507-1-sj@kernel.org
Link: https://lore.kernel.org/20261006065209.41507-2-sj@kernel.org
Link: https://lore.kernel.org/20260623011652.1354-1-sj@kernel.org [1]
Fixes: 198f0f4c58 ("mm/damon/vaddr,paddr: support pageout prioritization")
Signed-off-by: Jaeyeon Lee <jaeyeon.lee.dev@gmail.com>
Signed-off-by: SJ Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: SJ Park <sj@kernel.org>
Assisted-by: LLM
This commit is contained in:
Jaeyeon Lee 2026-10-05 23:52:05 -07:00 committed by Andrew Morton
parent e4dce9a62b
commit 0284dff76e
1 changed files with 3 additions and 2 deletions
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5
mm/damon/ops-common.c
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@ -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);