linux-kernel - [PATCH v2 3/5] mm/damon/ops-common: avoid divide-by-zero during region hotness calculation

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Message-Id: <20231020172317.64192-4-sj@kernel.org>
Date:   Fri, 20 Oct 2023 17:23:15 +0000
From:   SeongJae Park <sj@...nel.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     SeongJae Park <sj@...nel.org>, Jakub Acs <acsjakub@...zon.de>,
        damon@...ts.linux.dev, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: [PATCH v2 3/5] mm/damon/ops-common: avoid divide-by-zero during region hotness calculation

When calculating the hotness of each region for the under-quota regions
prioritization, DAMON divides some values by the maximum nr_accesses.
However, due to the type of the related variables, simple division-based
calculation of the divisor can return zero.  As a result, divide-by-zero
is possible.  Fix it by using damon_max_nr_accesses(), which handles the
case.

Reported-by: Jakub Acs <acsjakub@...zon.de>
Fixes: 198f0f4c58b9 ("mm/damon/vaddr,paddr: support pageout prioritization")
Cc: <stable@...r.kernel.org> # 5.16.x
Signed-off-by: SeongJae Park <sj@...nel.org>
---
 mm/damon/ops-common.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/mm/damon/ops-common.c b/mm/damon/ops-common.c
index ac1c3fa80f98..d25d99cb5f2b 100644
--- a/mm/damon/ops-common.c
+++ b/mm/damon/ops-common.c
@@ -73,7 +73,6 @@ void damon_pmdp_mkold(pmd_t *pmd, struct vm_area_struct *vma, unsigned long addr
 int damon_hot_score(struct damon_ctx *c, struct damon_region *r,
 			struct damos *s)
 {
-	unsigned int max_nr_accesses;
 	int freq_subscore;
 	unsigned int age_in_sec;
 	int age_in_log, age_subscore;
@@ -81,8 +80,8 @@ int damon_hot_score(struct damon_ctx *c, struct damon_region *r,
 	unsigned int age_weight = s->quota.weight_age;
 	int hotness;
 
-	max_nr_accesses = c->attrs.aggr_interval / c->attrs.sample_interval;
-	freq_subscore = r->nr_accesses * DAMON_MAX_SUBSCORE / max_nr_accesses;
+	freq_subscore = r->nr_accesses * DAMON_MAX_SUBSCORE /
+		damon_max_nr_accesses(&c->attrs);
 
 	age_in_sec = (unsigned long)r->age * c->attrs.aggr_interval / 1000000;
 	for (age_in_log = 0; age_in_log < DAMON_MAX_AGE_IN_LOG && age_in_sec;
-- 
2.34.1