linux-kernel - [PATCH 1/2] sched/fair: Fix integer underflow

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Message-ID: <20260205150846.1242134-2-pierre.gondois@arm.com>
Date: Thu,  5 Feb 2026 16:08:44 +0100
From: Pierre Gondois <pierre.gondois@....com>
To: linux-kernel@...r.kernel.org
Cc: Christian Loehle <christian.loehle@....com>,
	Pierre Gondois <pierre.gondois@....com>,
	stable@...r.kernel.org,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Juri Lelli <juri.lelli@...hat.com>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>,
	Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>,
	Rik van Riel <riel@...riel.com>
Subject: [PATCH 1/2] sched/fair: Fix integer underflow

(struct sg_lb_stats).idle_cpus is of type 'unsigned int'.
(local->idle_cpus - busiest->idle_cpus) can underflow to UINT_MAX
for instance, and max_t(long, 0, UINT_MAX) will output UINT_MAX.

Use lsub_positive() instead of max_t().

Fixes: 16b0a7a1a0af ("sched/fair: Ensure tasks spreading in LLC during LB")
cc: stable@...r.kernel.org
Signed-off-by: Pierre Gondois <pierre.gondois@....com>
Reviewed-by: Vincent Guittot <vincent.guittot@...aro.org>
---
 kernel/sched/fair.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index da46c31645378..aa14a9982b9f1 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -11249,8 +11249,8 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s
 			 * idle CPUs.
 			 */
 			env->migration_type = migrate_task;
-			env->imbalance = max_t(long, 0,
-					       (local->idle_cpus - busiest->idle_cpus));
+			env->imbalance = local->idle_cpus;
+			lsub_positive(&env->imbalance, busiest->idle_cpus);
 		}
 
 #ifdef CONFIG_NUMA
-- 
2.43.0