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

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Message-ID: <64ebef1d-5f22-44ed-a1f3-2f7cb432e10b@amd.com>
Date: Mon, 9 Feb 2026 09:43:39 +0530
From: K Prateek Nayak <kprateek.nayak@....com>
To: Pierre Gondois <pierre.gondois@....com>, <linux-kernel@...r.kernel.org>
CC: Christian Loehle <christian.loehle@....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: Re: [PATCH 1/2] sched/fair: Fix integer underflow

Hello Pierre,

On 2/5/2026 8:38 PM, Pierre Gondois wrote:
> (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")

Should this be commit 0b0695f2b34a ("sched/fair: Rework load_balance()")
since I could spot the max_t usage all the way back to to it?

Apart from that, feel free to include:

Reviewed-by: K Prateek Nayak <kprateek.nayak@....com>

> 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

-- 
Thanks and Regards,
Prateek