linux-kernel - Re: [PATCH v4] sched/numa: Fix divide by zero for sysctl_numa_balancing_scan

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Date:   Sat, 29 Apr 2023 16:56:35 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     chris hyser <chris.hyser@...cle.com>
Cc:     linux-kernel@...r.kernel.org, dietmar.eggemann@....com,
        vincent.guittot@...aro.org, Chen Yu <yu.c.chen@...el.com>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH v4] sched/numa: Fix divide by zero for
 sysctl_numa_balancing_scan_size.

On Thu, Apr 06, 2023 at 11:26:33AM -0400, chris hyser wrote:
> Commit 6419265899d9 ("sched/fair: Fix division by zero
> sysctl_numa_balancing_scan_size") prevented a divide by zero by using
> sysctl mechanisms to return EINVAL for a sysctl_numa_balancing_scan_size
> value of zero. When moved from a sysctl to a debugfs file, this checking
> was lost.
> 
> This patch puts zero checking back in place.
> 
> Cc: stable@...r.kernel.org
> Fixes: 8a99b6833c88 ("sched: Move SCHED_DEBUG sysctl to debugfs")
> Tested-by: Chen Yu <yu.c.chen@...el.com>
> Signed-off-by: Chris Hyser <chris.hyser@...cle.com>

I suppose.. but is it really worth the hassle? I mean, this is debug
stuff, just don't write 0 in then?

If we do find we want this (why?!) then should we not invest in a better
debugfs_create_u32_minmax() or something so that we don't get to add 40+
lines for everthing we want to add limits on?

> ---
>  kernel/sched/debug.c | 44 +++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 43 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
> index 1637b65ba07a..cc6a0172a598 100644
> --- a/kernel/sched/debug.c
> +++ b/kernel/sched/debug.c
> @@ -278,6 +278,48 @@ static const struct file_operations sched_dynamic_fops = {
>  
>  #endif /* CONFIG_PREEMPT_DYNAMIC */
>  
> +#ifdef CONFIG_NUMA_BALANCING
> +
> +static ssize_t sched_numa_scan_write(struct file *filp, const char __user *ubuf,
> +				     size_t cnt, loff_t *ppos)
> +{
> +	int err;
> +	unsigned int scan_size;
> +
> +	err = kstrtouint_from_user(ubuf, cnt, 10, &scan_size);
> +	if (err)
> +		return err;
> +
> +	if (!scan_size)
> +		return -EINVAL;
> +
> +	sysctl_numa_balancing_scan_size = scan_size;
> +
> +	*ppos += cnt;
> +	return cnt;
> +}
> +
> +static int sched_numa_scan_show(struct seq_file *m, void *v)
> +{
> +	seq_printf(m, "%d\n", sysctl_numa_balancing_scan_size);
> +	return 0;
> +}
> +
> +static int sched_numa_scan_open(struct inode *inode, struct file *filp)
> +{
> +	return single_open(filp, sched_numa_scan_show, NULL);
> +}
> +
> +static const struct file_operations sched_numa_scan_fops = {
> +	.open		= sched_numa_scan_open,
> +	.write		= sched_numa_scan_write,
> +	.read		= seq_read,
> +	.llseek		= seq_lseek,
> +	.release	= single_release,
> +};
> +
> +#endif /* CONFIG_NUMA_BALANCING */
> +
>  __read_mostly bool sched_debug_verbose;
>  
>  static const struct seq_operations sched_debug_sops;
> @@ -332,7 +374,7 @@ static __init int sched_init_debug(void)
>  	debugfs_create_u32("scan_delay_ms", 0644, numa, &sysctl_numa_balancing_scan_delay);
>  	debugfs_create_u32("scan_period_min_ms", 0644, numa, &sysctl_numa_balancing_scan_period_min);
>  	debugfs_create_u32("scan_period_max_ms", 0644, numa, &sysctl_numa_balancing_scan_period_max);
> -	debugfs_create_u32("scan_size_mb", 0644, numa, &sysctl_numa_balancing_scan_size);
> +	debugfs_create_file("scan_size_mb", 0644, numa, NULL, &sched_numa_scan_fops);
>  	debugfs_create_u32("hot_threshold_ms", 0644, numa, &sysctl_numa_balancing_hot_threshold);
>  #endif
>  
> -- 
> 2.31.1
>