linux-kernel - Re: [PATCH] mm: fix arithmetic for bdi min_ratio and max

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Date: Mon, 18 Dec 2023 10:21:17 -0800
From: Andrew Morton <akpm@...ux-foundation.org>
To: Jingbo Xu <jefflexu@...ux.alibaba.com>
Cc: shr@...kernel.io, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 joseph.qi@...ux.alibaba.com
Subject: Re: [PATCH] mm: fix arithmetic for bdi min_ratio and max_ratio

On Mon, 18 Dec 2023 11:16:40 +0800 Jingbo Xu <jefflexu@...ux.alibaba.com> wrote:

> bdi->[min|max]_ratio are both part per million.  Fix the wrong
> arithmetic when setting bdi's min_ratio and max_ratio.
> 
> Fixes: efc3e6ad53ea ("mm: split off __bdi_set_max_ratio() function")
> Fixes: 8021fb3232f2 ("mm: split off __bdi_set_min_ratio() function")

As we have two Fixes:, it would be better to have two patches please.

> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -692,7 +692,6 @@ static int __bdi_set_min_ratio(struct backing_dev_info *bdi, unsigned int min_ra
>  
>  	if (min_ratio > 100 * BDI_RATIO_SCALE)
>  		return -EINVAL;
> -	min_ratio *= BDI_RATIO_SCALE;
>  
>  	spin_lock_bh(&bdi_lock);
>  	if (min_ratio > bdi->max_ratio) {
> @@ -729,7 +728,8 @@ static int __bdi_set_max_ratio(struct backing_dev_info *bdi, unsigned int max_ra
>  		ret = -EINVAL;
>  	} else {
>  		bdi->max_ratio = max_ratio;
> -		bdi->max_prop_frac = (FPROP_FRAC_BASE * max_ratio) / 100;
> +		bdi->max_prop_frac = div64_u64(FPROP_FRAC_BASE * max_ratio,
> +					       100UL * BDI_RATIO_SCALE);
>  	}
>  	spin_unlock_bh(&bdi_lock);

Does this change have any observable runtime effects?  If so, what are they?