linux-kernel - Re: [PATCH v2 2/2] mm: fix arithmetic for max_prop_frac when setting max

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Date: Tue, 19 Dec 2023 04:06:34 +0000
From: Matthew Wilcox <willy@...radead.org>
To: Jingbo Xu <jefflexu@...ux.alibaba.com>
Cc: shr@...kernel.io, akpm@...ux-foundation.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, joseph.qi@...ux.alibaba.com,
	linux-fsdevel@...r.kernel.org, linux-block@...r.kernel.org
Subject: Re: [PATCH v2 2/2] mm: fix arithmetic for max_prop_frac when setting
 max_ratio

On Tue, Dec 19, 2023 at 10:42:46AM +0800, Jingbo Xu wrote:
>  	} 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,
> +					       100 * BDI_RATIO_SCALE);
>  	}

Why use div64_u64 here?

FPROP_FRAC_BASE is an unsigned long.  max_ratio is an unsigned int, so
the numerator is an unsigned long.  BDI_RATIO_SCALE is 10,000, so the
numerator is an unsigned int.  There's no 64-bit arithmetic needed here.