linux-kernel - Re: [PATCH v3 2/2] mm: fix arithmetic for max_prop_frac when setting max

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Date: Thu, 4 Jan 2024 18:41:01 +0100
From: Jan Kara <jack@...e.cz>
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,
	willy@...radead.org
Subject: Re: [PATCH v3 2/2] mm: fix arithmetic for max_prop_frac when setting
 max_ratio

On Tue 19-12-23 22:25:08, Jingbo Xu wrote:
> Since now bdi->max_ratio is part per million, fix the wrong arithmetic
> for max_prop_frac when setting max_ratio.  Otherwise the miscalculated
> max_prop_frac will affect the incrementing of writeout completion count
> when max_ratio is not 100%.
> 
> Fixes: efc3e6ad53ea ("mm: split off __bdi_set_max_ratio() function")
> Signed-off-by: Jingbo Xu <jefflexu@...ux.alibaba.com>

Good catch. Feel free to add:

Reviewed-by: Jan Kara <jack@...e.cz>

								Honza

> ---
>  mm/page-writeback.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 2140382dd768..05e5c425b3ff 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -728,7 +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 = (FPROP_FRAC_BASE * max_ratio) /
> +						(100 * BDI_RATIO_SCALE);
>  	}
>  	spin_unlock_bh(&bdi_lock);
>  
> -- 
> 2.19.1.6.gb485710b
> 
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR