linux-kernel - Re: [PATCH v3 1/2] mm: fix arithmetic for bdi min

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Date: Thu, 4 Jan 2024 18:33:19 +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 1/2] mm: fix arithmetic for bdi min_ratio

On Tue 19-12-23 22:25:07, Jingbo Xu wrote:
> Since now bdi->min_ratio is part per million, fix the wrong arithmetic.
> Otherwise it will fail with -EINVAL when setting a reasonable min_ratio,
> as it tries to set min_ratio to (min_ratio * BDI_RATIO_SCALE) in
> percentage unit, which exceeds 100% anyway.
> 
>     # cat /sys/class/bdi/253\:0/min_ratio
>     0
>     # cat /sys/class/bdi/253\:0/max_ratio
>     100
>     # echo 1 > /sys/class/bdi/253\:0/min_ratio
>     -bash: echo: write error: Invalid argument
> 
> Fixes: 8021fb3232f2 ("mm: split off __bdi_set_min_ratio() function")
> Reported-by: Joseph Qi <joseph.qi@...ux.alibaba.com>
> Signed-off-by: Jingbo Xu <jefflexu@...ux.alibaba.com>

Looks good. Feel free to add:

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

								Honza


> ---
>  mm/page-writeback.c | 1 -
>  1 file changed, 1 deletion(-)
> 
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index ee2fd6a6af40..2140382dd768 100644
> --- 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) {
> -- 
> 2.19.1.6.gb485710b
> 
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR