Re: [PATCH] mm: fix arithmetic for bdi min_ratio and max_ratio

[Jingbo Xu]

+cc fsdevel

On 12/18/23 11:16 AM, Jingbo Xu 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")
> Reported-by: Joseph Qi <joseph.qi@xxxxxxxxxxxxxxxxx>
> Signed-off-by: Jingbo Xu <jefflexu@xxxxxxxxxxxxxxxxx>
> ---
>  mm/page-writeback.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index ee2fd6a6af40..b393b3281ce9 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) {
> @@ -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);
>  

-- 
Thanks,
Jingbo