2023-12-19 14:28:12

by Jingbo Xu

Subject: [PATCH v3 2/2] mm: fix arithmetic for max_prop_frac when setting max_ratio

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 <[email protected]>
---
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

2024-01-04 17:41:23

by Jan Kara

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 <[email protected]>

Good catch. Feel free to add:

Reviewed-by: Jan Kara <[email protected]>

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 <[email protected]>
SUSE Labs, CR