2023-12-19 14:27:39

by Jingbo Xu

Subject: [PATCH v3 1/2] mm: fix arithmetic for bdi min_ratio

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 <[email protected]>
Signed-off-by: Jingbo Xu <[email protected]>
---
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

2024-01-04 17:33:44

by Jan Kara

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 <[email protected]>
> Signed-off-by: Jingbo Xu <[email protected]>

Looks good. Feel free to add:

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

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 <[email protected]>
SUSE Labs, CR