2023-12-18 03:17:00

by Jingbo Xu

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

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

--
2.19.1.6.gb485710b

2023-12-18 03:30:14

by Jingbo Xu

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

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

2023-12-18 18:21:30

by Andrew Morton

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

On Mon, 18 Dec 2023 11:16:40 +0800 Jingbo Xu <[email protected]> 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")

As we have two Fixes:, it would be better to have two patches please.

> --- 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);

Does this change have any observable runtime effects? If so, what are they?

2023-12-19 02:03:56

by Jingbo Xu

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

On 12/19/23 2:21 AM, Andrew Morton wrote:
> On Mon, 18 Dec 2023 11:16:40 +0800 Jingbo Xu <[email protected]> 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")
>
> As we have two Fixes:, it would be better to have two patches please.

Sounds reasonable. Thanks.

>
>> --- 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;

This indeed has visible runtime effect as follows:

# 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

Setting min_ratio will fail with -EINVAL, as the above code tries to set
min_ratio with (min_ratio * BDI_RATIO_SCALE)%, i.e. 10000% in the above
example.

>>
>> 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);

This one has no visible runtime effect, but I believe this would affect
the incrementing of writeout completion count when max_ratio is not 100%.


--
Thanks,
Jingbo