changes since v1:
- split the previous v1 patch into two separate patches with
corresponding "Fixes" tag (Andrew Morton)
- patch 2: remove "UL" suffix for the "100" constant
v1: https://lore.kernel.org/all/[email protected]/
Jingbo Xu (2):
mm: fix arithmetic for bdi min_ratio
mm: fix arithmetic for max_prop_frac when setting max_ratio
mm/page-writeback.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
--
2.19.1.6.gb485710b
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..dda59b368c01 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 = div64_u64(FPROP_FRAC_BASE * max_ratio,
+ 100 * BDI_RATIO_SCALE);
}
spin_unlock_bh(&bdi_lock);
--
2.19.1.6.gb485710b
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
On Tue, Dec 19, 2023 at 10:42:46AM +0800, Jingbo Xu wrote:
> } 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,
> + 100 * BDI_RATIO_SCALE);
> }
Why use div64_u64 here?
FPROP_FRAC_BASE is an unsigned long. max_ratio is an unsigned int, so
the numerator is an unsigned long. BDI_RATIO_SCALE is 10,000, so the
numerator is an unsigned int. There's no 64-bit arithmetic needed here.
On 12/19/23 12:06 PM, Matthew Wilcox wrote:
> On Tue, Dec 19, 2023 at 10:42:46AM +0800, Jingbo Xu wrote:
>> } 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,
>> + 100 * BDI_RATIO_SCALE);
>> }
>
> Why use div64_u64 here?
>
> FPROP_FRAC_BASE is an unsigned long. max_ratio is an unsigned int, so
> the numerator is an unsigned long. BDI_RATIO_SCALE is 10,000, so the
> numerator is an unsigned int. There's no 64-bit arithmetic needed here.
Yes, div64_u64() is actually not needed here. So it seems
bdi->max_prop_frac = FPROP_FRAC_BASE * max_ratio / 100 / BDI_RATIO_SCALE;
is adequate?
--
Thanks,
Jingbo
On Tue, Dec 19, 2023 at 01:58:21PM +0800, Jingbo Xu wrote:
> On 12/19/23 12:06 PM, Matthew Wilcox wrote:
> > On Tue, Dec 19, 2023 at 10:42:46AM +0800, Jingbo Xu wrote:
> >> } 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,
> >> + 100 * BDI_RATIO_SCALE);
> >> }
> >
> > Why use div64_u64 here?
> >
> > FPROP_FRAC_BASE is an unsigned long. max_ratio is an unsigned int, so
> > the numerator is an unsigned long. BDI_RATIO_SCALE is 10,000, so the
> > numerator is an unsigned int. There's no 64-bit arithmetic needed here.
>
> Yes, div64_u64() is actually not needed here. So it seems
>
> bdi->max_prop_frac = FPROP_FRAC_BASE * max_ratio / 100 / BDI_RATIO_SCALE;
>
> is adequate?
I'd rather spell that as:
bdi->max_prop_frac = (FPROP_FRAC_BASE * max_ratio) /
(100 * BDI_RATIO_SCALE);
It's closer to how you'd write it out mathematically and so it reads
more easily. At least for me.
On 12/19/23 9:01 PM, Matthew Wilcox wrote:
> On Tue, Dec 19, 2023 at 01:58:21PM +0800, Jingbo Xu wrote:
>> On 12/19/23 12:06 PM, Matthew Wilcox wrote:
>>> On Tue, Dec 19, 2023 at 10:42:46AM +0800, Jingbo Xu wrote:
>>>> } 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,
>>>> + 100 * BDI_RATIO_SCALE);
>>>> }
>>>
>>> Why use div64_u64 here?
>>>
>>> FPROP_FRAC_BASE is an unsigned long. max_ratio is an unsigned int, so
>>> the numerator is an unsigned long. BDI_RATIO_SCALE is 10,000, so the
>>> numerator is an unsigned int. There's no 64-bit arithmetic needed here.
>>
>> Yes, div64_u64() is actually not needed here. So it seems
>>
>> bdi->max_prop_frac = FPROP_FRAC_BASE * max_ratio / 100 / BDI_RATIO_SCALE;
>>
>> is adequate?
>
> I'd rather spell that as:
>
> bdi->max_prop_frac = (FPROP_FRAC_BASE * max_ratio) /
> (100 * BDI_RATIO_SCALE);
>
> It's closer to how you'd write it out mathematically and so it reads
> more easily. At least for me.
Thanks, I would send v3 soon.
--
Thanks,
Jingbo