[6.6-stable 0/2] mm: two fixes for bdi min_ratio and max_ratio knob

[6.6-stable 0/2] mm: two fixes for bdi min_ratio and max_ratio knob

[Jingbo Xu]

Have no idea why stable branch missed the "fix" tag.

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

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

[Jingbo Xu]

commit e0646b7590084a5bf3b056d3ad871d9379d2c25a upstream.

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

Link: https://lkml.kernel.org/r/20231219142508.86265-2-jefflexu@linux.alibaba.com
Fixes: 8021fb3232f2 ("mm: split off __bdi_set_min_ratio() function")
Signed-off-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Reported-by: Joseph Qi <joseph.qi@linux.alibaba.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Stefan Roesch <shr@devkernel.io>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---
 mm/page-writeback.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 9ceb841af819..3929bb0a7501 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -714,7 +714,6 @@ 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

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

[Jingbo Xu]

commit fa151a39a6879144b587f35c0dfcc15e1be9450f upstream.

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%.

Link: https://lkml.kernel.org/r/20231219142508.86265-3-jefflexu@linux.alibaba.com
Fixes: efc3e6ad53ea ("mm: split off __bdi_set_max_ratio() function")
Signed-off-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Cc: Joseph Qi <joseph.qi@linux.alibaba.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Stefan Roesch <shr@devkernel.io>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---
 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 3929bb0a7501..e7a7233f6d5d 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -750,7 +750,8 @@ 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

Re: [6.6-stable 0/2] mm: two fixes for bdi min_ratio and max_ratio knob

[Greg KH]

On Thu, Dec 11, 2025 at 10:35:05AM +0800, Jingbo Xu wrote:
> Have no idea why stable branch missed the "fix" tag.

Because that does not guarantee a patch will ever be backported.  See:
    https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html
for how to do this properly in the future.

thanks,

greg k-h

Re: [6.6-stable 0/2] mm: two fixes for bdi min_ratio and max_ratio knob

[Jingbo Xu]

On 1/8/26 7:07 PM, Greg KH wrote:
> On Thu, Dec 11, 2025 at 10:35:05AM +0800, Jingbo Xu wrote:
>> Have no idea why stable branch missed the "fix" tag.
> 
> Because that does not guarantee a patch will ever be backported.  See:
>     https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html
> for how to do this properly in the future.
> 

Sorry I though that a 'fix' tag suffices for stable tree.  Thanks for
the pointer.

-- 
Thanks,
Jingbo