Re: bdi_min_ratio never shrinks, ultimately preventing valid setting of min_ratio

[Peter Zijlstra <peterz@infradead.org>]

It really helps if you CC the relevant people.

On Tue, 2011-05-31 at 19:28 -0500, lkml@pengaru.com wrote:
> mm/page-writeback.c
> There is a static global bdi_min_ratio used for policing the setting of
> per-bdi min_ratio's, to ensure the sum doesn't cross 100.
> 
> There is no place in this listing where the value is decremented by the
> respective bdi's min_ratio when a bdi is torn down. 

There is, adding a negative number is equal to a subtraction.

                min_ratio -= bdi->min_ratio;
                if (bdi_min_ratio + min_ratio < 100) {
                        bdi_min_ratio += min_ratio;
                        bdi->min_ratio += min_ratio;
                }

is the relevant piece, note that bdi->min_ratio is the current setting,
this makes min_ratio the difference between the new and old setting, and
adding this to both bdi_min_ratio (the global sum) and bdi->min_ratio
dtrt regardless if the new value is larger or smaller than the old
value.

>  This looks like a bug
> to me, and I have a situation where I'm unable to set a min_ratio to 1
> where the sum of /sys/class/bdi/*/min_ratio does not add up to 100, which
> is what triggered this investigation.

Which of the two -EINVAL cases is triggered?