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

[lkml@pengaru.com]

On Thu, Jun 02, 2011 at 01:43:31PM +0200, Peter Zijlstra wrote:
> 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 accounts for the repeated setting of min_ratio on the same bdi.  But
does bdi_set_min_ratio() get entered with a min_ratio of 0 on bdi removal?
If not, we leak the non-zero min_ratio of a removed bdi.

> 
> >  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?

It's the bdi_min_ratio + min_ratio >= 100 case.

The system has many usb disks coming and going day to day, with their
respective bdi's having min_ratio set to 1 when inserted.  It works for
some time until eventually min_ratio can no longer be set, even when the
active set of bdi's seen in /sys/class/bdi/*/min_ratio doesn't add up to
anywhere near 100.

This then leads to an unrelated starvation problem caused by write-heavy
fuse mounts being used atop the usb disks, a problem the min_ratio
setting at the underlying devices bdi effectively prevents.

Regards,
Vito Caputo