Messages in this thread | | | Date | Thu, 2 Jun 2011 13:32:44 -0500 | From | lkml@pengaru ... | Subject | Re: bdi_min_ratio never shrinks, ultimately preventing valid setting of min_ratio |
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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
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