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SubjectRe: [PATCH 3/3] mm: when handling percpu_pagelist_fraction, use on_each_cpu() to set percpu pageset fields.
On 04/08/2013 05:20 AM, Gilad Ben-Yossef wrote:
> On Fri, Apr 5, 2013 at 11:33 PM, Cody P Schafer <cody@linux.vnet.ibm.com> wrote:
>> In free_hot_cold_page(), we rely on pcp->batch remaining stable.
>> Updating it without being on the cpu owning the percpu pageset
>> potentially destroys this stability.
>>
>> Change for_each_cpu() to on_each_cpu() to fix.
>
> Are you referring to this? -

This was the case I noticed.

>
> 1329 if (pcp->count >= pcp->high) {
> 1330 free_pcppages_bulk(zone, pcp->batch, pcp);
> 1331 pcp->count -= pcp->batch;
> 1332 }
>
> I'm probably missing the obvious but won't it be simpler to do this in
> free_hot_cold_page() -
>
> 1329 if (pcp->count >= pcp->high) {
> 1330 unsigned int batch = ACCESS_ONCE(pcp->batch);
> 1331 free_pcppages_bulk(zone, batch, pcp);
> 1332 pcp->count -= batch;
> 1333 }
>

Potentially, yes. Note that this was simply the one case I noticed,
rather than certainly the only case.

I also wonder whether there could be unexpected interactions between
->high and ->batch not changing together atomically. For example, could
adjusting this knob cause ->batch to rise enough that it is greater than
the previous ->high? If the code above then runs with the previous
->high, ->count wouldn't be correct (checking this inside
free_pcppages_bulk() might help on this one issue).

> Now the batch value used is stable and you don't have to IPI every CPU
> in the system just to change a config knob...

Is this really considered an issue? I wouldn't have expected someone to
adjust the config knob often enough (or even more than once) to cause
problems. Of course as a "It'd be nice" thing, I completely agree.



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