Re: [PATCH 3/3] mm: when handling percpu_pagelist_fraction, use on_each_cpu() to set percpu pageset fields.

[Cody P Schafer <cody@linux.vnet.ibm.com>]

On 04/08/2013 11:06 PM, Gilad Ben-Yossef wrote:
> On Tue, Apr 9, 2013 at 9:03 AM, Gilad Ben-Yossef <gilad@benyossef.com> wrote:
>
>>
>>> 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).
>>
>> You are right, but that can be treated in  setup_pagelist_highmark()  e.g.:
>>
>> 3993 static void setup_pagelist_highmark(struct per_cpu_pageset *p,
>> 3994                                 unsigned long high)
>> 3995 {
>> 3996         struct per_cpu_pages *pcp;
>>                  unsigned int batch;
>> 3997
>> 3998         pcp = &p->pcp;
>>                  /* We're about to mess with PCP in an non atomic fashion.
>>                     Put an intermediate safe value of batch and make sure it
>>                     is visible before any other change */
>>                  pcp->batch = 1UL;
>>                  smb_mb();
>>
>> 3999         pcp->high = high;
>
> and i think I missed another needed barrier here:
>                    smp_mb();
>
>>
>> 4000         batch = max(1UL, high/4);
>> 4001         if ((high/4) > (PAGE_SHIFT * 8))
>> 4002                 batch = PAGE_SHIFT * 8;
>>
>>                 pcp->batch = batch;
>> 4003 }
>>
>

Yep, that appears to work, provided no additional users of ->batch and 
->high show up. It seems we'll also need some locking to prevent 
concurrent updaters, but that is relatively light weight.

I'll roll up a new patchset that uses this methodology.

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