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

[Cody P Schafer]

On 04/08/2013 11:06 PM, Gilad Ben-Yossef wrote:
> On Tue, Apr 9, 2013 at 9:03 AM, Gilad Ben-Yossef <gilad@xxxxxxxxxxxxx> 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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