linux-kernel - Re: [PATCH 3/3] mm: when handling percpu_pagelist_fraction, use on_each

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Date:	Tue, 9 Apr 2013 09:06:16 +0300
From:	Gilad Ben-Yossef <gilad@...yossef.com>
To:	Cody P Schafer <cody@...ux.vnet.ibm.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>, Linux MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/3] mm: when handling percpu_pagelist_fraction, use
 on_each_cpu() to set percpu pageset fields.

On Tue, Apr 9, 2013 at 9:03 AM, Gilad Ben-Yossef <gilad@...yossef.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 }
>

-- 
Gilad Ben-Yossef
Chief Coffee Drinker
gilad@...yossef.com
Israel Cell: +972-52-8260388
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