linux-kernel - Re: [PATCH v3 00/11] mm: fixup changers of per cpu pageset's ->high and ->batch

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Message-ID: <5165D8DE.5090801@linux.vnet.ibm.com>
Date:	Wed, 10 Apr 2013 14:25:50 -0700
From:	Cody P Schafer <cody@...ux.vnet.ibm.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Gilad Ben-Yossef <gilad@...yossef.com>,
	Simon Jeons <simon.jeons@...il.com>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	Mel Gorman <mgorman@...e.de>, Linux MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 00/11] mm: fixup changers of per cpu pageset's ->high
 and ->batch

On 04/10/2013 02:23 PM, Andrew Morton wrote:
> On Wed, 10 Apr 2013 11:23:28 -0700 Cody P Schafer <cody@...ux.vnet.ibm.com> wrote:
>
>> "Problems" with the current code:
>>   1. there is a lack of synchronization in setting ->high and ->batch in
>>      percpu_pagelist_fraction_sysctl_handler()
>>   2. stop_machine() in zone_pcp_update() is unnecissary.
>>   3. zone_pcp_update() does not consider the case where percpu_pagelist_fraction is non-zero
>>
>> To fix:
>>   1. add memory barriers, a safe ->batch value, an update side mutex when
>>      updating ->high and ->batch, and use ACCESS_ONCE() for ->batch users that
>>      expect a stable value.
>>   2. avoid draining pages in zone_pcp_update(), rely upon the memory barriers added to fix #1
>>   3. factor out quite a few functions, and then call the appropriate one.
>>
>> Note that it results in a change to the behavior of zone_pcp_update(), which is
>> used by memory_hotplug. I'm rather certain that I've diserned (and preserved)
>> the essential behavior (changing ->high and ->batch), and only eliminated
>> unneeded actions (draining the per cpu pages), but this may not be the case.
>>
>> Further note that the draining of pages that previously took place in
>> zone_pcp_update() occured after repeated draining when attempting to offline a
>> page, and after the offline has "succeeded". It appears that the draining was
>> added to zone_pcp_update() to avoid refactoring setup_pageset() into 2
>> funtions.
>
> There hasn't been a ton of review activity for this patchset :(
>
> I'm inclined to duck it until after 3.9.  Do the patches fix any
> noticeably bad userspace behavior?

No, all the bugs are theoretical. Waiting should be fine.

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