Subject: Re: [PATCH] Revert "mm/page_alloc: protect pcp->batch accesses with
 ACCESS_ONCE"
To: Hekuang <hekuang@huawei.com>, Michal Hocko <mhocko@kernel.org>
References: <1459333327-89720-1-git-send-email-hekuang@huawei.com>
 <20160330103839.GA4773@techsingularity.net> <56FBAFA0.3010604@huawei.com>
 <20160330111044.GA4324@dhcp22.suse.cz> <56FC7A02.1080201@huawei.com>
CC: Mel Gorman <mgorman@techsingularity.net>, <akpm@linux-foundation.org>,
        <vbabka@suse.cz>, <rientjes@google.com>, <cody@linux.vnet.ibm.com>,
        <gilad@benyossef.com>, <kosaki.motohiro@gmail.com>, <mgorman@suse.de>,
        <penberg@kernel.org>, <wangnan0@huawei.com>, <linux-mm@kvack.org>,
        <linux-kernel@vger.kernel.org>
From: Zefan Li <lizefan@huawei.com>
Message-ID: <56FC7FD2.9000203@huawei.com>
Date: Thu, 31 Mar 2016 09:39:30 +0800
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On 2016/3/31 9:14, Hekuang wrote:
> Hi
> 
> 在 2016/3/30 19:10, Michal Hocko 写道:
>> On Wed 30-03-16 18:51:12, Hekuang wrote:
>>> hi
>>>
>>> 在 2016/3/30 18:38, Mel Gorman 写道:
>>>> On Wed, Mar 30, 2016 at 10:22:07AM +0000, He Kuang wrote:
>>>>> This reverts commit 998d39cb236fe464af86a3492a24d2f67ee1efc2.
>>>>>
>>>>> When local irq is disabled, a percpu variable does not change, so we can
>>>>> remove the access macros and let the compiler optimize the code safely.
>>>>>
>>>> batch can be changed from other contexts. Why is this safe?
>>>>
>>> I've mistakenly thought that per_cpu variable can only be accessed by that
>>> cpu.
>> git blame would point you to 998d39cb236f ("mm/page_alloc: protect
>> pcp->batch accesses with ACCESS_ONCE"). I haven't looked into the code
>> deeply to confirm this is still the case but it would be a good lead
>> that this is not that simple. ACCESS_ONCE resp. {READ,WRITE}_ONCE are
>> usually quite subtle so I would encourage you or anybody else who try to
>> remove them to study the code and the history deeper before removing
>> them.
>>
> Thank you for responding, I've read that commit and related articles and not sending
> mail casually, though you may think it's a stupid patch. I'm a beginner and I think
> sending mails to maillist is a effective way to learn kernel, And, sure i'll be more careful and
> well prepared next time :)
> 

pcp->batch can be changed in a different cpu. You may read percpu_pagelist_fraction_sysctl_handler()
to see how that can happen.