linux-kernel - Re: [PATCH] mm: mmu_notifier: re-fix freed page still mapped in secondary MMU

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Message-ID: <516EECDB.6090400@linux.vnet.ibm.com>
Date:	Thu, 18 Apr 2013 02:41:31 +0800
From:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
To:	Robin Holt <holt@....com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Gleb Natapov <gleb@...hat.com>,
	Avi Kivity <avi.kivity@...il.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>,
	Linux Memory Management List <linux-mm@...ck.org>
Subject: Re: [PATCH] mm: mmu_notifier: re-fix freed page still mapped in secondary
 MMU

On 04/17/2013 10:10 PM, Robin Holt wrote:
> On Wed, Apr 17, 2013 at 10:55:26AM +0800, Xiao Guangrong wrote:
>> On 04/17/2013 02:08 AM, Robin Holt wrote:
>>> On Tue, Apr 16, 2013 at 09:07:20PM +0800, Xiao Guangrong wrote:
>>>> On 04/16/2013 07:43 PM, Robin Holt wrote:
>>>>> Argh.  Taking a step back helped clear my head.
>>>>>
>>>>> For the -stable releases, I agree we should just go with your
>>>>> revert-plus-hlist_del_init_rcu patch.  I will give it a test
>>>>> when I am in the office.
>>>>
>>>> Okay. Wait for your test report. Thank you in advance.
>>>>
>>>>>
>>>>> For the v3.10 release, we should work on making this more
>>>>> correct and completely documented.
>>>>
>>>> Better document is always welcomed.
>>>>
>>>> Double call ->release is not bad, like i mentioned it in the changelog:
>>>>
>>>> it is really rare (e.g, can not happen on kvm since mmu-notify is unregistered
>>>> after exit_mmap()) and the later call of multiple ->release should be
>>>> fast since all the pages have already been released by the first call.
>>>>
>>>> But, of course, it's great if you have a _light_ way to avoid this.
>>>
>>> Getting my test environment set back up took longer than I would have liked.
>>>
>>> Your patch passed.  I got no NULL-pointer derefs.
>>
>> Thanks for your test again.
>>
>>>
>>> How would you feel about adding the following to your patch?
>>
>> I prefer to make these changes as a separate patch, this change is the
>> improvement, please do not mix it with bugfix.
> 
> I think your "improvement" classification is a bit deceiving.  My previous
> patch fixed the bug in calling release multiple times.  Your patch without
> this will reintroduce that buggy behavior.  Just because the bug is already
> worked around by KVM does not mean it is not a bug.

As your tested, calling ->release() multiple times can work, but just make your
testcase more _slower_. So your changes is trying to speed it up - it is a
improvement.

Well, _if_ it is really a bug, could you please do not fix two bugs in one patch?

Thanks!

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