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

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Date:	Thu, 18 Apr 2013 02:52:31 +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 04/18/2013 02:45 AM, Robin Holt wrote:

>>>>>>> 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?
> 
> The code, as is, does not call ->release() multiple times.  Your code
> changes the behavior to call it multiple times.  You are introducing the
> bug by your code changes.  Why not fix the bug you create in the patch
> which creates it?

Andrew, your thought?


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