Re: + mm-hugetlb-fix-race-when-migrate-pages.patch added to -mm tree

From: zhong jiang <zhongjiang@huawei.com>
To: Michal Hocko <mhocko@kernel.org>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
	akpm@linux-foundation.org, qiuxishi@huawei.com, vbabka@suse.cz,
	mm-commits@vger.kernel.org,
	Mike Kravetz <mike.kravetz@oracle.com>,
	Mel Gorman <mgorman@suse.de>,
	linux-mm@kvack.org
Subject: Re: + mm-hugetlb-fix-race-when-migrate-pages.patch added to -mm tree
Date: Thu, 21 Jul 2016 22:33:47 +0800	[thread overview]
Message-ID: <5790DD4B.2060000@huawei.com> (raw)
In-Reply-To: <20160721142722.GP26379@dhcp22.suse.cz>

On 2016/7/21 22:27, Michal Hocko wrote:
> On Thu 21-07-16 22:13:55, zhong jiang wrote:
>> On 2016/7/21 22:01, Michal Hocko wrote:
>>> On Thu 21-07-16 21:58:23, zhong jiang wrote:
>>>> On 2016/7/21 21:40, Michal Hocko wrote:
>>>>> On Thu 21-07-16 21:25:38, zhong jiang wrote:
>>>>>> On 2016/7/21 20:55, Michal Hocko wrote:
>>>>> [...]
>>>>>>> OK, now I understand what you mean. So you mean that a different process
>>>>>>> initiates the migration while this path copies to pte. That is certainly
>>>>>>> possible but I still fail to see what is the problem about that.
>>>>>>> huge_pte_alloc will return the identical pte whether it is regular or
>>>>>>> migration one. So what exactly is the problem?
>>>>>>>
>>>>>> copy_hugetlb_page_range obtain the shared dst_pte, it may be not equal
>>>>>> to the src_pte.  The dst_pte can come from other process sharing the
>>>>>> mapping.
>>>>> So you mean that the parent doesn't have the shared pte while the child
>>>>> would get one?
>>>>>  
>>>>  no, parent must have the shared pte because the the child copy the
>>>> parent. but parent is not the only source pte we can get. when we
>>>> scan the maping->i_mmap, firstly ,it can obtain a shared pte from
>>>> other process. but I am not sure.
>>> But then all the shared ptes should be identical, no? Or am I missing
>>> something?
>>  all the shared ptes should be identical, but  there is  a possibility that new process
>>  want to share the pte from other process ,  other than the parent,  For the first time
>>  the process is about to share pte with it.   is it possiblity?
> I do not see how. They are opperating on the same mapping so I really do
> not see how different process makes any difference.
>
   ok , In a words . the new process get the shared pte, The shared pte not come from the parent process.
  so , src_pte is not equal to dst_pte.  because src_pte come from the parent, while dst_pte come from
  other process.    obviously, it is not same. 

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