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List:       linux-mm
Subject:    Re: mm: kernel BUG at include/linux/swapops.h:131!
From:       Wanpeng Li <liwanp () linux ! vnet ! ibm ! com>
Date:       2013-12-26 6:27:26
Message-ID: 52bbcc56.280d420a.2c84.ffffc456SMTPIN_ADDED_BROKEN () mx ! google ! com
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On Thu, Dec 26, 2013 at 03:18:58PM +0900, Konstantin Khlebnikov wrote:
>Bob Liu <bob.liu@oracle.com> wrote:
>>On 12/24/2013 03:45 PM, Joonsoo Kim wrote:
>>> On Tue, Dec 24, 2013 at 03:07:05PM +0900, Joonsoo Kim wrote:
>>>> On Mon, Dec 23, 2013 at 10:01:10PM -0500, Sasha Levin wrote:
>>>>> On 12/23/2013 09:51 PM, Joonsoo Kim wrote:
>>>>>> On Mon, Dec 23, 2013 at 12:24:02PM -0500, Sasha Levin wrote:
>>>>>>>> Ping?
>>>>>>>>
>>>>>>>> I've also Cc'ed the "this page shouldn't be locked at all" team.
>>>>>> Hello,
>>>>>>
>>>>>> I can't find the reason of this problem.
>>>>>> If it is reproducible, how about bisecting?
>>>>>
>>>>> While it reproduces under fuzzing it's pretty hard to bisect it
>>with
>>>>> the amount of issues uncovered by trinity recently.
>>>>>
>>>>> I can add any debug code to the site of the BUG if that helps.
>>>>
>>>> Good!
>>>> It will be helpful to add dump_page() in migration_entry_to_page().
>>>>
>>>> Thanks.
>>>>
>>> 
>>> Minchan teaches me that there is possible race condition between
>>> fork and migration.
>>> 
>>> Please consider following situation.
>>> 
>>> 
>>> Process A (do migration)			Process B (parents) Process C (child)
>>> 
>>> try_to_unmap() for migration <begin>		fork
>>> setup migration entry to B's vma
>>> ...
>>> try_to_unmap() for migration <end>
>>> move_to_new_page()
>>> 
>>> 						link new vma
>>> 						    into interval tree
>>> remove_migration_ptes() <begin>
>>> check and clear migration entry on C's vma
>>> ...						copy_one_pte:
>>> ...						    now, B and C have migration entry
>>> ...
>>> ...
>>> check and clear migration entry on B's vma
>>> ...
>>> ...
>>> remove_migration_ptes() <end>
>>> 
>>> 
>>> Eventually, migration entry on C's vma is left.
>>> And then, when C exits, above BUG_ON() can be triggered.
>>> 
>>
>>Yes, Looks like this is a potential race condition.
>>
>>> I'm not sure the I am right, so please think of it together. :)
>>> And I'm not sure again that above assumption is related to this
>>trigger report,
>>> since this may exist for a long time.
>>> 
>>> So my question to mm folks is is above assumption possible and do we
>>have
>>> any protection mechanism on this race?
>>> 
>>
>>I think we can down_read(&mm->mmap_sem) before remove_migration_ptes()
>>to fix this issue, but I don't have time to verify it currently.
>
>Hmm. This kind of race looks impossible: dup_mmap() always places child's
>vma in into rmap tree after parent's one. For file-vma it's done explicitly
>(vma_interval_tree_insert_after), for anon vma it's true because rb-tree
>insert function goes to right branch if elements are equal.
>
>Thus remove_migration_ptes() sees parent's pte first:
>If child has the copy this function will check it after that.
>And they are already synchronized with parent's and child's pte locks.
>

Agreed. 

>Sorry for double posting, gmail cannot into plain text =)
>
>-- 
>Sent from my Android device with K-9 Mail. Please excuse my brevity.
>
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