Re: mm: kernel BUG at include/linux/swapops.h:131!

Konstantin Khlebnikov
2013-12-26T06:19:14Z

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.﻿

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