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

[Bob Liu]

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.

-- 
Regards,
-Bob
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