Re: mm: kernel BUG at mm/huge_memory.c:2785!

[Sasha Levin <sasha.levin@oracle.com>]

On 03/07/2014 07:18 AM, Kirill A. Shutemov wrote:
> On Thu, Mar 06, 2014 at 08:32:48PM -0500, Sasha Levin wrote:
>> On 03/04/2014 10:53 PM, Sasha Levin wrote:
>>> On 03/04/2014 10:16 PM, Bob Liu wrote:
>>>> On Thu, Feb 27, 2014 at 11:03 PM, Kirill A. Shutemov
>>>> <kirill.shutemov@linux.intel.com> wrote:
>>>>> Sasha Levin wrote:
>>>>>> Hi all,
>>>>>>
>>>>>> While fuzzing with trinity inside a KVM tools guest running latest -next kernel I've stumbled on the
>>>>>> following spew:
>>>>>>
>>>>>> [ 1428.146261] kernel BUG at mm/huge_memory.c:2785!
>>>>>
>>>>> Hm, interesting.
>>>>>
>>>>> It seems we either failed to split huge page on vma split or it
>>>>> materialized from under us. I don't see how it can happen:
>>>>>
>>>>>    - it seems we do the right thing with vma_adjust_trans_huge() in
>>>>>      __split_vma();
>>>>>    - we hold ->mmap_sem all the way from vm_munmap(). At least I don't see
>>>>>      a place where we could drop it;
>>>>>
>>>>
>>>> Enable CONFIG_DEBUG_VM may show some useful information, at least we
>>>> can confirm weather rwsem_is_locked(&tlb->mm->mmap_sem) before
>>>> split_huge_page_pmd().
>>>
>>> I have CONFIG_DEBUG_VM enabled and that code you're talking is not triggering, so mmap_sem
>>> is locked.
>>
>> Guess what. I've just hit it.
>
> I think this particular traceback is not a real problem: by time of
> exit_mm() we shouldn't race with anybody for the mm_struct.
>
> We probably could drop ->mmap_sem later in mmput() rather then in
> exit_mm() to fix this false positive.
>
>> It's worth keeping in mind that this is the first time I see it.
>
> Hm. That's strange exit_mmap() is called without holding ->mmap_sem.
>

This issues does happen quite often and is very easy to reproduce, I could try
anything you can thing of.


Thanks,
Sasha

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