Re: mm: kernel BUG at mm/mlock.c:82!

[Wanpeng Li <liwanp@linux.vnet.ibm.com>]

Hi Sasha,
On Tue, Dec 17, 2013 at 01:46:54AM -0500, Sasha Levin wrote:
>On 12/17/2013 12:39 AM, Bob Liu wrote:
>>cc'd more people.
>>
>>On 12/17/2013 09:04 AM, 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.
>>>
>>>Codewise, it's pretty straightforward. In try_to_unmap_cluster():
>>>
>>>                 page = vm_normal_page(vma, address, *pte);
>>>                 BUG_ON(!page || PageAnon(page));
>>>
>>>                 if (locked_vma) {
>>>                         mlock_vma_page(page);   /* no-op if already
>>>mlocked */
>>>                         if (page == check_page)
>>>                                 ret = SWAP_MLOCK;
>>>                         continue;       /* don't unmap */
>>>                 }
>>>
>>>And the BUG triggers once we see that 'page' isn't locked.
>>>
>>
>>Yes, I didn't see any place locked the corresponding page in
>>try_to_unmap_cluster().
>>
>>I'm afraid adding lock_page() over there may cause potential deadlock.
>>How about just remove the BUG_ON() in mlock_vma_page()?
>
>Welp, it's been there for 5 years now - there should be a good reason to justify removing it.
>

Page should be locked before invoke try_to_unmap(), this check can't be removed 
since this bug is just triggered by confirm !check page hold page lock in virtual 
scan during nolinear VMAs pages aging. Avoid to confirm !check page hold page 
lock is acceptable.

Regards,
Wanpeng Li 

>
>Thanks,
>Sasha
>
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