Re: mm, something wring in page_lock_anon_vma_read()?

[Vlastimil Babka <vbabka@suse.cz>]

On 05/23/2017 11:21 AM, zhong jiang wrote:
> On 2017/5/23 0:51, Vlastimil Babka wrote:
>> On 05/20/2017 05:01 AM, zhong jiang wrote:
>>> On 2017/5/20 10:40, Hugh Dickins wrote:
>>>> On Sat, 20 May 2017, Xishi Qiu wrote:
>>>>> Here is a bug report form redhat: https://bugzilla.redhat.com/show_bug.cgi?id=1305620
>>>>> And I meet the bug too. However it is hard to reproduce, and 
>>>>> 624483f3ea82598("mm: rmap: fix use-after-free in __put_anon_vma") is not help.
>>>>>
>>>>> From the vmcore, it seems that the page is still mapped(_mapcount=0 and _count=2),
>>>>> and the value of mapping is a valid address(mapping = 0xffff8801b3e2a101),
>>>>> but anon_vma has been corrupted.
>>>>>
>>>>> Any ideas?
>>>> Sorry, no.  I assume that _mapcount has been misaccounted, for example
>>>> a pte mapped in on top of another pte; but cannot begin tell you where
>>>> in Red Hat's kernel-3.10.0-229.4.2.el7 that might happen.
>>>>
>>>> Hugh
>>>>
>>>> .
>>>>
>>> Hi, Hugh
>>>
>>> I find the following message from the dmesg.
>>>
>>> [26068.316592] BUG: Bad rss-counter state mm:ffff8800a7de2d80 idx:1 val:1
>>>
>>> I can prove that the __mapcount is misaccount.  when task is exited. the rmap
>>> still exist.
>> Check if the kernel in question contains this commit: ad33bb04b2a6 ("mm:
>> thp: fix SMP race condition between THP page fault and MADV_DONTNEED")
>   HI, Vlastimil
>  
>   I miss the patch.

Try applying it then, there's good chance the error and crash will go
away. Even if your workload doesn't actually run any madvise(MADV_DONTNEED).

> when I read the patch. I find the following issue. but I am sure it is right.
> 
>       if (unlikely(pmd_trans_unstable(pmd)))
>         return 0;
>     /*
>      * A regular pmd is established and it can't morph into a huge pmd
>      * from under us anymore at this point because we hold the mmap_sem
>      * read mode and khugepaged takes it in write mode. So now it's
>      * safe to run pte_offset_map().
>      */
>     pte = pte_offset_map(pmd, address);
> 
>   after pmd_trans_unstable call,  without any protect method.  by the comments,
>   it think the pte_offset_map is safe.    before pte_offset_map call, it still may be
>   unstable. it is possible?

IIRC it's "unstable" wrt possible none->huge->none transition. But once
we've seen it's a regular pmd via pmd_trans_unstable(), we're safe as a
transition from regular pmd can't happen.

>   Thanks
> zhongjiang
>>> Thanks
>>> zhongjiang
>>>
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>>
>> .
>>
> 
> 
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