Re: mm, something wring in page_lock_anon_vma_read()?

[zhong jiang]

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.  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?

  Thanks
zhongjiang
>> Thanks
>> zhongjiang
>>
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