Re: + mm-hugetlb-fix-race-when-migrate-pages.patch added to -mm tree

[zhong jiang <zhongjiang@huawei.com>]

On 2016/7/21 19:27, Michal Hocko wrote:
> On Thu 21-07-16 18:54:09, zhong jiang wrote:
>> On 2016/7/21 15:43, Michal Hocko wrote:
>>> We have further discussed the patch and I believe it is not correct. See [1].
>>> I am proposing the following alternative.
>>>
>>> [1] http://lkml.kernel.org/r/20160720132431.GM11249@dhcp22.suse.cz
>>> ---
>>> >From b1e9b3214f1859fdf7d134cdcb56f5871933539c Mon Sep 17 00:00:00 2001
>>> From: Michal Hocko <mhocko@suse.com>
>>> Date: Thu, 21 Jul 2016 09:28:13 +0200
>>> Subject: [PATCH] mm, hugetlb: fix huge_pte_alloc BUG_ON
>>>
>>> Zhong Jiang has reported a BUG_ON from huge_pte_alloc hitting when he
>>> runs his database load with memory online and offline running in
>>> parallel. The reason is that huge_pmd_share might detect a shared pmd
>>> which is currently migrated and so it has migration pte which is
>>> !pte_huge.
>>>
>>> There doesn't seem to be any easy way to prevent from the race and in
>>> fact seeing the migration swap entry is not harmful. Both callers of
>>> huge_pte_alloc are prepared to handle them. copy_hugetlb_page_range
>>> will copy the swap entry and make it COW if needed. hugetlb_fault will
>>> back off and so the page fault is retries if the page is still under
>>> migration and waits for its completion in hugetlb_fault.
>>>
>>> That means that the BUG_ON is wrong and we should update it. Let's
>>> simply check that all present ptes are pte_huge instead.
>>>
>>> Reported-by: zhongjiang <zhongjiang@huawei.com>
>>> Signed-off-by: Michal Hocko <mhocko@suse.com>
>>> ---
>>>  mm/hugetlb.c | 2 +-
>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>>> index 34379d653aa3..31dd2b8b86b3 100644
>>> --- a/mm/hugetlb.c
>>> +++ b/mm/hugetlb.c
>>> @@ -4303,7 +4303,7 @@ pte_t *huge_pte_alloc(struct mm_struct *mm,
>>>  				pte = (pte_t *)pmd_alloc(mm, pud, addr);
>>>  		}
>>>  	}
>>> -	BUG_ON(pte && !pte_none(*pte) && !pte_huge(*pte));
>>> +	BUG_ON(pte && pte_present(*pte) && !pte_huge(*pte));
>>>  
>>>  	return pte;
>>>  }
>>   I don't think that the patch can fix the question.   The explain is as follow.
>>
>>                cpu0                                                                                      cpu1
>> copy_hugetlb_page_range                                                       try_to_unmap_one
>>              huge_pte_alloc  #pmd may be shared                           
>>              lock dst_pte     #dst_pte may be migrate                    
>>             lock src_pte     #src_pte may be normal pt1       
>>            set_huge_pte_at    #dst_pte points to normal
>>            spin_unlock (src_pt1)
>>                                                                                                           lock src_pte
>>            spin_unlock(dst_pt1)                                                          set src_pte migrate entry
>>                                                                                                          spin_unlock(src_pte)
>>    *       dst_pte is a normal pte, but corresponding to the
>>             pfn is under migrate.  it is dangerous.
>>
>> The race may occur. is right ?  if the scenario exist.  we should think about more.
> Can this happen at all? copy_hugetlb_page_range does the following to
> rule out shared page table entries. At least that is my understanding of
> c5c99429fa57 ("fix hugepages leak due to pagetable page sharing")
>
> 		/* If the pagetables are shared don't copy or take references */
> 		if (dst_pte == src_pte)
> 			continue;
  vm_file points to mapping should be shared,  I am not sure, if it is so,  the  possibility is exist.
  of course, src_pte is the same as the dst_pte.

  when dst_pte is migrate entry and src pte is normal entry,  if  try_to_unmap_one is successful,
   then exec copy_hugetlb_page_range,   it will lead to the dst_pte is under dangerous. 
 

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