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

[zhong jiang <zhongjiang@huawei.com>]

On 2016/7/21 20:30, Michal Hocko wrote:
> On Thu 21-07-16 20:14:41, zhong jiang wrote:
>> 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.
> I am not sure I understand. This is a fork path where the ptes are
> copied over from the parent to the child. So how would vm_file differ?
  I think you can misunderstand my meaning.  A file refers to the mapping field can be shared by other process,
  parent process have the mapping , but is not only.  This is only my viewpoint. is right ??

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