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

[zhong jiang <zhongjiang@huawei.com>]

On 2016/7/29 19:27, Michal Hocko wrote:
> On Thu 21-07-16 09:43:40, Michal Hocko wrote:
>> We have further discussed the patch and I believe it is not correct. See [1].
>> I am proposing the following alternative.
> Andrew, please drop the mm-hugetlb-fix-race-when-migrate-pages.patch. It
> is clearly racy. Whether the BUG_ON update is really the right and
> sufficient fix is not 100% clear yet and we are waiting for Zhong Jiang
> testing.
  The issue is very hard  to recur.   Without attaching any patch to kernel code. up to now,
   it still not happens to it.

  Thanks
  zhongjiang
>> [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;
>>  }
>> -- 
>> 2.8.1
>>
>> -- 
>> Michal Hocko
>> SUSE Labs


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