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

[Hillf Danton]

> 
> From b1e9b3214f1859fdf7d134cdcb56f5871933539c Mon Sep 17 00:00:00 2001
> From: Michal Hocko <mhocko@xxxxxxxx>
> 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@xxxxxxxxxx>
> Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
> ---
Acked-by: Hillf Danton <hillf.zj@xxxxxxxxxxxxxxx>

>  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
>