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Date: Fri, 29 Jul 2016 13:27:07 +0200
From: Michal Hocko <mhocko@kernel.org>
Subject: Re: + mm-hugetlb-fix-race-when-migrate-pages.patch added to -mm tree
Message-ID: <20160729112707.GB8031@dhcp22.suse.cz>
References: <578eb28b.YbRUDGz5RloTVlrE%akpm@linux-foundation.org>
 <20160721074340.GA26398@dhcp22.suse.cz>
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Cc: zhongjiang@huawei.com, qiuxishi@huawei.com, vbabka@suse.cz, mm-commits@vger.kernel.org, Mike Kravetz <mike.kravetz@oracle.com>, Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>, Mel Gorman <mgorman@suse.de>, linux-mm@kvack.org

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.

> [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

-- 
Michal Hocko
SUSE Labs

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