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From:   "Huang, Ying" <ying.huang@intel.com>
To:     Miaohe Lin <linmiaohe@huawei.com>
Cc:     <akpm@linux-foundation.org>, <ziy@nvidia.com>,
        <baolin.wang@linux.alibaba.com>, <songmuchun@bytedance.com>,
        <apopple@nvidia.com>, <linux-mm@kvack.org>,
        <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2 11/11] mm/migration: fix possible do_pages_stat_array
 racing with memory offline
References: <20220318111709.60311-1-linmiaohe@huawei.com>
        <20220318111709.60311-12-linmiaohe@huawei.com>
Date:   Fri, 18 Mar 2022 14:40:36 +0800
In-Reply-To: <20220318111709.60311-12-linmiaohe@huawei.com> (Miaohe Lin's
        message of "Fri, 18 Mar 2022 19:17:09 +0800")
Message-ID: <878rt7oja3.fsf@yhuang6-desk2.ccr.corp.intel.com>
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Miaohe Lin <linmiaohe@huawei.com> writes:

> When follow_page peeks a page, the page could be migrated and then be
> offlined while it's still being used by the do_pages_stat_array().
> Use FOLL_GET to hold the page refcnt to fix this potential race.
>
> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>

Acked-by: "Huang, Ying" <ying.huang@intel.com>

> ---
>  mm/migrate.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index dbd91fbdb127..cd85ba0ab592 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1807,13 +1807,18 @@ static void do_pages_stat_array(struct mm_struct *mm, unsigned long nr_pages,
>  			goto set_status;
>  
>  		/* FOLL_DUMP to ignore special (like zero) pages */
> -		page = follow_page(vma, addr, FOLL_DUMP);
> +		page = follow_page(vma, addr, FOLL_GET | FOLL_DUMP);
>  
>  		err = PTR_ERR(page);
>  		if (IS_ERR(page))
>  			goto set_status;
>  
> -		err = page ? page_to_nid(page) : -ENOENT;
> +		if (page) {
> +			err = page_to_nid(page);
> +			put_page(page);
> +		} else {
> +			err = -ENOENT;
> +		}
>  set_status:
>  		*status = err;