Re: [PATCH v2 11/11] mm/migration: fix possible do_pages_stat_array racing with memory offline

[Huang, Ying]

Miaohe Lin <linmiaohe@xxxxxxxxxx> 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@xxxxxxxxxx>

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

> ---
>  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;