linux-kernel - Re: [PATCH 1/6] mm: migrate: do not migrate HugeTLB page whose refcount is one

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Date:   Wed, 6 Jan 2021 17:11:35 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     mike.kravetz@...cle.com, akpm@...ux-foundation.org,
        hillf.zj@...baba-inc.com, n-horiguchi@...jp.nec.com,
        ak@...ux.intel.com, yongjun_wei@...ndmicro.com.cn,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/6] mm: migrate: do not migrate HugeTLB page whose
 refcount is one

On Mon 04-01-21 14:58:38, Muchun Song wrote:
> If the refcount is one when it is migrated, it means that the page
> was freed from under us. So we are done and do not need to migrate.

Is this common enough that it would warrant the explicit check for each
migration?

> Signed-off-by: Muchun Song <songmuchun@...edance.com>
> ---
>  mm/migrate.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 4385f2fb5d18..a6631c4eb6a6 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1279,6 +1279,12 @@ static int unmap_and_move_huge_page(new_page_t get_new_page,
>  		return -ENOSYS;
>  	}
>  
> +	if (page_count(hpage) == 1) {
> +		/* page was freed from under us. So we are done. */
> +		putback_active_hugepage(hpage);
> +		return MIGRATEPAGE_SUCCESS;
> +	}
> +
>  	new_hpage = get_new_page(hpage, private);
>  	if (!new_hpage)
>  		return -ENOMEM;
> -- 
> 2.11.0

-- 
Michal Hocko
SUSE Labs