linux-kernel - Re: [PATCH] mm/memcg: use get_page() for device private pages in mc_handle_swap

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Message-ID: <ZLIY+ZwrLvpapGE6@casper.infradead.org>
Date:   Sat, 15 Jul 2023 04:56:41 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Miaohe Lin <linmiaohe@...wei.com>
Cc:     akpm@...ux-foundation.org, hannes@...xchg.org, mhocko@...nel.org,
        roman.gushchin@...ux.dev, shakeelb@...gle.com,
        muchun.song@...ux.dev, linux-mm@...ck.org, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/memcg: use get_page() for device private pages in
 mc_handle_swap_pte()

On Sat, Jul 15, 2023 at 11:28:02AM +0800, Miaohe Lin wrote:
> When page table locked is held, the page can't be freed from under us.

But the page isn't mapped into the page table ... there's a swap entry
in the page table, so I don't think your logic holds.

> So use get_page() to get the extra page reference to simplify the code.
> No functional change intended.
> 
> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
> ---
>  mm/memcontrol.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 93e3cc581b51..4ca382efb1ca 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -5670,8 +5670,9 @@ static struct page *mc_handle_swap_pte(struct vm_area_struct *vma,
>  	 */
>  	if (is_device_private_entry(ent)) {
>  		page = pfn_swap_entry_to_page(ent);
> -		if (!get_page_unless_zero(page))
> -			return NULL;
> +		/* Get a page reference while we know the page can't be freed. */
> +		get_page(page);
> +
>  		return page;
>  	}
>  
> -- 
> 2.33.0
> 
>