linux-kernel - Re: [PATCH -next 1/8] mm: memory_hotplug: add pfn_to_online

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Date:   Tue, 13 Dec 2022 12:40:26 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Kefeng Wang <wangkefeng.wang@...wei.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Oscar Salvador <osalvador@...e.de>,
        SeongJae Park <sj@...nel.org>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        damon@...ts.linux.dev, vishal.moola@...il.com, willy@...radead.org
Subject: Re: [PATCH -next 1/8] mm: memory_hotplug: add pfn_to_online_folio()

On 13.12.22 10:27, Kefeng Wang wrote:
> Introduce a wrapper function pfn_to_online_folio(), which calls
> pfn_to_online_page() and returns the folio of the page found,
> or null if no page.
> 
> Signed-off-by: Kefeng Wang <wangkefeng.wang@...wei.com>
> ---
>   include/linux/memory_hotplug.h | 7 +++++++
>   1 file changed, 7 insertions(+)
> 
> diff --git a/include/linux/memory_hotplug.h b/include/linux/memory_hotplug.h
> index 9fcbf5706595..e841e4fb52a7 100644
> --- a/include/linux/memory_hotplug.h
> +++ b/include/linux/memory_hotplug.h
> @@ -265,6 +265,13 @@ static inline void pgdat_kswapd_unlock(pg_data_t *pgdat) {}
>   static inline void pgdat_kswapd_lock_init(pg_data_t *pgdat) {}
>   #endif /* ! CONFIG_MEMORY_HOTPLUG */
>   
> +static inline struct folio *pfn_to_online_folio(unsigned long pfn)
> +{
> +	struct page *page = pfn_to_online_page(pfn);
> +
> +	return page ? page_folio(page) : NULL;
> +}

Who guarantees that page_folio() is safe and stable at that point?

IIRC, that's very tricky. We could have the page concurrently getting 
freed and the folio dissolved.

-- 
Thanks,

David / dhildenb