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

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Date:   Tue, 13 Dec 2022 20:13:31 +0800
From:   Kefeng Wang <wangkefeng.wang@...wei.com>
To:     David Hildenbrand <david@...hat.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 2022/12/13 19:40, David Hildenbrand wrote:
> 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.
So the caller should consider this,  lock or get the folio in the caller?