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Subject: Re: [PATCH -next 1/8] mm: memory_hotplug: add pfn_to_online_folio()
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To:     David Hildenbrand <david@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Oscar Salvador <osalvador@suse.de>,
        SeongJae Park <sj@kernel.org>
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        <willy@infradead.org>
References: <20221213092735.187924-1-wangkefeng.wang@huawei.com>
 <20221213092735.187924-2-wangkefeng.wang@huawei.com>
 <f07d1c95-f13c-682b-0519-0b137ccdc631@redhat.com>
From:   Kefeng Wang <wangkefeng.wang@huawei.com>
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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@huawei.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?