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

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Message-ID: <6d33b510-e913-5738-1e87-9f70da2ccee0@redhat.com>
Date:   Tue, 13 Dec 2022 15:42:55 +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 13:13, Kefeng Wang wrote:
> 
> 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?
> 

I think the helper pfn_to_online_folio() as is might be wrong approach. 
The caller should convert to a folio after taking precautions that it is 
actually safe to do that (e.g., succeeded grabbing a reference first).

-- 
Thanks,

David / dhildenb