linux-kernel - Re: [PATCH 1/2] mm: Fix parameter passed to page_mapcount_is

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Message-ID: <908c37d2-2da7-4892-bd07-4ec7ffb8fc3f@redhat.com>
Date: Fri, 21 Mar 2025 21:26:47 +1000
From: Gavin Shan <gshan@...hat.com>
To: David Hildenbrand <david@...hat.com>, linux-mm@...ck.org
Cc: linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
 willy@...radead.org, vbabka@...e.cz, osalvador@...e.de, gehao@...inos.cn,
 shan.gavin@...il.com
Subject: Re: [PATCH 1/2] mm: Fix parameter passed to page_mapcount_is_type()

On 3/21/25 8:13 PM, David Hildenbrand wrote:
> On 21.03.25 06:31, Gavin Shan wrote:
>> As the comments of page_mapcount_is_type() indicate, the parameter
>> passed to the function should be one more than page->__mapcount.
>> However, page->__mapcount (equivalent to page->page_type) is passed to
>> the function by commit 4ffca5a96678 ("mm: support only one page_type per
>> page") where page_type_has_type() is replaced by page_mapcount_is_type(),
>> but the parameter isn't adjusted.
>>
>> Fix the parameter passed to page_mapcount_is_type() to be (page->__mapcount
>> + 1).
>>
>> Fixes: 4ffca5a96678 ("mm: support only one page_type per page")
>> Cc: stable@...r.kernel.org # v6.12+
>> Signed-off-by: Gavin Shan <gshan@...hat.com>
>> ---
>>   include/linux/page-flags.h | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
>> index 36d283552f80..ad87b4cf1f9a 100644
>> --- a/include/linux/page-flags.h
>> +++ b/include/linux/page-flags.h
>> @@ -950,7 +950,7 @@ static inline bool page_mapcount_is_type(unsigned int mapcount)
>>   static inline bool page_has_type(const struct page *page)
>>   {
>> -    return page_mapcount_is_type(data_race(page->page_type));
>> +    return page_mapcount_is_type(data_race(page->page_type) + 1);
> 
> Probably we should just call page_type_has_type() instead?
> 

Yes, page_type_has_type() is better. It will be used in v2.

Thanks,
Gavin