linux-kernel - Re: [PATCH v2 2/2] mm/debug: Fix parameter passed to page_mapcount_is

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Message-ID: <3e6040c1-81f4-4977-bbb7-70908104aa45@redhat.com>
Date: Fri, 21 Mar 2025 16:38:29 +0100
From: David Hildenbrand <david@...hat.com>
To: Vlastimil Babka <vbabka@...e.cz>, Gavin Shan <gshan@...hat.com>,
 linux-mm@...ck.org
Cc: linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
 willy@...radead.org, linmiaohe@...wei.com, gehao@...inos.cn,
 shan.gavin@...il.com
Subject: Re: [PATCH v2 2/2] mm/debug: Fix parameter passed to
 page_mapcount_is_type()

On 21.03.25 16:19, Vlastimil Babka wrote:
> On 3/21/25 13:02, 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 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 for page_mapcount_is_type() to be (page->__mapcount
>> + 1). Note that the issue doesn't cause any visible impacts due to the
>> safety gap introduced by PGTY_mapcount_underflow limit.
>>
>> Fixes: 4ffca5a96678 ("mm: support only one page_type per page")
>> Signed-off-by: Gavin Shan <gshan@...hat.com>
>> Acked-by: David Hildenbrand <david@...hat.com>
> 
> Acked-by: Vlastimil Babka <vbabka@...e.cz>
> 
>> ---
>>   mm/debug.c | 4 ++--
>>   1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/debug.c b/mm/debug.c
>> index 8d2acf432385..b6bd9555ec7b 100644
>> --- a/mm/debug.c
>> +++ b/mm/debug.c
>> @@ -71,10 +71,10 @@ static void __dump_folio(struct folio *folio, struct page *page,
>>   		unsigned long pfn, unsigned long idx)
>>   {
>>   	struct address_space *mapping = folio_mapping(folio);
>> -	int mapcount = atomic_read(&page->_mapcount);
>> +	int mapcount = atomic_read(&page->_mapcount) + 1;
>>   	char *type = "";
>>   
>> -	mapcount = page_mapcount_is_type(mapcount) ? 0 : mapcount + 1;
>> +	mapcount = page_mapcount_is_type(mapcount) ? 0 : mapcount;
> 
> At this point it would be perhaps more obvious:
> 
> if (page_mapcount_is_type(mapcount))
> 	mapcount = 0;

Agreed, maybe Andrew can fix that up.

-- 
Cheers,

David / dhildenb