Re: [PATCH v2 2/2] mm/debug: Fix parameter passed to page_mapcount_is_type()

[Vlastimil Babka <vbabka@suse.cz>]

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@redhat.com>
> Acked-by: David Hildenbrand <david@redhat.com>

Acked-by: Vlastimil Babka <vbabka@suse.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;

But doesn't matter much.

>  	pr_warn("page: refcount:%d mapcount:%d mapping:%p index:%#lx pfn:%#lx\n",
>  			folio_ref_count(folio), mapcount, mapping,
>  			folio->index + idx, pfn);