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

[David Hildenbrand]

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@xxxxxxxxxx>
> > Acked-by: David Hildenbrand <david@xxxxxxxxxx>
>
> Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
>
> > ---
> >   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