mm/debug: fix parameter passed to page_mapcount_is_type()

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 4ffca5a ("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.

[[email protected]: simplify __dump_folio(), per David]
Link: https://lkml.kernel.org/r/[email protected]
Fixes: 4ffca5a ("mm: support only one page_type per page")
Signed-off-by: Gavin Shan <[email protected]>
Acked-by: David Hildenbrand <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
Cc: gehao <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: Miaohe Lin <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: Gavin Shan

mm/debug.c

@@ -71,10 +71,12 @@ 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;
+	if (page_mapcount_is_type(mapcount))
+		mapcount = 0;
+
 	pr_warn("page: refcount:%d mapcount:%d mapping:%p index:%#lx pfn:%#lx\n",
 			folio_ref_count(folio), mapcount, mapping,
 			folio->index + idx, pfn);