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 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.

[akpm@linux-foundation.org: simplify __dump_folio(), per David]
Link: https://lkml.kernel.org/r/20250321120222.1456770-3-gshan@redhat.com
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>
Cc: gehao <gehao@kylinos.cn>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/debug.c b/mm/debug.c
index db83e381a8ae35a556a1b37e2e94df9302894f77..907382257062c0c8cb779acd050556feea16c251 100644 (file)
--- a/mm/debug.c
+++ b/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);