mm: introduce page_mapcount_is_type()

Resolve the awkward "and add one to this opaque constant" test into a
self-documenting inline function.

Link: https://lkml.kernel.org/r/20240821173914.2270383-3-willy@infradead.orgSigned-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Cc: Kent Overstreet <kent.overstreet@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

fs/proc/internal.h

@@ -166,8 +166,7 @@ static inline int folio_precise_page_mapcount(struct folio *folio,
 {
 	int mapcount = atomic_read(&page->_mapcount) + 1;
 
-	/* Handle page_has_type() pages */
-	if (mapcount < PAGE_MAPCOUNT_RESERVE + 1)
+	if (page_mapcount_is_type(mapcount))
 		mapcount = 0;
 	if (folio_test_large(folio))
 		mapcount += folio_entire_mapcount(folio);

include/linux/mm.h

@@ -1228,8 +1228,7 @@ static inline int folio_mapcount(const struct folio *folio)
 
 	if (likely(!folio_test_large(folio))) {
 		mapcount = atomic_read(&folio->_mapcount) + 1;
-		/* Handle page_has_type() pages */
-		if (mapcount < PAGE_MAPCOUNT_RESERVE + 1)
+		if (page_mapcount_is_type(mapcount))
 			mapcount = 0;
 		return mapcount;
 	}

include/linux/page-flags.h

@@ -956,12 +956,18 @@ enum pagetype {
 #define folio_test_type(folio, flag)					\
 	((READ_ONCE(folio->page.page_type) & (PAGE_TYPE_BASE | flag))  == PAGE_TYPE_BASE)
 
-static inline int page_type_has_type(unsigned int page_type)
+static inline bool page_type_has_type(int page_type)
 {
-	return (int)page_type < PAGE_MAPCOUNT_RESERVE;
+	return page_type < PAGE_MAPCOUNT_RESERVE;
 }
 
-static inline int page_has_type(const struct page *page)
+/* This takes a mapcount which is one more than page->_mapcount */
+static inline bool page_mapcount_is_type(unsigned int mapcount)
+{
+	return page_type_has_type(mapcount - 1);
+}
+
+static inline bool page_has_type(const struct page *page)
 {
 	return page_type_has_type(READ_ONCE(page->page_type));
 }