mm: factor out large folio handling from folio_nr_pages() into folio_large_nr_pages()

Let's factor it out into a simple helper function.  This helper will also
come in handy when working with code where we know that our folio is
large.

While at it, let's consistently return a "long" value from all these
similar functions.  Note that we cannot use "unsigned int" (even though
_folio_nr_pages is of that type), because it would break some callers that
do stuff like "-folio_nr_pages()".  Both "int" or "unsigned long" would
work as well.

Maybe in the future we'll have the nr_pages readily available for all
large folios, maybe even for small folios, or maybe for none.

Link: https://lkml.kernel.org/r/20250303163014.1128035-3-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Andy Lutomirks^H^Hski <luto@kernel.org>
Cc: Borislav Betkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jann Horn <jannh@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Lance Yang <ioworker0@gmail.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Matthew Wilcow (Oracle) <willy@infradead.org>
Cc: Michal Koutn <mkoutny@suse.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: tejun heo <tj@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Zefan Li <lizefan.x@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: davidhildenbrand

include/linux/mm.h

@@ -1199,6 +1199,18 @@ static inline unsigned int folio_large_order(const struct folio *folio)
 	return folio->_flags_1 & 0xff;
 }
 
+#ifdef CONFIG_64BIT
+static inline long folio_large_nr_pages(const struct folio *folio)
+{
+	return folio->_folio_nr_pages;
+}
+#else
+static inline long folio_large_nr_pages(const struct folio *folio)
+{
+	return 1L << folio_large_order(folio);
+}
+#endif
+
 /*
  * compound_order() can be called without holding a reference, which means
  * that niceties like page_folio() don't work.  These callers should be
@@ -2147,11 +2159,7 @@ static inline long folio_nr_pages(const struct folio *folio)
 {
 	if (!folio_test_large(folio))
 		return 1;
-#ifdef CONFIG_64BIT
-	return folio->_folio_nr_pages;
-#else
-	return 1L << folio_large_order(folio);
-#endif
+	return folio_large_nr_pages(folio);
 }
 
 /* Only hugetlbfs can allocate folios larger than MAX_ORDER */
@@ -2166,24 +2174,20 @@ static inline long folio_nr_pages(const struct folio *folio)
  * page.  compound_nr() can be called on a tail page, and is defined to
  * return 1 in that case.
  */
-static inline unsigned long compound_nr(struct page *page)
+static inline long compound_nr(struct page *page)
 {
 	struct folio *folio = (struct folio *)page;
 
 	if (!test_bit(PG_head, &folio->flags))
 		return 1;
-#ifdef CONFIG_64BIT
-	return folio->_folio_nr_pages;
-#else
-	return 1L << folio_large_order(folio);
-#endif
+	return folio_large_nr_pages(folio);
 }
 
 /**
  * thp_nr_pages - The number of regular pages in this huge page.
  * @page: The head page of a huge page.
  */
-static inline int thp_nr_pages(struct page *page)
+static inline long thp_nr_pages(struct page *page)
 {
 	return folio_nr_pages((struct folio *)page);
 }