mm: Turn head_compound_mapcount() into folio_entire_mapcount()

Adjust documentation to be more clear.
Signed-off-by: NMatthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: NChristoph Hellwig <hch@lst.de>

include/linux/mm.h

@@ -776,21 +776,26 @@ static inline int is_vmalloc_or_module_addr(const void *x)
 }
 #endif
 
-static inline int head_compound_mapcount(struct page *head)
+/*
+ * How many times the entire folio is mapped as a single unit (eg by a
+ * PMD or PUD entry).  This is probably not what you want, except for
+ * debugging purposes; look at folio_mapcount() or page_mapcount()
+ * instead.
+ */
+static inline int folio_entire_mapcount(struct folio *folio)
 {
-	return atomic_read(compound_mapcount_ptr(head)) + 1;
+	VM_BUG_ON_FOLIO(!folio_test_large(folio), folio);
+	return atomic_read(folio_mapcount_ptr(folio)) + 1;
 }
 
 /*
  * Mapcount of compound page as a whole, does not include mapped sub-pages.
  *
- * Must be called only for compound pages or any their tail sub-pages.
+ * Must be called only for compound pages.
  */
 static inline int compound_mapcount(struct page *page)
 {
-	VM_BUG_ON_PAGE(!PageCompound(page), page);
-	page = compound_head(page);
-	return head_compound_mapcount(page);
+	return folio_entire_mapcount(page_folio(page));
 }
 
 /*

mm/debug.c

@@ -48,7 +48,8 @@ const struct trace_print_flags vmaflag_names[] = {
 
 static void __dump_page(struct page *page)
 {
-	struct page *head = compound_head(page);
+	struct folio *folio = page_folio(page);
+	struct page *head = &folio->page;
 	struct address_space *mapping;
 	bool compound = PageCompound(page);
 	/*
@@ -76,6 +77,7 @@ static void __dump_page(struct page *page)
 		else
 			mapping = (void *)(tmp & ~PAGE_MAPPING_FLAGS);
 		head = page;
+		folio = (struct folio *)page;
 		compound = false;
 	} else {
 		mapping = page_mapping(page);
@@ -94,7 +96,7 @@ static void __dump_page(struct page *page)
 	if (compound) {
 		pr_warn("head:%p order:%u compound_mapcount:%d compound_pincount:%d\n",
 				head, compound_order(head),
-				head_compound_mapcount(head),
+				folio_entire_mapcount(folio),
 				head_compound_pincount(head));
 	}