[PATCH 2/4] mm: introduce struct head_page and compound_head_t

[Kirill A. Shutemov]

This patch creates new type that is compatible with struct page on
memory layout, but distinct from C point of view.

compound_head_t() has the same functionality as compound_head(), but
returns pointer on struct head_page.

Not-yet-signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
---
 include/linux/mm_types.h   | 4 ++++
 include/linux/page-flags.h | 9 ++++++++-
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 944b2b37313b..247e86adaa1c 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -225,6 +225,10 @@ struct page {
 #endif
 ;
 
+struct head_page {
+	struct page page;
+};
+
 struct page_frag {
 	struct page *page;
 #if (BITS_PER_LONG > 32) || (PAGE_SIZE >= 65536)
diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index d111caad2a22..54801253b85c 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -133,7 +133,9 @@ enum pageflags {
 
 #ifndef __GENERATING_BOUNDS_H
 
-struct page;	/* forward declaration */
+/* forward declaration */
+struct page;
+struct head_page;
 
 static inline struct page *compound_head(struct page *page)
 {
@@ -144,6 +146,11 @@ static inline struct page *compound_head(struct page *page)
 	return page;
 }
 
+static inline struct head_page *compound_head_t(struct page *page)
+{
+	return (struct head_page *)compound_head(page);
+}
+
 static __always_inline int PageTail(struct page *page)
 {
 	return READ_ONCE(page->compound_head) & 1;
-- 
2.8.0.rc3