mm: add thp_head

This is like compound_head() but compiles away when
CONFIG_TRANSPARENT_HUGEPAGE is not enabled.

Signed-off-by: Matthew Wilcox (Oracle) <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Reviewed-by: William Kucharski <[email protected]>
Reviewed-by: Zi Yan <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Mike Kravetz <[email protected]>
Cc: "Kirill A. Shutemov" <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Linus Torvalds <[email protected]>

Author: Matthew Wilcox (Oracle)

include/linux/huge_mm.h

@@ -259,6 +259,15 @@ static inline spinlock_t *pud_trans_huge_lock(pud_t *pud,
 		return NULL;
 }
 
+/**
+ * thp_head - Head page of a transparent huge page.
+ * @page: Any page (tail, head or regular) found in the page cache.
+ */
+static inline struct page *thp_head(struct page *page)
+{
+	return compound_head(page);
+}
+
 /**
  * thp_order - Order of a transparent huge page.
  * @page: Head page of a transparent huge page.
@@ -335,6 +344,12 @@ static inline struct list_head *page_deferred_list(struct page *page)
 #define HPAGE_PUD_MASK ({ BUILD_BUG(); 0; })
 #define HPAGE_PUD_SIZE ({ BUILD_BUG(); 0; })
 
+static inline struct page *thp_head(struct page *page)
+{
+	VM_BUG_ON_PGFLAGS(PageTail(page), page);
+	return page;
+}
+
 static inline unsigned int thp_order(struct page *page)
 {
 	VM_BUG_ON_PGFLAGS(PageTail(page), page);