2 files changed, 40 insertions, 38 deletions

diff --git a/include/linux/mm.h b/include/linux/mm.h
index ec6e33d..26fd168 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -355,36 +355,39 @@ static inline struct page *compound_head(struct page *page)
 	return page;
 }
 
+/*
+ * The atomic page->_mapcount, starts from -1: so that transitions
+ * both from it and to it can be tracked, using atomic_inc_and_test
+ * and atomic_add_negative(-1).
+ */
+static inline void reset_page_mapcount(struct page *page)
+{
+	atomic_set(&(page)->_mapcount, -1);
+}
+
+static inline int page_mapcount(struct page *page)
+{
+	return atomic_read(&(page)->_mapcount) + 1;
+}
+
 static inline int page_count(struct page *page)
 {
 	return atomic_read(&compound_head(page)->_count);
 }
 
+extern bool __get_page_tail(struct page *page);
+
 static inline void get_page(struct page *page)
 {
+	if (unlikely(PageTail(page)))
+		if (likely(__get_page_tail(page)))
+			return;
 	/*
 	 * Getting a normal page or the head of a compound page
-	 * requires to already have an elevated page->_count. Only if
-	 * we're getting a tail page, the elevated page->_count is
-	 * required only in the head page, so for tail pages the
-	 * bugcheck only verifies that the page->_count isn't
-	 * negative.
+	 * requires to already have an elevated page->_count.
 	 */
-	VM_BUG_ON(atomic_read(&page->_count) < !PageTail(page));
+	VM_BUG_ON(atomic_read(&page->_count) <= 0);
 	atomic_inc(&page->_count);
-	/*
-	 * Getting a tail page will elevate both the head and tail
-	 * page->_count(s).
-	 */
-	if (unlikely(PageTail(page))) {
-		/*
-		 * This is safe only because
-		 * __split_huge_page_refcount can't run under
-		 * get_page().
-		 */
-		VM_BUG_ON(atomic_read(&page->first_page->_count) <= 0);
-		atomic_inc(&page->first_page->_count);
-	}
 }
 
 static inline struct page *virt_to_head_page(const void *x)
@@ -803,21 +806,6 @@ static inline pgoff_t page_index(struct page *page)
 }
 
 /*
- * The atomic page->_mapcount, like _count, starts from -1:
- * so that transitions both from it and to it can be tracked,
- * using atomic_inc_and_test and atomic_add_negative(-1).
- */
-static inline void reset_page_mapcount(struct page *page)
-{
-	atomic_set(&(page)->_mapcount, -1);
-}
-
-static inline int page_mapcount(struct page *page)
-{
-	return atomic_read(&(page)->_mapcount) + 1;
-}
-
-/*
  * Return true if this page is mapped into pagetables.
  */
 static inline int page_mapped(struct page *page)
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 027935c..059839c 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -36,10 +36,24 @@ struct page {
 					 * updated asynchronously */
 	atomic_t _count;		/* Usage count, see below. */
 	union {
-		atomic_t _mapcount;	/* Count of ptes mapped in mms,
-					 * to show when page is mapped
-					 * & limit reverse map searches.
-					 */
+		/*
+		 * Count of ptes mapped in
+		 * mms, to show when page is
+		 * mapped & limit reverse map
+		 * searches.
+		 *
+		 * Used also for tail pages
+		 * refcounting instead of
+		 * _count. Tail pages cannot
+		 * be mapped and keeping the
+		 * tail page _count zero at
+		 * all times guarantees
+		 * get_page_unless_zero() will
+		 * never succeed on tail
+		 * pages.
+		 */
+		atomic_t _mapcount;
+
 		struct {		/* SLUB */
 			u16 inuse;
 			u16 objects;