---
 mm/memory.c |   18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

Index: linux-2.6.21-rc7/mm/memory.c
===================================================================
--- linux-2.6.21-rc7.orig/mm/memory.c	2007-04-20 10:55:16.000000000 -0700
+++ linux-2.6.21-rc7/mm/memory.c	2007-04-20 10:55:29.000000000 -0700
@@ -374,6 +374,12 @@ static inline int is_cow_mapping(unsigne
  * used to manipulate the page state. Use compound_head() if
  * operations (like getting a ref count) are necessary.
  *
+ * NOTE! For compound pages it may get to a tail page (maybe we
+ * only deal with a portion of a compound page after all). This
+ * means that the result of vm_normal_page may not directly be
+ * used to manipulate the page state. Use compound_head() if
+ * operations (like getting a ref count) are necessary.
+ *
  * NOTE! Some mappings do not have "struct pages". A raw PFN mapping
  * will have each page table entry just pointing to a raw page frame
  * number, and as far as the VM layer is concerned, those do not have
@@ -998,6 +1004,14 @@ no_page_table:
 	return page;
 }
 
+/*
+ * Get user pages return a series of pointers to page struct.
+ * Note that these may contain compound head and tail pages (necessary
+ * to support direct I/O). Beware: It is not trivial to perform operations
+ * on tail pages. Dirtying etc should be performed on compound_head(page)
+ * and is performed by get_user_pages() (via follow_page) only on the head
+ * page alone.
+ */
 int get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
 		unsigned long start, int len, int write, int force,
 		struct page **pages, struct vm_area_struct **vmas)
@@ -1628,6 +1642,7 @@ static int do_wp_page(struct mm_struct *
 		goto unlock;
 	}
 
+	head_page = compound_head(old_page);
 	/*
 	 * Ok, we need to copy. Oh, well..
 	 */
@@ -2220,7 +2235,7 @@ static int do_no_page(struct mm_struct *
 		int write_access)
 {
 	spinlock_t *ptl;
-	struct page *new_page;
+	struct page *new_page, *head_page;
 	struct address_space *mapping = NULL;
 	pte_t entry;
 	unsigned int sequence = 0;
@@ -2238,6 +2253,7 @@ static int do_no_page(struct mm_struct *
 	}
 retry:
 	new_page = vma->vm_ops->nopage(vma, address & PAGE_MASK, &ret);
+
 	/*
 	 * No smp_rmb is needed here as long as there's a full
 	 * spin_lock/unlock sequence inside the ->nopage callback