Make "install_page()" able to handle truncated pages.

This makes it much easier on the callers, no need to
worry about races with vmtruncate() and friends, since
"install_page()" will just cleanly handle that case
and tell the caller about it.

mm/fremap.c

@@ -61,12 +61,6 @@ int install_page(struct mm_struct *mm, struct vm_area_struct *vma,
 	pmd_t *pmd;
 	pte_t pte_val;
 
-	/*
-	 * We use page_add_file_rmap below: if install_page is
-	 * ever extended to anonymous pages, this will warn us.
-	 */
-	BUG_ON(!page_mapping(page));
-
 	pgd = pgd_offset(mm, addr);
 	spin_lock(&mm->page_table_lock);
 
@@ -78,6 +72,14 @@ int install_page(struct mm_struct *mm, struct vm_area_struct *vma,
 	if (!pte)
 		goto err_unlock;
 
+	/*
+	 * This page may have been truncated. Tell the
+	 * caller about it.
+	 */
+	err = -EAGAIN;
+	if (!page_mapping(page))
+		goto err_unlock;
+
 	zap_pte(mm, vma, addr, pte);
 
 	mm->rss++;