[PATCH] mm: extend prefault helpers to fault in more than PAGE_SIZE

@@ -436,7 +436,7 @@ i915_gem_shmem_pread(struct drm_device *dev,
 		mutex_unlock(&dev->struct_mutex);
 
 		if (!prefaulted) {
-			ret = fault_in_pages_writeable(user_data, remain);
+			ret = fault_in_multipages_writeable(user_data, remain);
 			/* Userspace is tricking us, but we've already clobbered
 			 * its pages with the prefault and promised to write the
 			 * data up to the first fault. Hence ignore any errors

@@ -822,8 +822,8 @@ i915_gem_pwrite_ioctl(struct drm_device *dev, void *data,
 		       args->size))
 		return -EFAULT;
 
-	ret = fault_in_pages_readable((char __user *)(uintptr_t)args->data_ptr,
-				      args->size);
+	ret = fault_in_multipages_readable((char __user *)(uintptr_t)args->data_ptr,
+					   args->size);
 	if (ret)
 		return -EFAULT;
 

@@ -955,7 +955,7 @@ validate_exec_list(struct drm_i915_gem_exec_object2 *exec,
 		if (!access_ok(VERIFY_WRITE, ptr, length))
 			return -EFAULT;
 
-		if (fault_in_pages_readable(ptr, length))
+		if (fault_in_multipages_readable(ptr, length))
 			return -EFAULT;
 	}
 

@@ -426,7 +426,7 @@ static inline int fault_in_pages_writeable(char __user *uaddr, int size)
 		 */
 		if (((unsigned long)uaddr & PAGE_MASK) !=
 				((unsigned long)end & PAGE_MASK))
-		 	ret = __put_user(0, end);
+			ret = __put_user(0, end);
 	}
 	return ret;
 }

@@ -445,13 +445,71 @@ static inline int fault_in_pages_readable(const char __user *uaddr, int size)
 
 		if (((unsigned long)uaddr & PAGE_MASK) !=
 				((unsigned long)end & PAGE_MASK)) {
-		 	ret = __get_user(c, end);
+			ret = __get_user(c, end);
 			(void)c;
 		}
 	}
 	return ret;
 }
 
+/* Multipage variants of the above prefault helpers, useful if more than
+ * PAGE_SIZE of date needs to be prefaulted. These are separate from the above
+ * functions (which only handle up to PAGE_SIZE) to avoid clobbering the
+ * filemap.c hotpaths. */
+static inline int fault_in_multipages_writeable(char __user *uaddr, int size)
+{
+	int ret;
+	const char __user *end = uaddr + size - 1;
+
+	if (unlikely(size == 0))
+		return 0;
+
+	/*
+	 * Writing zeroes into userspace here is OK, because we know that if
+	 * the zero gets there, we'll be overwriting it.
+	 */
+	while (uaddr <= end) {
+		ret = __put_user(0, uaddr);
+		if (ret != 0)
+			return ret;
+		uaddr += PAGE_SIZE;
+	}
+
+	/* Check whether the range spilled into the next page. */
+	if (((unsigned long)uaddr & PAGE_MASK) ==
+			((unsigned long)end & PAGE_MASK))
+		ret = __put_user(0, end);
+
+	return ret;
+}
+
+static inline int fault_in_multipages_readable(const char __user *uaddr,
+					       int size)
+{
+	volatile char c;
+	int ret;
+	const char __user *end = uaddr + size - 1;
+
+	if (unlikely(size == 0))
+		return 0;
+
+	while (uaddr <= end) {
+		ret = __get_user(c, uaddr);
+		if (ret != 0)
+			return ret;
+		uaddr += PAGE_SIZE;
+	}
+
+	/* Check whether the range spilled into the next page. */
+	if (((unsigned long)uaddr & PAGE_MASK) ==
+			((unsigned long)end & PAGE_MASK)) {
+		ret = __get_user(c, end);
+		(void)c;
+	}
+
+	return ret;
+}
+
 int add_to_page_cache_locked(struct page *page, struct address_space *mapping,
 				pgoff_t index, gfp_t gfp_mask);
 int add_to_page_cache_lru(struct page *page, struct address_space *mapping,