mm: tighten fault_in_pages_writeable()

copy_page_to_iter_iovec() is currently the only user of
fault_in_pages_writeable(), and it definitely can use fragments from
high order pages.

Make sure fault_in_pages_writeable() is only touching two adjacent pages
at most, as claimed.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/pagemap.h

@@ -518,33 +518,27 @@ void page_endio(struct page *page, int rw, int err);
 extern void add_page_wait_queue(struct page *page, wait_queue_t *waiter);
 
 /*
- * Fault a userspace page into pagetables.  Return non-zero on a fault.
- *
- * This assumes that two userspace pages are always sufficient.
+ * Fault one or two userspace pages into pagetables.
+ * Return -EINVAL if more than two pages would be needed.
+ * Return non-zero on a fault.
  */
 static inline int fault_in_pages_writeable(char __user *uaddr, int size)
 {
-	int ret;
+	int span, ret;
 
 	if (unlikely(size == 0))
 		return 0;
 
+	span = offset_in_page(uaddr) + size;
+	if (span > 2 * PAGE_SIZE)
+		return -EINVAL;
 	/*
 	 * Writing zeroes into userspace here is OK, because we know that if
 	 * the zero gets there, we'll be overwriting it.
 	 */
 	ret = __put_user(0, uaddr);
-	if (ret == 0) {
-		char __user *end = uaddr + size - 1;
-
-		/*
-		 * If the page was already mapped, this will get a cache miss
-		 * for sure, so try to avoid doing it.
-		 */
-		if (((unsigned long)uaddr & PAGE_MASK) !=
-				((unsigned long)end & PAGE_MASK))
-			ret = __put_user(0, end);
-	}
+	if (ret == 0 && span > PAGE_SIZE)
+		ret = __put_user(0, uaddr + size - 1);
 	return ret;
 }