cow_user_page: fix page alignment

High Dickins points out that the user virtual address passed to the page
fault handler isn't necessarily page-aligned.

Also, add a comment on why the copy could fail for the user address case.

Signed-off-by: Linus Torvalds <[e-mail]>

mm/memory.c

···
 	 */
 	if (unlikely(!src)) {
 		void *kaddr = kmap_atomic(dst, KM_USER0);
-		unsigned long left = __copy_from_user_inatomic(kaddr, (void __user *)va, PAGE_SIZE);
-		if (left)
+		void __user *uaddr = (void __user *)(va & PAGE_MASK);
+
+		/*
+		 * This really shouldn't fail, because the page is there
+		 * in the page tables. But it might just be unreadable,
+		 * in which case we just give up and fill the result with
+		 * zeroes.
+		 */
+		if (__copy_from_user_inatomic(kaddr, uaddr, PAGE_SIZE))
 			memset(kaddr, 0, PAGE_SIZE);
 		kunmap_atomic(kaddr, KM_USER0);
 		return;