On Wed, 9 Jul 2014, Andrey Ryabinin wrote:

+
+Markers of unaccessible bytes could be found in mm/kasan/kasan.h header:
+
+#define KASAN_FREE_PAGE         0xFF  /* page was freed */
+#define KASAN_PAGE_REDZONE      0xFE  /* redzone for kmalloc_large allocations */
+#define KASAN_SLAB_REDZONE      0xFD  /* Slab page redzone, does not belong to any slub object */

We call these zones "PADDING". Redzones are associated with an object.
Padding is there because bytes are left over, unusable or necessary for
alignment.