1 files changed, 12 insertions, 1 deletions

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 93a849f742db..ebfc238cc243 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -91,10 +91,21 @@ struct per_cpu_pageset {
  * be 8 (2 ** 3) zonelists.  GFP_ZONETYPES defines the number of possible
  * combinations of zone modifiers in "zone modifier space".
  *
+ * As an optimisation any zone modifier bits which are only valid when
+ * no other zone modifier bits are set (loners) should be placed in
+ * the highest order bits of this field.  This allows us to reduce the
+ * extent of the zonelists thus saving space.  For example in the case
+ * of three zone modifier bits, we could require up to eight zonelists.
+ * If the left most zone modifier is a "loner" then the highest valid
+ * zonelist would be four allowing us to allocate only five zonelists.
+ * Use the first form for GFP_ZONETYPES when the left most bit is not
+ * a "loner", otherwise use the second.
+ *
  * NOTE! Make sure this matches the zones in <linux/gfp.h>
  */
 #define GFP_ZONEMASK    0x07
-#define GFP_ZONETYPES   5
+/* #define GFP_ZONETYPES       (GFP_ZONEMASK + 1) */           /* Non-loner */
+#define GFP_ZONETYPES  ((GFP_ZONEMASK + 1) / 2 + 1)            /* Loner */
 
 /*
  * On machines where it is needed (eg PCs) we divide physical memory