[PATCH 1/2] GFP_ZONETYPES add commentry on how to calculate

[Andy Whitcroft]

GFP_ZONETYPES define using GFP_ZONEMASK and add commentry

Add commentry explaining the optimisation that we can apply to
GFP_ZONETYPES when the lest most bit is a 'loaner', it can only be
set in isolation.

Signed-off-by: Andy Whitcroft <apw@xxxxxxxxxxxx>
---
 mmzone.h |    8 ++++++++
 1 file changed, 8 insertions(+)
diff -upN reference/include/linux/mmzone.h current/include/linux/mmzone.h
--- reference/include/linux/mmzone.h
+++ current/include/linux/mmzone.h
@@ -91,6 +91,14 @@ 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.
+ *
  * NOTE! Make sure this matches the zones in <linux/gfp.h>
  */
 #define GFP_ZONEMASK	0x07
-
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