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authorAndy Whitcroft <apw@shadowen.org>2006-02-01 03:05:26 -0800
committerLinus Torvalds <torvalds@g5.osdl.org>2006-02-01 08:53:14 -0800
commit79046ae07ae21245520ca0aab985ee6678a879f8 (patch)
tree8a980747ab9eeaa3407e2aa2366a4c6e95dee9d0
parent42c722d4cb4022e56ff200f3f5a58c0dfd7edac6 (diff)
[PATCH] GFP_ZONETYPES: add commentry on how to calculate
GFP_ZONETYPES define using GFP_ZONEMASK and add commentry Add commentry explaining the optimisation that we can apply to GFP_ZONETYPES when the leftmost bit is a 'loaner', it can only be set in isolation. Signed-off-by: Andy Whitcroft <apw@shadowen.org> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
-rw-r--r--include/linux/mmzone.h8
1 files changed, 8 insertions, 0 deletions
diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 93a849f742d..88c30f844ab 100644
--- a/include/linux/mmzone.h
+++ b/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