[patch 3/3] ipc: increase IPCMNI_MAX

[Nick Piggin <npiggin@suse.de>]

Just wondering whether there is a good reason to have a full 16 bits of
sequence in ipc ids? 32K indexes is pretty easy to overflow, if only in
stress tests for now. I was doing some big aim7 stress testing, which
required this patch, but it's not exactly a realistic workload :) 

But the sequence seems like it just helps slightly with buggy apps, and
if the app is buggy then it can by definition mess up its own ids
anyway? So I don't see that such amount of seq is required.

Index: linux-2.6/ipc/util.h
===================================================================
--- linux-2.6.orig/ipc/util.h
+++ linux-2.6/ipc/util.h
@@ -14,7 +14,16 @@
 #include <linux/err.h>
 
 /* IPCMNI_MAX should be <= MAX_INT, absolute limit for ipc arrays */
-#define IPCMNI_MAX_SHIFT	15
+/*
+ * IPC ids consist of an index into the idr, which allocates from the bottom
+ * up, and a sequence number which is continually incremented. Valid indexes
+ * are from 0..IPCMNI_MAX (or further constrained by sysctls or other limits).
+ * The sequence number prevents ids from being reused quickly. The sequence
+ * number resides in the top part of the 'int' after IPCMNI_MAX.
+ *
+ * Increasing IPCMNI_MAX reduces the sequence wrap interval.
+ */
+#define IPCMNI_MAX_SHIFT	20
 #define IPCMNI_MAX		(1 << IPCMNI_MAX_SHIFT)
 
 #define SEQ_SHIFT		IPCMNI_MAX_SHIFT