Re: [PATCH 1/8] Scaling msgmni to the amount of lowmem

From: Michael Kerrisk
Date: Tue May 20 2008 - 10:28:38 EST

Hello Nadia,

Regarding your:

[PATCH 1/8] Scaling msgmni to the amount of lowmem
which I see has made its way in 2.6.26-rc

Your patch has the following change:

-#define MSGPOOL (MSGMNI*MSGMNB/1024) /* size in kilobytes of message pool */
+#define MSGPOOL (MSGMNI * MSGMNB) /* size in bytes of message pool */

Since this constitutes a kernel-userland interface change, so please
do CC me, so that I can change the man pages if needed.

The man page (
does indeed say that msgpool is "unused". But that meant "unused by
the kernel" (sorry -- I probably should have worded that text better).
And, as you spotted, the page also wrongly said the value is in

However, making this change affects the ABI. A userspace application
that was previously using msgctl(IPC_INFO) to retrieve the msgpool
field will be affected by the factor-of-1024 change. I strongly
suspect that there no such applications, or certainly none that care
(since this value is unused by the kernel). But was there a reason
for making this change, aside from the fact that the code and the man
page didn't agree?



Michael Kerrisk
Linux man-pages maintainer;
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