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

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Message-ID: <cfd18e0f0805200728j1f38d90s1f6355b71e2d76@mail.gmail.com>
Date:	Tue, 20 May 2008 16:28:27 +0200
From:	"Michael Kerrisk" <mtk.manpages@...glemail.com>
To:	Nadia.Derbey@...l.net
Cc:	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
	linux-mm@...r.kernel.org
Subject: Re: [PATCH 1/8] Scaling msgmni to the amount of lowmem

Hello Nadia,

Regarding your:

[PATCH 1/8] Scaling msgmni to the amount of lowmem
http://article.gmane.org/gmane.linux.kernel/637849/
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 (http://www.kernel.org/doc/man-pages/online/pages/man2/msgctl.2.html)
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
bytes.

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?

Cheers,

Michael

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Found a bug? http://www.kernel.org/doc/man-pages/reporting_bugs.html
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