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

From: Michael Kerrisk
Date: Tue May 20 2008 - 10:57:12 EST

[Fixing the bad list address in my initial mail: CC += linux-mm@xxxxxxxxx]

On Tue, May 20, 2008 at 4:45 PM, Nadia Derbey <Nadia.Derbey@xxxxxxxx> wrote:
> Michael Kerrisk wrote:
>> 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.
> Oops, sorry for not doing it: I misunderstood the "unused"
>> 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
>> 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?
> No, that was the only reason.
> Should I repost a patch to set it back as it used to be?

On the one hand, I'd be inclined to leave things as they were pre
2.6.26. On the other hand, I believe that on other systems that have
the limit, msgpool is a limit in bytes. (But documentation of these
details on other systems is very thin on the ground.) I wonder if
anyone else has some knowledge here?

Michael Kerrisk
Linux man-pages maintainer;
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