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Subject: Re: [PATCH 1/1] IPC - Do not use a negative value to re-enable
	msgmni automatic recomputing
From: Nadia Derbey <Nadia.Derbey@bull.net>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: matthltc@us.ibm.com, linux-kernel@vger.kernel.org,
       Solofo.Ramangalahy@bull.net
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Date: Thu, 21 Aug 2008 08:27:29 +0200
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On Tue, 2008-07-22 at 03:34 -0700, Andrew Morton wrote:
> On Fri, 04 Jul 2008 08:37:16 +0200 Nadia.Derbey@bull.net wrote:
> 
> > Resending after fixing the issues pointed out by Matt.
> > Now applies to 2.6.26-rc8-mm1.
> > 
> > [PATCH 01/01]
> > 
> > This patch proposes an alternative to the "magical positive-versus-negative
> > number trick" Andrew complained about last week in
> > http://lkml.org/lkml/2008/6/24/418.
> > 
> > This had been introduced with the patches that scale msgmni to the amount of
> > lowmem. With these patches, msgmni has a registered notification routine
> > that recomputes msgmni value upon memory add/remove or ipc namespace creation/
> > removal.
> > 
> > When msgmni is changed from user space (i.e. value written to the proc file),
> > that notification routine is unregistered, and the way to make it registered
> > back is to write a negative value into the proc file. This is the "magical
> > positive-versus-negative number trick".
> > 
> > To fix this, a new proc file is introduced: /proc/sys/kernel/auto_msgmni.
> > This file acts as ON/OFF for msgmni automatic recomputing.
> > 
> > With this patch, the process is the following:
> > 1) kernel boots in "automatic recomputing mode"
> >    /proc/sys/kernel/msgmni contains the value that has been computed (depends
> >                            on lowmem)
> >    /proc/sys/kernel/automatic_msgmni contains "1"
> > 
> > 2) echo <val> > /proc/sys/kernel/msgmni
> >    . sets msg_ctlmni to <val>
> >    . de-activates automatic recomputing (i.e. if, say, some memory is added
> >      msgmni won't be recomputed anymore)
> >    . /proc/sys/kernel/automatic_msgmni now contains "0"
> > 
> > 3) echo "0" > /proc/sys/kernel/automatic_msgmni
> >    . de-activates msgmni automatic recomputing
> >      this has the same effect as 2) except that msg_ctlmni's value stays
> >      blocked at its current value)
> > 
> > 3) echo "1" > /proc/sys/kernel/automatic_msgmni
> >    . recomputes msgmni's value based on the current available memory size
> >      and number of ipc namespaces
> >    . re-activates automatic recomputing for msgmni.
> > 
> 
> Sigh.  I suppose it's a bit better.
> 
> > ---
> >  include/linux/ipc_namespace.h |    3 +
> >  ipc/ipc_sysctl.c              |   72 ++++++++++++++++++++++++++++++++++--------
> >  ipc/ipcns_notifier.c          |   20 ++++++++---
> 
> Could we get this all documented for our poor users please?
> 
> Documentation/filesystems/proc.txt is probably the most appropriate
> place.
> 
> 

Sorry for answering so late, but I just came back from a looooong
vacation.
Actually, I first thought of documenting this new file in proc.txt, but
didn't find anything related to the ipc tunables there, that's why I
finally gave up with the idea. 
I'll send an update of the proc.txt as soon as possible.

Regards,
Nadia

-- 
Nadia Derbey <Nadia.Derbey@bull.net>

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