Re: [PATCH 1/1] IPC - Do not use a negative value to re-enable msgmni automatic recomputing

On Tue, 2008-07-22 at 03:34 -0700, Andrew Morton wrote:
On Fri, 04 Jul 2008 08:37:16 +0200 Nadia.Derbey@xxxxxxxx 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@xxxxxxxx>

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