linux-kernel - Re: [RFC][PATCH v2 7/7] Do not recompute msgmni anymore if explicitely set by user

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Date:	Tue, 05 Feb 2008 15:55:10 +0100
From:	Nadia Derbey <Nadia.Derbey@...l.net>
To:	Yasunori Goto <y-goto@...fujitsu.com>
Cc:	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	linux-mm@...ck.org, containers@...ts.linux-foundation.org,
	matthltc@...ibm.com
Subject: Re: [RFC][PATCH v2 7/7] Do not recompute msgmni anymore if explicitely
 set by user

Yasunori Goto wrote:
> Thanks Nadia-san.
> 
> I tested this patch set on my box. It works well.
> I have only one comment.
> 
> 
> 
>>---
>> ipc/ipc_sysctl.c |   43 +++++++++++++++++++++++++++++++++++++++++--
>> 1 file changed, 41 insertions(+), 2 deletions(-)
>>
>>Index: linux-2.6.24/ipc/ipc_sysctl.c
>>===================================================================
>>--- linux-2.6.24.orig/ipc/ipc_sysctl.c	2008-01-29 16:55:04.000000000 +0100
>>+++ linux-2.6.24/ipc/ipc_sysctl.c	2008-01-31 13:13:14.000000000 +0100
>>@@ -34,6 +34,24 @@ static int proc_ipc_dointvec(ctl_table *
>> 	return proc_dointvec(&ipc_table, write, filp, buffer, lenp, ppos);
>> }
>> 
>>+static int proc_ipc_callback_dointvec(ctl_table *table, int write,
>>+	struct file *filp, void __user *buffer, size_t *lenp, loff_t *ppos)
>>+{
>>+	size_t lenp_bef = *lenp;
>>+	int rc;
>>+
>>+	rc = proc_ipc_dointvec(table, write, filp, buffer, lenp, ppos);
>>+
>>+	if (write && !rc && lenp_bef == *lenp)
>>+		/*
>>+		 * Tunable has successfully been changed from userland:
>>+		 * disable its automatic recomputing.
>>+		 */
>>+		unregister_ipcns_notifier(current->nsproxy->ipc_ns);
>>+
>>+	return rc;
>>+}
>>+
> 
> 
> 
> Hmmm. I suppose this may be side effect which user does not wish.
> 
> I would like to recommend there should be a switch which can turn on/off
> automatic recomputing.
> If user would like to change this value, it should be turned off.
> Otherwise, his requrest will be rejected with some messages.
> 
> Probably, user can understand easier than this side effect.
> 

Hi Yasunori,

Hope you're feeling better!

Well the idea behind this was the following: if msgmni is changed say 
via procfs it is usually to increase it, in order for applications that 
need more msg queues to be able to run.
So even if a new ipc namespace is created or memory is removed, the 
application that has set that new value doesn't care: it wants msgmni to 
be unchanged. I agree with you that unconditionally turning the 
recomputing off may appear coarse, but I'm afraid that adding the 
functionality of turning that recomputing on/off will make things more 
complicated:
1) manage the tunable recomputing state: it shouldn't be turned on twice
2) adds more things to do at the user level.

I'll try to think about it more deeply and may be come up with an 
intermediate solution.

Regards,
Nadia



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