linux-kernel - Re: [PATCH] IPC initialize shmmax and shmall from the current value not the default

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Date:	Sun, 04 May 2014 13:17:40 +0200
From:	Manfred Spraul <manfred@...orfullife.com>
To:	Davidlohr Bueso <davidlohr@...com>, Marian Marinov <mm@...com>
CC:	akpm@...ux-foundation.org, n-horiguchi@...jp.nec.com,
	Greg KH <gregkh@...uxfoundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linux Containers <containers@...ts.linux-foundation.org>
Subject: Re: [PATCH] IPC initialize shmmax and shmall from the current value
 not the default

Hi Marian,

Note: The limits will soon be increased to (nearly) ULONG_MAX.
I.e.: If you propose the patch because you are running into issues with 
a too small SEMMAX after an unshare(CLONE_NEWIPC), then this will be 
fixed soon.


On 05/04/2014 01:53 AM, Davidlohr Bueso wrote:
> On Sun, 2014-05-04 at 01:48 +0300, Marian Marinov wrote:
>> When we are creating new IPC namespace that should be cloned from the current namespace it is a good idea to copy the
>> values of the current shmmax and shmall to the new namespace.
The idea sounds reasonable:
If an admin has reduced the limits, then the reduction should also apply 
after a unshare(CLONE_NEWIPC).

But:
Your patch doesn't use the current shmmax, it uses the shmmax from 
init_ipc_ns.
Would it be possible to use the current values?

> Why is this a good idea?
>
> This would break userspace that relies on the current behavior.
> Furthermore we've recently changed the default value of both these
> limits to be as large as you can get, thus deprecating them. I don't
> like the idea of this being replaced by namespaces.
Davidlohr: We are not deprecating them, we make the default huge.
The limits should stay as usable as they were.

With regards to breaking user space, I must think about it a bit more.
Right now, each new namespace starts with SEMMAX=32MB, i.e. an often 
unusable default.

--
     Manfred
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