linux-kernel - Re: [PATCH 2/3] make clone

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Date:	Wed, 14 Dec 2011 11:09:14 +0400
From:	Glauber Costa <glommer@...allels.com>
To:	Li Zefan <lizf@...fujitsu.com>
CC:	Tejun Heo <tj@...nel.org>, <linux-kernel@...r.kernel.org>,
	<jbottomley@...allels.com>, <cgroups@...r.kernel.org>,
	<bsingharora@...il.com>, <devel@...nvz.org>,
	<kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [PATCH 2/3] make clone_children a flag

On 12/14/2011 06:29 AM, Li Zefan wrote:
> Tejun Heo wrote:
>> On Sun, Dec 11, 2011 at 03:45:37PM +0100, Glauber Costa wrote:
>>> There is no reason to have a flags field, and then a separate
>>> bool field just to indicate if the clone_children flag is set.
>>> Make it a flag
>>>
>>> Signed-off-by: Glauber Costa<glommer@...allels.com>
>>
>> Doesn't this change how remount conditions are checked?
>>

Well, I was thinking it wouldn't, because I patched all callers. But I 
forget life is not always that simple: After you mentioned, I checked 
and we do test for changes in the flag field explicitly on remount. So I 
missed that, indeed.

> Right. Currently we can do this:
>
> 	# mount -t cgroup xxx /mnt
> 	# mount -o remount,clone_children /mnt
>
> with this patch, the above remount will fail.
>
> But..the current bevaiour of remount is a bit confusing in that remount
> with/without "clone_children" has no effect on anything:
>
> 	# mount -t cgroup -o clone_children xxx /mnt
> 	# cat /mnt/cgroup.clone_children
> 	1
> 	# mount -o remount xxx /mnt
> 	# mount | grep cgroup
> 	xxx on /mnt type cgroup (rw,clone_children)
> 	# cat /mnt/cgroup.clone_children
> 	1

That's indeed confusing, and it comes from the fact that we always 
inherit clone_children from the parent - which is sane, IMHO. So this 
flag only has any value in establishing the initial behaviour of the top 
root cgroup. I wonder then if it wouldn't better to just be explicit and 
fail in this case ?

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