linux-kernel - Re: [PATCH v2 2/3] clone3: allow spawning processes into cgroups

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Message-ID: <20200116235721.tnadjblifqirg5bo@wittgenstein>
Date:   Fri, 17 Jan 2020 00:57:22 +0100
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Michal Koutný <mkoutny@...e.com>
Cc:     linux-api@...r.kernel.org, linux-kernel@...r.kernel.org,
        Tejun Heo <tj@...nel.org>, Ingo Molnar <mingo@...hat.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Li Zefan <lizefan@...wei.com>,
        Peter Zijlstra <peterz@...radead.org>, cgroups@...r.kernel.org
Subject: Re: [PATCH v2 2/3] clone3: allow spawning processes into cgroups

On Wed, Jan 08, 2020 at 07:10:49PM +0100, Christian Brauner wrote:
> On Wed, Jan 08, 2020 at 05:01:02PM +0100, Michal Koutný wrote:
> > On Mon, Dec 23, 2019 at 07:15:03AM +0100, Christian Brauner <christian.brauner@...ntu.com> wrote:
> > > This adds support for creating a process in a different cgroup than its
> > > parent.
> > Binding fork and migration together looks useful.
> > 
> > > --- a/kernel/cgroup/cgroup.c
> > > +++ b/kernel/cgroup/cgroup.c
> > > @@ -5882,21 +5882,176 @@ void cgroup_fork(struct task_struct *child)
> > >  	INIT_LIST_HEAD(&child->cg_list);
> > Just a nitpick, I noticed the comment for cgroup_fork should be updated
> > too (generic migration happens in cgroup_post_fork).
> 
> Thanks.
> 
> > 
> > > --- a/kernel/fork.c
> > > +++ b/kernel/fork.c
> > > [...]
> > > @@ -2279,8 +2278,7 @@ static __latent_entropy struct task_struct *copy_process(
> > >  	write_unlock_irq(&tasklist_lock);
> > >  
> > >  	proc_fork_connector(p);
> > > -	cgroup_post_fork(p);
> > > -	cgroup_threadgroup_change_end(current);
> > > +	cgroup_post_fork(current, p, args);
> > I can see that when CLONE_INTO_CGROUP | CLONE_NEWCGROUP is passed, then
> > the child's cgroup NS will be rooted at parent's css set
> > (copy_namespaces precedes cgroup_post_fork).
> > 
> > Wouldn't it make better sense if this flags combination resulted in
> > child's NS rooted in its css set?
> 
> I need to take a closer look but it sounds like we should move the
> copying of the cgroup namespace to a later point; but again I need to
> look into this.

Ok, this is way simpler I think, we just set the root_cset of the new
cgroup namespace to the child's cset in cgroup_post_fork() if
CLONE_NEWCGROUP is requested.

Christian