linux-kernel - Re: [PATCH 05/10] mm, oom: skip vforked tasks from being selected

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Date:	Thu, 16 Jun 2016 08:24:34 +0200
From:	Michal Hocko <mhocko@...nel.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	linux-mm@...ck.org,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	David Rientjes <rientjes@...gle.com>,
	Vladimir Davydov <vdavydov@...allels.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 05/10] mm, oom: skip vforked tasks from being selected

On Wed 15-06-16 16:51:06, Oleg Nesterov wrote:
> On 06/09, Michal Hocko wrote:
> >
> > --- a/include/linux/sched.h
> > +++ b/include/linux/sched.h
> > @@ -1883,6 +1883,32 @@ extern int arch_task_struct_size __read_mostly;
> >  #define TNF_FAULT_LOCAL	0x08
> >  #define TNF_MIGRATE_FAIL 0x10
> >  
> > +static inline bool in_vfork(struct task_struct *tsk)
> > +{
> > +	bool ret;
> > +
> > +	/*
> > +	 * need RCU to access ->real_parent if CLONE_VM was used along with
> > +	 * CLONE_PARENT.
> > +	 *
> > +	 * We check real_parent->mm == tsk->mm because CLONE_VFORK does not
> > +	 * imply CLONE_VM
> > +	 *
> > +	 * CLONE_VFORK can be used with CLONE_PARENT/CLONE_THREAD and thus
> > +	 * ->real_parent is not necessarily the task doing vfork(), so in
> > +	 * theory we can't rely on task_lock() if we want to dereference it.
> > +	 *
> > +	 * And in this case we can't trust the real_parent->mm == tsk->mm
> > +	 * check, it can be false negative. But we do not care, if init or
> > +	 * another oom-unkillable task does this it should blame itself.
> > +	 */
> > +	rcu_read_lock();
> > +	ret = tsk->vfork_done && tsk->real_parent->mm == tsk->mm;
> > +	rcu_read_unlock();
> > +
> > +	return ret;
> > +}
> 
> ACK, but why sched.h ? It has a single caller in oom_kill.c.

It felt like generally reusable helper to get subtle details about the
vfork right.
-- 
Michal Hocko
SUSE Labs