Re: [PATCH 1/1] mm, oom_adj: don't loop through tasks in __set_oom_adj when not necessary

[Michal Hocko]

On Thu 20-08-20 13:30:23, Christian Brauner wrote:
> On Thu, Aug 20, 2020 at 01:13:49PM +0200, Michal Hocko wrote:
> > On Thu 20-08-20 12:55:56, Oleg Nesterov wrote:
> > > On 08/19, Suren Baghdasaryan wrote:
> > > >
> > > > Since the combination of CLONE_VM and !CLONE_SIGHAND is rarely
> > > > used the additional mutex lock in that path of the clone() syscall should
> > > > not affect its overall performance. Clearing the MMF_PROC_SHARED flag
> > > > (when the last process sharing the mm exits) is left out of this patch to
> > > > keep it simple and because it is believed that this threading model is
> > > > rare.
> > > 
> > > vfork() ?
> > 
> > Could you be more specific?
> 
> vfork() implies CLONE_VM but !CLONE_THREAD. The way this patch is
> written the mutex lock will be taken every time you do a vfork().

OK, I see. We definietely do not want to impact vfork so we likely have
to check for CLONE_VFORK as well. Ohh, well our clone flags are really
clear as mud.

> (It's honestly also debatable whether it's that rare. For one, userspace
> stuff I maintain uses it too (see [1]).
> [1]: https://github.com/lxc/lxc/blob/9d3b7c97f0443adc9f0b0438437657ab42f5a1c3/src/lxc/start.c#L1676
> )
> 
> > 
> > > > --- a/kernel/fork.c
> > > > +++ b/kernel/fork.c
> > > > @@ -1403,6 +1403,15 @@ static int copy_mm(unsigned long clone_flags, struct task_struct *tsk)
> > > >  	if (clone_flags & CLONE_VM) {
> > > >  		mmget(oldmm);
> > > >  		mm = oldmm;
> > > > +		if (!(clone_flags & CLONE_SIGHAND)) {
> > > 
> > > I agree with Christian, you need CLONE_THREAD
> > 
> > This was my suggestion to Suren, likely because I've misrememberd which
> > clone flag is responsible for the signal delivery. But now, after double
> > checking we do explicitly disallow CLONE_SIGHAND && !CLONE_VM. So
> > CLONE_THREAD is the right thing to check.
> > 
> > > > +			/* We need to synchronize with __set_oom_adj */
> > > > +			mutex_lock(&oom_adj_lock);
> > > > +			set_bit(MMF_PROC_SHARED, &mm->flags);
> > > > +			/* Update the values in case they were changed after copy_signal */
> > > > +			tsk->signal->oom_score_adj = current->signal->oom_score_adj;
> > > > +			tsk->signal->oom_score_adj_min = current->signal->oom_score_adj_min;
> > > > +			mutex_unlock(&oom_adj_lock);
> > > 
> > > I don't understand how this can close the race with __set_oom_adj...
> > > 
> > > What if __set_oom_adj() is called right after mutex_unlock() ? It will see
> > > MMF_PROC_SHARED, but for_each_process() won't find the new child until
> > > copy_process() does list_add_tail_rcu(&p->tasks, &init_task.tasks) ?
> > 
> > Good point. Then we will have to move this thing there.
> 
> I was toying with moving this into sm like:
> 
> static inline copy_oom_score(unsigned long flags, struct task_struct *tsk)
> 
> trying to rely on set_bit() and test_bit() in copy_mm() being atomic and
> then calling it where Oleg said after the point of no return.

No objections.

-- 
Michal Hocko
SUSE Labs