Re: [PATCH] fs, mm: account filp and names caches to kmemcg

[Johannes Weiner <hannes@cmpxchg.org>]

On Tue, Oct 10, 2017 at 04:24:34PM +0200, Michal Hocko wrote:
> On Tue 10-10-17 10:17:33, Johannes Weiner wrote:
> > On Tue, Oct 10, 2017 at 11:14:30AM +0200, Michal Hocko wrote:
> > > On Mon 09-10-17 16:26:13, Johannes Weiner wrote:
> > > > It's consistent in the sense that only page faults enable the memcg
> > > > OOM killer. It's not the type of memory that decides, it's whether the
> > > > allocation context has a channel to communicate an error to userspace.
> > > > 
> > > > Whether userspace is able to handle -ENOMEM from syscalls was a voiced
> > > > concern at the time this patch was merged, although there haven't been
> > > > any reports so far,
> > > 
> > > Well, I remember reports about MAP_POPULATE breaking or at least having
> > > an unexpected behavior.
> > 
> > Hm, that slipped past me. Did we do something about these? Or did they
> > fix userspace?
> 
> Well it was mostly LTP complaining. I have tried to fix that but Linus
> was against so we just documented that this is possible and MAP_POPULATE
> is not a guarantee.

Okay, makes sense. I wouldn't really count that as a regression.

> > > Well, we should be able to do that with the oom_reaper. At least for v2
> > > which doesn't have synchronous userspace oom killing.
> > 
> > I don't see how the OOM reaper is a guarantee as long as we have this:
> > 
> > 	if (!down_read_trylock(&mm->mmap_sem)) {
> > 		ret = false;
> > 		trace_skip_task_reaping(tsk->pid);
> > 		goto unlock_oom;
> > 	}
> 
> And we will simply mark the victim MMF_OOM_SKIP and hide it from the oom
> killer if we fail to get the mmap_sem after several attempts. This will
> allow to find a new victim. So we shouldn't deadlock.

It's less likely to deadlock, but not exactly deadlock-free. There
might not BE any other mm's holding significant amounts of memory.

> > What do you mean by 'v2'?
> 
> cgroup v2 because the legacy memcg allowed sync wait for the oom killer
> and that would be a bigger problem from a deep callchains for obevious
> reasons.

Actually, the async oom killing code isn't dependent on cgroup
version. cgroup1 doesn't wait inside the charge context, either.

> > > > > c) Overcharge kmem to oom memcg and queue an async memcg limit checker,
> > > > >    which will oom kill if needed.
> > > > 
> > > > This makes the most sense to me. Architecturally, I imagine this would
> > > > look like b), with an OOM handler at the point of return to userspace,
> > > > except that we'd overcharge instead of retrying the syscall.
> > > 
> > > I do not think we should break the hard limit semantic if possible. We
> > > can currently allow that for allocations which are very short term (oom
> > > victims) or too important to fail but allowing that for kmem charges in
> > > general sounds like too easy to runaway.
> > 
> > I'm not sure there is a convenient way out of this.
> > 
> > If we want to respect the hard limit AND guarantee allocation success,
> > the OOM killer has to free memory reliably - which it doesn't. But if
> > it did, we could also break the limit temporarily and have the OOM
> > killer replenish the pool before that userspace app can continue. The
> > allocation wouldn't have to be short-lived, since memory is fungible.
> 
> If we can guarantee the oom killer is started then we can allow temporal
> access to reserves which is already implemented even for memcg. The
> thing is we do not invoke the oom killer...

You lost me here. Which reserves?

All I'm saying is that, when the syscall-context fails to charge, we
should do mem_cgroup_oom() to set up the async OOM killer, let the
charge succeed over the hard limit - since the OOM killer will most
likely get us back below the limit - then mem_cgroup_oom_synchronize()
before the syscall returns to userspace.

That would avoid returning -ENOMEM from syscalls without the risk of
the hard limit deadlocking - at the risk of sometimes overrunning the
hard limit, but that seems like the least problematic behavior out of
the three.

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