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

[Michal Hocko <mhocko@kernel.org>]

On Thu 12-10-17 15:03:12, Johannes Weiner wrote:
> On Tue, Oct 10, 2017 at 04:24:34PM +0200, Michal Hocko wrote:
[...]
> > 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.

true, try_charge would have to return with failure when out_of_memory
returns with false of course.

> > > 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.

Sorry, I was just not clear. What I meant to say, would couldn't make v1
wait inside the try_charge path because async oom killing wouldn't help
for the oom disabled case (aka user space oom handling).

> > > > > > 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.

OK, then we are on the same page now. Your initial wording didn't
mention async OOM killer. This makes more sense. Although I would argue
that we can retry the charge as long as out_of_memory finds a victim.
This would return ENOMEM to the pathological cases where no victims
could be found.

-- 
Michal Hocko
SUSE Labs