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

[Michal Hocko <mhocko@kernel.org>]

On Tue 24-10-17 12:06:37, Johannes Weiner wrote:
> On Fri, Oct 13, 2017 at 05:24:21PM +0200, Michal Hocko wrote:
> > Well, it actually occured to me that this would trigger the global oom
> > killer in case no memcg specific victim can be found which is definitely
> > not something we would like to do. This should work better. I am not
> > sure we can trigger this corner case but we should cover it and it
> > actually doesn't make the code much worse.
> > ---
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index d5f3a62887cf..7b370f070b82 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -1528,26 +1528,40 @@ static void memcg_oom_recover(struct mem_cgroup *memcg)
> >  
> >  static void mem_cgroup_oom(struct mem_cgroup *memcg, gfp_t mask, int order)
> >  {
> > -	if (!current->memcg_may_oom)
> > -		return;
> >  	/*
> >  	 * We are in the middle of the charge context here, so we
> >  	 * don't want to block when potentially sitting on a callstack
> >  	 * that holds all kinds of filesystem and mm locks.
> >  	 *
> > -	 * Also, the caller may handle a failed allocation gracefully
> > -	 * (like optional page cache readahead) and so an OOM killer
> > -	 * invocation might not even be necessary.
> > +	 * cgroup v1 allowes sync users space handling so we cannot afford
> > +	 * to get stuck here for that configuration. That's why we don't do
> > +	 * anything here except remember the OOM context and then deal with
> > +	 * it at the end of the page fault when the stack is unwound, the
> > +	 * locks are released, and when we know whether the fault was overall
> > +	 * successful.
> 
> How about
> 
> "cgroup1 allows disabling the OOM killer and waiting for outside
> handling until the charge can succeed; remember the context and put
> the task to sleep at the end of the page fault when all locks are
> released."

OK

> and then follow it directly with the branch that handles this:
> 
> 	if (memcg->oom_kill_disable) {
> 		css_get(&memcg->css);
> 		current->memcg_in_oom = memcg;
> 		...
> 		return false;
> 	}
> 
> 	return mem_cgroup_out_of_memory(memcg, mask, order);	
> 
> > +	 * On the other hand, in-kernel OOM killer allows for an async victim
> > +	 * memory reclaim (oom_reaper) and that means that we are not solely
> > +	 * relying on the oom victim to make a forward progress so we can stay
> > +	 * in the the try_charge context and keep retrying as long as there
> > +	 * are oom victims to select.
> 
> I would put that part into try_charge, where that decision is made.

OK

> >  	 *
> > -	 * That's why we don't do anything here except remember the
> > -	 * OOM context and then deal with it at the end of the page
> > -	 * fault when the stack is unwound, the locks are released,
> > -	 * and when we know whether the fault was overall successful.
> > +	 * Please note that mem_cgroup_oom_synchronize might fail to find a
> > +	 * victim and then we have rely on mem_cgroup_oom_synchronize otherwise
> > +	 * we would fall back to the global oom killer in pagefault_out_of_memory
> 
> Ah, that's why... Ugh, that's really duct-tapey.

As you know, I really hate the #PF OOM path. We should get rid of it.
 
> >  	 */
> > +	if (!memcg->oom_kill_disable &&
> > +			mem_cgroup_out_of_memory(memcg, mask, order))
> > +		return true;
> > +
> > +	if (!current->memcg_may_oom)
> > +		return false;
> >  	css_get(&memcg->css);
> >  	current->memcg_in_oom = memcg;
> >  	current->memcg_oom_gfp_mask = mask;
> >  	current->memcg_oom_order = order;
> > +
> > +	return false;
> >  }
> >  
> >  /**
> > @@ -2007,8 +2021,11 @@ static int try_charge(struct mem_cgroup *memcg, gfp_t gfp_mask,
> >  
> >  	mem_cgroup_event(mem_over_limit, MEMCG_OOM);
> >  
> > -	mem_cgroup_oom(mem_over_limit, gfp_mask,
> > -		       get_order(nr_pages * PAGE_SIZE));
> > +	if (mem_cgroup_oom(mem_over_limit, gfp_mask,
> > +		       get_order(nr_pages * PAGE_SIZE))) {
> > +		nr_retries = MEM_CGROUP_RECLAIM_RETRIES;
> > +		goto retry;
> > +	}
> 
> As per the previous email, this has to goto force, otherwise we return
> -ENOMEM from syscalls once in a blue moon, which makes verification an
> absolute nightmare. The behavior should be reliable, without weird p99
> corner cases.
>
> I think what we should be doing here is: if a charge fails, set up an
> oom context and force the charge; add mem_cgroup_oom_synchronize() to
> the end of syscalls and kernel-context faults.

What would prevent a runaway in case the only process in the memcg is
oom unkillable then?
-- 
Michal Hocko
SUSE Labs