[Devel] Re: [PATCH v5 06/14] memcg: kmem controller infrastructure

[David Rientjes]

On Thu, 18 Oct 2012, Glauber Costa wrote:

> >> @@ -2630,6 +2634,171 @@ static void __mem_cgroup_commit_charge(struct mem_cgroup *memcg,
> >>  	memcg_check_events(memcg, page);
> >>  }
> >>  
> >> +#ifdef CONFIG_MEMCG_KMEM
> >> +static inline bool memcg_can_account_kmem(struct mem_cgroup *memcg)
> >> +{
> >> +	return !mem_cgroup_disabled() && !mem_cgroup_is_root(memcg) &&
> >> +		(memcg->kmem_accounted & KMEM_ACCOUNTED_MASK);
> >> +}
> >> +
> >> +static int memcg_charge_kmem(struct mem_cgroup *memcg, gfp_t gfp, u64 size)
> >> +{
> >> +	struct res_counter *fail_res;
> >> +	struct mem_cgroup *_memcg;
> >> +	int ret = 0;
> >> +	bool may_oom;
> >> +
> >> +	ret = res_counter_charge(&memcg->kmem, size, &fail_res);
> >> +	if (ret)
> >> +		return ret;
> >> +
> >> +	/*
> >> +	 * Conditions under which we can wait for the oom_killer.
> >> +	 * We have to be able to wait, but also, if we can't retry,
> >> +	 * we obviously shouldn't go mess with oom.
> >> +	 */
> >> +	may_oom = (gfp & __GFP_WAIT) && !(gfp & __GFP_NORETRY);
> > 
> > What about gfp & __GFP_FS?
> >
> 
> Do you intend to prevent or allow OOM under that flag? I personally
> think that anything that accepts to be OOM-killed should have GFP_WAIT
> set, so that ought to be enough.
> 

The oom killer in the page allocator cannot trigger without __GFP_FS 
because direct reclaim has little chance of being very successful and 
thus we end up needlessly killing processes, and that tends to happen 
quite a bit if we dont check for it.  Seems like this would also happen 
with memcg if mem_cgroup_reclaim() has a large probability of failing?