Re: [patch] mm: memcg: do not declare OOM from __GFP_NOFAIL allocations

Johannes Weiner
2013-11-27T16:39:16Z

On Tue, Nov 26, 2013 at 07:33:12PM -0800, David Rientjes wrote:
> On Tue, 26 Nov 2013, David Rientjes wrote:
>
> > On Fri, 22 Nov 2013, Johannes Weiner wrote:
> >
> > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > > index 13b9d0f..cc4f9cb 100644
> > > --- a/mm/memcontrol.c
> > > +++ b/mm/memcontrol.c
> > > @@ -2677,6 +2677,9 @@ static int __mem_cgroup_try_charge(struct mm_struct *mm,
> > > if (unlikely(task_in_memcg_oom(current)))
> > > goto bypass;
> > >
> > > + if (gfp_mask & __GFP_NOFAIL)
> > > + oom = false;
> > > +
> > > /*
> > > * We always charge the cgroup the mm_struct belongs to.
> > > * The mm_struct's mem_cgroup changes on task migration if the
> >
> > Sorry, I don't understand this. What happens in the following scenario:
> >
> > - memory.usage_in_bytes == memory.limit_in_bytes,
> >
> > - memcg reclaim fails to reclaim memory, and
> >
> > - all processes (perhaps only one) attached to the memcg are doing one of
> > the over dozen __GFP_NOFAIL allocations in the kernel?
> >
> > How do we make forward progress if you cannot oom kill something?

Bypass the limit.

> Ah, this is because of 3168ecbe1c04 ("mm: memcg: use proper memcg in limit
> bypass") which just bypasses all of these allocations and charges the root
> memcg. So if allocations want to bypass memcg isolation they just have to
> be __GFP_NOFAIL?

I don't think we have another option.