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SubjectRe: [PATCH memcg] memcg: prohibit unconditional exceeding the limit of dying tasks
On Mon 13-09-21 10:51:37, Vasily Averin wrote:
> On 9/10/21 3:39 PM, Vasily Averin wrote:
> > The kernel currently allows dying tasks to exceed the memcg limits.
> > The allocation is expected to be the last one and the occupied memory
> > will be freed soon.
> > This is not always true because it can be part of the huge vmalloc
> > allocation. Allowed once, they will repeat over and over again.
>
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 389b5766e74f..67195fcfbddf 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -2622,15 +2625,6 @@ static int try_charge_memcg(struct mem_cgroup *memcg, gfp_t gfp_mask,
> > if (gfp_mask & __GFP_ATOMIC)
> > goto force;
> >
> > - /*
> > - * Unlike in global OOM situations, memcg is not in a physical
> > - * memory shortage. Allow dying and OOM-killed tasks to
> > - * bypass the last charges so that they can exit quickly and
> > - * free their memory.
> > - */
> > - if (unlikely(should_force_charge()))
> > - goto force;
> > -
>
> Should we keep current behaviour for (current->flags & PF_EXITING) case perhaps?

Why?

> It is set inside do_exit only and (I hope) cannot trigger huge vmalloc allocations.

Allocations in this code path should be rare but it is not like they are
non-existent. This is rather hard to review area spread at many places
so if we are deciding to make the existing model simpler (no bypassing)
then I would rather have no exceptions unless they are reaaly necessary
and document them if they are.
--
Michal Hocko
SUSE Labs

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