Re: [PATCH v2] memcg: Fix memcg_kmem_bypass() for remote memcg charging

Roman Gushchin
2020-05-15T17:31:10Z

On Fri, May 15, 2020 at 09:22:25AM -0700, Shakeel Butt wrote:
> On Fri, May 15, 2020 at 1:35 AM Michal Hocko <mhocko-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org> wrote:
> >
> > On Fri 15-05-20 16:20:04, Li Zefan wrote:
> > > On 2020/5/15 14:56, Michal Hocko wrote:
> > > > On Thu 14-05-20 15:52:59, Roman Gushchin wrote:
> > [...]
> > > >>> I thought the user should ensure not do this, but now I think it makes sense to just bypass
> > > >>> the interrupt case.
> > > >>
> > > >> I think now it's mostly a legacy of the opt-out kernel memory accounting.
> > > >> Actually we can relax this requirement by forcibly overcommit the memory cgroup
> > > >> if the allocation is happening from the irq context, and punish it afterwards.
> > > >> Idk how much we wanna this, hopefully nobody is allocating large non-temporarily
> > > >> objects from an irq.
> > > >
> > > > I do not think we want to pretend that remote charging from the IRQ
> > > > context is supported. Why don't we simply WARN_ON(in_interrupt()) there?
> > > >
> > >
> > > How about:
> > >
> > > static inline bool memcg_kmem_bypass(void)
> > > {
> > > if (in_interrupt()) {
> > > WARN_ON(current->active_memcg);
> > > return true;
> > > }
> >
> > Why not simply
> >
> > if (WARN_ON_ONCE(in_interrupt())
> > return true;
> >
> > the idea is that we want to catch any __GFP_ACCOUNT user from IRQ
> > context because this just doesn't work and we do not plan to support it
> > for now and foreseeable future.

Actually, why not?
It should be fairly simple, especially after the rework of the slab controller.

> If this is reduced only to active_memcg
> > then we are only getting a partial picture.
> >
>
> There are SLAB_ACCOUNT kmem caches which do allocations in IRQ context
> (see sk_prot_alloc()), so, either we make charging work in IRQ or no
> warnings at all.

I agree. Actually, there is nothing wrong to warn about, it's just a limitation
of the current accounting implementation.

Thanks!