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

Michal Hocko
2020-05-15T08:34:58Z

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. If this is reduced only to active_memcg
then we are only getting a partial picture.

>
> /* Allow remote memcg charging in kthread contexts. */
> if ((!current->mm || (current->flags & PF_KTHREAD)) && !current->active_memcg)
> return true;
> return false;
> }

--
Michal Hocko
SUSE Labs