On Fri, May 10, 2024 at 03:26:35PM +0200, Michal Hocko wrote:
> On Wed 08-05-24 20:41:35, Roman Gushchin wrote:
> [...]
> > @@ -1747,106 +1623,14 @@ static bool mem_cgroup_oom(struct mem_cgroup *memcg, gfp_t mask, int order)
> >
> > memcg_memory_event(memcg, MEMCG_OOM);
> >
> > - /*
> > - * We are in the middle of the charge context here, so we
> > - * don't want to block when potentially sitting on a callstack
> > - * that holds all kinds of filesystem and mm locks.
> > - *
> > - * cgroup1 allows disabling the OOM killer and waiting for outside
> > - * handling until the charge can succeed; remember the context and put
> > - * the task to sleep at the end of the page fault when all locks are
> > - * released.
> > - *
> > - * On the other hand, in-kernel OOM killer allows for an async victim
> > - * memory reclaim (oom_reaper) and that means that we are not solely
> > - * relying on the oom victim to make a forward progress and we can
> > - * invoke the oom killer here.
> > - *
> > - * Please note that mem_cgroup_out_of_memory might fail to find a
> > - * victim and then we have to bail out from the charge path.
> > - */
> > - if (READ_ONCE(memcg->oom_kill_disable)) {
> > - if (current->in_user_fault) {
> > - css_get(&memcg->css);
> > - current->memcg_in_oom = memcg;
> > - current->memcg_oom_gfp_mask = mask;
> > - current->memcg_oom_order = order;
> > - }
> > + if (!mem_cgroup_v1_oom_prepare(memcg, mask, order, &locked))
> > return false;
> > - }
> > -
> > - mem_cgroup_mark_under_oom(memcg);
> > -
> > - locked = mem_cgroup_oom_trylock(memcg);
>
> This really confused me because this looks like the oom locking is
> removed for v2 but this is not the case because
> mem_cgroup_v1_oom_prepare is not really v1 only code - in other words
> this is not going to be just return false for CONFIG_MEMCG_V1=n.
>
> It makes sense to move the userspace oom handling out to the v1 file. I
> would keep mem_cgroup_mark_under_oom here.
Hm, I don't see any usages of memcg->under_oom outside of v1-specific
context. I probably miss something, can you, please, clarify?
> I am not sure about the oom
> locking thing because I think we can make it v1 only. For v2 I guess we
> can go without this locking as the oom path is already locked and it
> implements overkilling prevention (oom_evaluate_task) as it walks all
> processes in the oom hierarchy.
It's a good point and not obvious if we really need anything of this on v2.
I guess no, but will think a bit more.
Thank you!
