Re: [PATCH memcg 0/1] false global OOM triggered by memcg-limited task

Michal Hocko
2021-10-19T12:04:22Z

On Tue 19-10-21 13:54:42, Michal Hocko wrote:
> On Tue 19-10-21 13:30:06, Vasily Averin wrote:
> > On 19.10.2021 11:49, Michal Hocko wrote:
> > > On Tue 19-10-21 09:30:18, Vasily Averin wrote:
> > > [...]
> > >> With my patch ("memcg: prohibit unconditional exceeding the limit of dying tasks") try_charge_memcg() can fail:
> > >> a) due to fatal signal
> > >> b) when mem_cgroup_oom -> mem_cgroup_out_of_memory -> out_of_memory() returns false (when select_bad_process() found nothing)
> > >>
> > >> To handle a) we can follow to your suggestion and skip excution of out_of_memory() in pagefault_out_of memory()
> > >> To handle b) we can go to retry: if mem_cgroup_oom() return OOM_FAILED.
> >
> > > How is b) possible without current being killed? Do we allow remote
> > > charging?
> >
> > out_of_memory for memcg_oom
> > select_bad_process
> > mem_cgroup_scan_tasks
> > oom_evaluate_task
> > oom_badness
> >
> > /*
> > * Do not even consider tasks which are explicitly marked oom
> > * unkillable or have been already oom reaped or the are in
> > * the middle of vfork
> > */
> > adj = (long)p->signal->oom_score_adj;
> > if (adj == OOM_SCORE_ADJ_MIN ||
> > test_bit(MMF_OOM_SKIP, &p->mm->flags) ||
> > in_vfork(p)) {
> > task_unlock(p);
> > return LONG_MIN;
> > }
> >
> > This time we handle userspace page fault, so we cannot be kenrel thread,
> > and cannot be in_vfork().
> > However task can be marked as oom unkillable,
> > i.e. have p->signal->oom_score_adj == OOM_SCORE_ADJ_MIN
>
> You are right. I am not sure there is a way out of this though. The task
> can only retry for ever in this case. There is nothing actionable here.
> We cannot kill the task and there is no other way to release the memory.

Btw. don't we force the charge in that case?
--
Michal Hocko
SUSE Labs