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

Vasily Averin
2021-10-19T10:30:06Z

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

It can be set via oom_score_adj_write().

Thank you,
Vasily Averin