Re: [RFC PATCH 2/2] memcg: do not report racy no-eligible OOM tasks

From: Tetsuo Handa
Date: Fri Oct 26 2018 - 21:18:06 EST

On 2018/10/27 4:25, Michal Hocko wrote:
>> out_of_memory() bails on task_will_free_mem(current), which
>> specifically *excludes* already reaped tasks. Why are we then adding a
>> separate check before that to bail on already reaped victims?
> 696453e66630a has introduced the bail out.
>> Do we want to bail if current is a reaped victim or not?
>> I don't see how we could skip it safely in general: the current task
>> might have been killed and reaped and gotten access to the memory
>> reserve and still fail to allocate on its way out. It needs to kill
>> the next task if there is one, or warn if there isn't another
>> one. Because we're genuinely oom without reclaimable tasks.
> Yes, this would be the case for the global case which is a real OOM
> situation. Memcg oom is somehow more relaxed because the oom is local.

We can handle possibility of genuinely OOM without reclaimable tasks.
Only __GFP_NOFAIL OOM has to select next OOM victim. There is no need to
select next OOM victim unless __GFP_NOFAIL. Commit 696453e66630ad45
("mm, oom: task_will_free_mem should skip oom_reaped tasks") was too simple.

On 2018/10/27 4:33, Michal Hocko wrote:
> On Fri 26-10-18 21:25:51, Michal Hocko wrote:
>> On Fri 26-10-18 10:25:31, Johannes Weiner wrote:
> [...]
>>> There is of course the scenario brought forward in this thread, where
>>> multiple threads of a process race and the second one enters oom even
>>> though it doesn't need to anymore. What the global case does to catch
>>> this is to grab the oom lock and do one last alloc attempt. Should
>>> memcg lock the oom_lock and try one more time to charge the memcg?
>> That would be another option. I agree that making it more towards the
>> global case makes it more attractive. My tsk_is_oom_victim is more
>> towards "plug this particular case".
> Nevertheless let me emphasise that tsk_is_oom_victim will close the race
> completely, while mem_cgroup_margin will always be racy. So the question
> is whether we want to close the race because it is just too easy for
> userspace to hit it or keep the global and memcg oom handling as close
> as possible.

Yes, adding tsk_is_oom_victim(current) before calling out_of_memory() from
both global OOM and memcg OOM paths can close the race completely. (But
note that tsk_is_oom_victim(current) for global OOM path needs to check for
__GFP_NOFAIL in order to handle genuinely OOM case.)