Re: [PATCH] mm,oom: kill duplicated oom_unkillable_task() checks.

[Michal Hocko <mhocko@kernel.org>]

On Sat 20-02-16 01:01:36, Tetsuo Handa wrote:
> Michal Hocko wrote:
> > On Fri 19-02-16 23:33:31, Tetsuo Handa wrote:
> > > Currently, oom_unkillable_task() is called for twice for each thread,
> > > once at oom_scan_process_thread() and again at oom_badness().
> > > 
> > > The reason oom_scan_process_thread() needs to call oom_unkillable_task()
> > > is to skip TIF_MEMDIE test and oom_task_origin() test if that thread is
> > > not OOM-killable.
> > > 
> > > But there is a problem with this ordering, for oom_task_origin() == true
> > > will unconditionally select that thread regardless of oom_score_adj.
> > > When we merge the OOM reaper, the OOM reaper will mark already reaped
> > > process as OOM-unkillable by updating oom_score_adj. In order to avoid
> > > falling into infinite loop, oom_score_adj needs to be checked before
> > > doing oom_task_origin() test.
> > 
> > What would be the infinite loop?
> 
> Sequence until we merge the OOM reaper:
> 
>  (1) select_bad_process() returns p due to oom_task_origin(p) == true.
>  (2) oom_kill_process() sends SIGKILL to p and sets TIF_MEMDIE on p.
>  (3) p gets stuck at down_read(&mm->mmap_sem) in exit_mm().
>  (4) The OOM killer will ignore TIF_MEMDIE on p after some timeout expires.
>  (5) select_bad_process() returns p again due to oom_task_origin(p) == true &&
>      p->mm != NULL.

And one more thing. The task will stop being oom_task_origin right
after it detects signal pending and retuns from try_to_unuse resp.
unmerge_and_remove_all_rmap_items.
-- 
Michal Hocko
SUSE Labs

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>