On Thu 15-06-17 14:03:35, Michal Hocko wrote:
> On Thu 15-06-17 20:32:39, Tetsuo Handa wrote:
> > Michal Hocko wrote:
> [...]
> > > An alternative would be to allow reaping and exit_mmap race. The unmap
> > > part should just work I guess. We just have to be careful to not race
> > > with free_pgtables and that shouldn't be too hard to implement (e.g.
> > > (ab)use mmap_sem for write there). I haven't thought that through
> > > completely though so I might miss something of course.
> >
> > I think below one is simpler.
> [...]
> > @@ -556,25 +553,21 @@ static void oom_reap_task(struct task_struct *tsk)
> > struct mm_struct *mm = tsk->signal->oom_mm;
> >
> > /* Retry the down_read_trylock(mmap_sem) a few times */
> > - while (attempts++ < MAX_OOM_REAP_RETRIES && !__oom_reap_task_mm(tsk, mm))
> > + while (__oom_reap_task_mm(tsk, mm), !test_bit(MMF_OOM_SKIP, &mm->flags)
> > + && attempts++ < MAX_OOM_REAP_RETRIES)
> > schedule_timeout_idle(HZ/10);
> >
> > - if (attempts <= MAX_OOM_REAP_RETRIES)
> > - goto done;
> > -
> > -
> > - pr_info("oom_reaper: unable to reap pid:%d (%s)\n",
> > - task_pid_nr(tsk), tsk->comm);
> > - debug_show_all_locks();
> > -
> > -done:
> > - tsk->oom_reaper_list = NULL;
> > -
> > /*
> > * Hide this mm from OOM killer because it has been either reaped or
> > * somebody can't call up_write(mmap_sem).
> > */
> > - set_bit(MMF_OOM_SKIP, &mm->flags);
> > + if (!test_and_set_bit(MMF_OOM_SKIP, &mm->flags)) {
> > + pr_info("oom_reaper: unable to reap pid:%d (%s)\n",
> > + task_pid_nr(tsk), tsk->comm);
> > + debug_show_all_locks();
> > + }
> > +
>
> How does this _solve_ anything? Why would you even retry when you
> _know_ that the reference count dropped to zero. It will never
> increment. So the above is basically just schedule_timeout_idle(HZ/10) *
> MAX_OOM_REAP_RETRIES before we set MMF_OOM_SKIP.
Just to make myself more clear. The above assumes that the victim hasn't
passed exit_mmap and MMF_OOM_SKIP in __mmput. Which is the case we want to
address here.
--
Michal Hocko
SUSE Labs
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