David Rientjes wrote:
> On Thu, 14 Jan 2016, Tetsuo Handa wrote:
>
> > > > > > @@ -171,7 +195,7 @@ unsigned long oom_badness(struct task_struct *p, struct mem_cgroup *memcg,
> > > > > > if (oom_unkillable_task(p, memcg, nodemask))
> > > > > > return 0;
> > > > > >
> > > > > > - p = find_lock_task_mm(p);
> > > > > > + p = find_lock_non_victim_task_mm(p);
> > > > > > if (!p)
> > > > > > return 0;
> > > > > >
> > > > >
> > > > > I understand how this may make your test case pass, but I simply don't
> > > > > understand how this could possibly be the correct thing to do. This would
> > > > > cause oom_badness() to return 0 for any process where a thread has
> > > > > TIF_MEMDIE set. If the oom killer is called from the page allocator,
> > > > > kills a thread, and it is recalled before that thread may exit, then this
> > > > > will panic the system if there are no other eligible processes to kill.
> > > > >
> > > > Why? oom_badness() is called after oom_scan_process_thread() returned OOM_SCAN_OK.
> > > > oom_scan_process_thread() returns OOM_SCAN_ABORT if a thread has TIF_MEMDIE set.
> > > >
> > >
> > > oom_scan_process_thread() checks for TIF_MEMDIE on p, not on p's threads.
> > > If one of p's threads has TIF_MEMDIE set and p does not, we actually want
> > > to set TIF_MEMDIE for p. That's the current behavior since it will lead
> > > to p->mm memory freeing. Your patch is excluding such processes entirely
> > > and selecting another process to kill unnecessarily.
> > >
> >
> > I think p's threads are checked by oom_scan_process_thread() for TIF_MEMDIE
> > even if p does not have TIF_MEMDIE. What am I misunderstanding about what
> > for_each_process_thread(g, p) is doing?
> >
> > #define for_each_process_thread(p, t) for_each_process(p) for_each_thread(p, t)
> >
> > select_bad_process() {
> > for_each_process_thread(g, p) {
> > oom_scan_process_thread(oc, p, totalpages));
> > oom_badness(p);
> > }
> > }
> >
>
> Yes, select_bad_process() iterates over threads, that is obvious. The
> point is that today it can select a thread independent of whether any of
> its other threads have TIF_MEMDIE set, which is the desired behavior per
> the above. With your change, that is no longer possible because we
> disregard _all_ threads if one of them has TIF_MEMDIE set.
>
I still cannot understand. Today select_bad_process() can select a thread
independent of whether any of its other threads have TIF_MEMDIE set. But
select_bad_process() after all ignores that thread selected by oom_badness()
logic and aborts the iterate loop as soon as oom_scan_process_thread() finds
a TIF_MEMDIE thread from all threads. Changing oom_badness() logic to skip
processes with TIF_MEMDIE threads does not change the task select_bad_process()
logic will finally return.
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