Re: [PATCH] mm, oom: allow oom reaper to race with exit_mmap

[Andrea Arcangeli <aarcange@redhat.com>]

On Wed, Jul 26, 2017 at 07:45:33AM +0200, Michal Hocko wrote:
> Yes, exit_aio is the only blocking call I know of currently. But I would
> like this to be as robust as possible and so I do not want to rely on
> the current implementation. This can change in future and I can
> guarantee that nobody will think about the oom path when adding
> something to the final __mmput path.

I think ksm_exit may block too waiting for allocations, the generic
idea is those calls before exit_mmap can cause a problem yes.

> > exit_mmap would have no issue, if there was enough time in the
> > lifetime CPU to allocate the memory, sure the memory will also be
> > freed in finite amount of time by exit_mmap.
> 
> I am not sure I understand. Say that any call prior to unmap_vmas blocks
> on a lock which is held by another call path which cannot proceed with
> the allocation...

What I meant was, if three was no prior call to exit_mmap->unmap_vmas.

> I really do not want to rely on any timing. This just too fragile. Once
> we have killed a task then we shouldn't pick another victim until it
> passed exit_mmap or the oom_reaper did its job. Otherwise we just risk
> false positives while we have already disrupted the workload.

On smaller systems lack or parallelism in OOM killing surely isn't a
problem.

> This will work more or less the same to what we have currently.
> 
> [victim]		[oom reaper]				[oom killer]
> do_exit			__oom_reap_task_mm
>   mmput
>     __mmput
> 			  mmget_not_zero
> 			    test_and_set_bit(MMF_OOM_SKIP)
> 			    					oom_evaluate_task
> 								   # select next victim 
> 			  # reap the mm
>       unmap_vmas
>
> so we can select a next victim while the current one is still not
> completely torn down.

How does oom_evaluate_task possibly run at the same time of
test_and_set_bit in __oom_reap_task_mm considering both are running
under the oom_lock? It's hard to see how what you describe above could
materialize as second and third column cannot run in parallel because
of the oom_lock.

I don't think there was any issue, but then you pointed out the
locking on signal->oom_mm that is protected by the task_lock vs
current->mm NULL check, so I can replace in my patch the
test_and_set_bit with set_bit on one side and the oom_mm task_lock
protected locking on the other side. This way I can put back a set_bit
in the __mmput fast path (instead of test_and_set_bit) and it's even
more efficient. With such a change, I'll also stop depending on the
oom_lock to prevent second and third column to run in parallel.

I still didn't remove the oom_lock outright that seems orthogonal
change unrelated to this issue but now you could remove it as far as
the above is concerned.

> I hope 3f70dc38cec2 ("mm: make sure that kthreads will not refault oom
> reaped memory") will clarify this code. If not please start a new thread
> so that we do not conflate different things together.

I'll look into that, thanks.
Andrea

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