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Re: [Xen-devel] [PATCH] mm, oom: distinguish blockable mode for mmu notifiers

On Mon, Jul 16, 2018 at 04:12:49PM -0700, Andrew Morton wrote:
> On Mon, 16 Jul 2018 13:50:58 +0200 Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> > From: Michal Hocko <mhocko@xxxxxxxx>
> >
> > There are several blockable mmu notifiers which might sleep in
> > mmu_notifier_invalidate_range_start and that is a problem for the
> > oom_reaper because it needs to guarantee a forward progress so it cannot
> > depend on any sleepable locks.
> >
> > Currently we simply back off and mark an oom victim with blockable mmu
> > notifiers as done after a short sleep. That can result in selecting a
> > new oom victim prematurely because the previous one still hasn't torn
> > its memory down yet.
> >
> > We can do much better though. Even if mmu notifiers use sleepable locks
> > there is no reason to automatically assume those locks are held.
> > Moreover majority of notifiers only care about a portion of the address
> > space and there is absolutely zero reason to fail when we are unmapping an
> > unrelated range. Many notifiers do really block and wait for HW which is
> > harder to handle and we have to bail out though.
> >
> > This patch handles the low hanging fruid. 
> > __mmu_notifier_invalidate_range_start
> > gets a blockable flag and callbacks are not allowed to sleep if the
> > flag is set to false. This is achieved by using trylock instead of the
> > sleepable lock for most callbacks and continue as long as we do not
> > block down the call chain.
> I assume device driver developers are wondering "what does this mean
> for me".  As I understand it, the only time they will see
> blockable==false is when their driver is being called in response to an
> out-of-memory condition, yes?  So it is a very rare thing.

I can't say for everyone, but at least for me (mlx5), it is not rare event.
I'm seeing OOM very often while I'm running my tests in low memory VMs.


> Any suggestions regarding how the driver developers can test this code
> path?  I don't think we presently have a way to fake an oom-killing
> event?  Perhaps we should add such a thing, given the problems we're
> having with that feature.

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