Re: RFC [PATCH v4 2/7] Enable balloon drivers to report inflated memory

[David Hildenbrand]

> > > > Sounds to me that all you want is some notifier to be called from
> > > > adjust_managed_page_count(). What am I missing?
> > >
> > > Notifier will act as an accumulator to report size of change and it will make things easier for the drivers and users wrt locking.
> > > Notifier is similar to the memory hotplug notifier.
> >
> > Overall, I am not convinced that there is any value of separating the value
> > and the notifier. You can batch both or not batch both. In addition, as I
> > mentioned, having two values seems racy.
>
> I have identified two users so far above - may be more to come.
> One type needs the value to adjust. Also having the value is necessary
> to report it to users and oom. There are options with callbacks and so
> on but it will complicate things with no real gain. You are right about
> the atomicity but i guess if that's a problem for some user it could
> find a way to ensure it. i am yet to find such place.

I haven't followed the whole discussion, but I just wanted to raise that 
having a generic mechanism to notify on such changes could be valuable.

For example, virtio-mem also uses adjust_managed_page_count() and might 
sometimes not trigger memory hotplug notifiers when adding more memory 
(essentially, when it fake-adds memory part of an already added Linux 
memory block).

What might make sense is schedule some kind of deferred notification on 
adjust_managed_page_count() changes. This way, we could notify without 
caring about locking and would naturally batch notifications.

adjust_managed_page_count() users would not require changes.

--
Thanks,

David / dhildenb