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Date:   Tue, 11 Oct 2022 11:23:05 +0200
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Subject: Re: RFC [PATCH v4 2/7] Enable balloon drivers to report inflated
 memory
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To:     Alexander Atanasov <alexander.atanasov@virtuozzo.com>,
        Nadav Amit <nadav.amit@gmail.com>
Cc:     "Michael S. Tsirkin" <mst@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>, kernel@openvz.org,
        Linux Virtualization <virtualization@lists.linux-foundation.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux MM <linux-mm@kvack.org>
References: <20221005090158.2801592-1-alexander.atanasov@virtuozzo.com>
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 <7ba328e5-3bc8-cb22-f00c-eddb8aea9a06@virtuozzo.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
In-Reply-To: <7ba328e5-3bc8-cb22-f00c-eddb8aea9a06@virtuozzo.com>
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>>>> 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