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

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Date:   Tue, 11 Oct 2022 11:23:05 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Alexander Atanasov <alexander.atanasov@...tuozzo.com>,
        Nadav Amit <nadav.amit@...il.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>, kernel@...nvz.org,
        Linux Virtualization <virtualization@...ts.linux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>
Subject: Re: RFC [PATCH v4 2/7] Enable balloon drivers to report inflated
 memory

>>>> 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