Re: Balloon pressuring page cache

[David Hildenbrand <david@redhat.com>]

[...]

>>
>> Issue 2: When called via the shrinker, (but also to fix Issue 1), it could be
>> that we do have VIRTIO_BALLOON_F_MUST_TELL_HOST. I assume this means
>> (-ENOCLUE) that we have to wait until the hypervisor notifies us via the STOP? Or
>> for which event do we have to wait? Because there is no way to *tell host* here
>> that we want to reuse a page. The hypervisor will *tell us* when we can reuse pages.
>> For the shrinker it is simple: Don't use the shrinker with
>> VIRTIO_BALLOON_F_MUST_TELL_HOST :) . But to fix Issue 1, we *would* have to wait
>> until we get a STOP signal. That is not really possible because it might
>> take an infinite amount of time.
>>
>> Michael, any clue on which event we have to wait with
>> VIRTIO_BALLOON_F_MUST_TELL_HOST? IMHO, I don't think
>> VIRTIO_BALLOON_F_MUST_TELL_HOST applies to VIRTIO_BALLOON_F_FREE_PAGE_HINT and
>> we'd better document that. It introduces complexity with no clear benefit.
> 
> I meant that we must wait for host to see the hint. Signalled via using
> the buffer.  But maybe that's too far in the meaning from
> VIRTIO_BALLOON_F_MUST_TELL_HOST and we need a separate new flag for

Yes, that's what I think.

> that. Then current code won't be broken (yay!) but we need to
> document another flag that's pretty similar.

I mean, do we need a flag at all as long as there is no user?
Introducing a flag and documenting it if nobody uses it does not sound
like a work I will enjoy :)

We can simply document "VIRTIO_BALLOON_F_MUST_TELL_HOST does not apply
to FREE_PAGE_HINTING" and "with FREE_PAGE_HINTING, the guest can reuse
pages any time, without waiting for a response/ack from the hypervisor".

Thoughts?

-- 
Thanks,

David / dhildenb