Re: Balloon pressuring page cache

[David Hildenbrand <david@redhat.com>]

On 04.02.20 09:40, Michael S. Tsirkin wrote:
> On Tue, Feb 04, 2020 at 09:35:21AM +0100, David Hildenbrand wrote:
>>>>> I would say reverting probably makes sense. I'm not sure there is much
>>>>> value to having a shrinker running deflation when you are actively trying
>>>>> to increase the balloon. It would make more sense to wait until you are
>>>>> actually about to start hitting oom.
>>>>
>>>> I think the shrinker makes sense for free page hinting feature
>>>> (everything on free_page_list).
>>>>
>>>> So instead of only reverting, I think we should split it up and always
>>>> register the shrinker for VIRTIO_BALLOON_F_FREE_PAGE_HINT and the OOM
>>>> notifier (as before) for VIRTIO_BALLOON_F_MUST_TELL_HOST.
>>
>> s/VIRTIO_BALLOON_F_MUST_TELL_HOST/VIRTIO_BALLOON_F_DEFLATE_ON_OOM/
>>
>> :)
> 
> Well VIRTIO_BALLOON_F_MUST_TELL_HOST is also broken by shrinker
> with VIRTIO_BALLOON_F_FREE_PAGE_HINT as that code adds buffers
> but does not wait for them to be used even with VIRTIO_BALLOON_F_MUST_TELL_HOST.
> We never noticed because QEMU does not advertize
> VIRTIO_BALLOON_F_MUST_TELL_HOST.

So, I am trying to understand how the code is intended to work, but I
am afraid I am missing something (or to rephrase: I think I found a BUG :) and
there is lack of proper documentation about this feature).

a) We allocate pages and add them to the list as long as we are told to do so.
   We send these pages to the host one by one.
b) We free all pages once we get a STOP signal. Until then, we keep pages allocated.
c) When called via the shrinker, we want to free pages from the list, even
though the hypervisor did not notify us to do so.


Issue 1: When we unload the balloon driver in the guest in an unlucky event,
we won't free the pages. We are missing something like (if I am not wrong):

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index b1d2068fa2bd..e2b0925e1e83 100644
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -929,6 +929,10 @@ static void remove_common(struct virtio_balloon *vb)
                leak_balloon(vb, vb->num_pages);
        update_balloon_size(vb);
 
+       /* There might be free pages that are being reported: release them. */
+       if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT))
+               return_free_pages_to_mm(vb, ULONG_MAX);
+
        /* Now we reset the device so we can clean up the queues. */
        vb->vdev->config->reset(vb->vdev);
 

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.

-- 
Thanks,

David / dhildenb