Re: [Qemu-devel] [PATCH] hw/virtio/balloon: Fixes for different host page sizes

[Thomas Huth <thuth@redhat.com>]

On 13.04.2016 19:07, Michael S. Tsirkin wrote:
> On Wed, Apr 13, 2016 at 04:51:49PM +0200, Thomas Huth wrote:
>> On 13.04.2016 15:15, Michael S. Tsirkin wrote:
>>> On Wed, Apr 13, 2016 at 01:52:44PM +0200, Thomas Huth wrote:
...
>>>> Then, there's yet another problem: If the host page size is bigger
>>>> than the 4k balloon page size, we can not simply call madvise() on
>>>> each of the 4k balloon addresses that we get from the guest - since
>>>> the madvise() always evicts the whole host page, not only a 4k area!
>>>>
>>>> So in this case, we've got to track the 4k fragments of a host page
>>>> and only call madvise(DONTNEED) when all fragments have been collected.
>>>> This of course only works fine if the guest sends consecutive 4k
>>>> fragments - which is the case in the most important scenarios that
>>>> I try to address here (like a ppc64 guest with 64k page size that
>>>> is running on a ppc64 host with 64k page size). In case the guest
>>>> uses a page size that is smaller than the host page size, we might
>>>> need to add some more additional logic here later to increase the
>>>> probability of being able to release memory, but at least the guest
>>>> should now not crash anymore due to unintentionally evicted pages.
>> ...
>>>>  static void virtio_balloon_instance_init(Object *obj)
>>>> diff --git a/include/hw/virtio/virtio-balloon.h b/include/hw/virtio/virtio-balloon.h
>>>> index 35f62ac..04b7c0c 100644
>>>> --- a/include/hw/virtio/virtio-balloon.h
>>>> +++ b/include/hw/virtio/virtio-balloon.h
>>>> @@ -43,6 +43,9 @@ typedef struct VirtIOBalloon {
>>>>      int64_t stats_last_update;
>>>>      int64_t stats_poll_interval;
>>>>      uint32_t host_features;
>>>> +    void *current_addr;
>>>> +    unsigned long *fragment_bits;
>>>> +    int fragment_bits_size;
>>>>  } VirtIOBalloon;
>>>>  
>>>>  #endif
>>>
>>> It looks like fragment_bits would have to be migrated.
>>> Which is a lot of complexity.
...
>>> How about we just skip madvise if host page size is > balloon
>>> page size, for 2.6?
>>
>> That would mean a regression compared to what we have today. Currently,
>> the ballooning is working OK for 64k guests on a 64k ppc host - rather
>> by chance than on purpose, but it's working. The guest is always sending
>> all the 4k fragments of a 64k page, and QEMU is trying to call madvise()
>> for every one of them, but the kernel is ignoring madvise() on
>> non-64k-aligned addresses, so we end up with a situation where the
>> madvise() frees a whole 64k page which is also declared as free by the
>> guest.
>>
>> I think we should either take this patch as it is right now (without
>> adding extra code for migration) and later update it to the bitmap code
>> by Jitendra Kolhe, or omit it completely (leaving 4k guests broken) and
>> fix it properly after the bitmap code has been applied. But disabling
>> the balloon code for 64k guests on 64k hosts completely does not sound
>> very appealing to me. What do you think?
>>
>>  Thomas
> 
> True. As simple a hack - how about disabling madvise when host page size >
> target page size?

That could work - but is there a generic way in QEMU to get the current
page size from a guest (since this might differ from TARGET_PAGE_SIZE)?
Or would that mean to pollute the virtio-balloon code with ugly #ifdefs?

 Thomas