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

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Apr 13, 2016 at 07:38:12PM +0200, Thomas Huth wrote:
> 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

let's just use TARGET_PAGE_SIZE, that's the best I can think of.