Re: [PATCH v1 3/3] virtio-balloon: Switch back to OOM handler for VIRTIO_BALLOON_F_DEFLATE_ON_OOM

[Michael S. Tsirkin]

On Fri, Feb 14, 2020 at 10:51:43AM +0100, David Hildenbrand wrote:
> On 05.02.20 17:34, David Hildenbrand wrote:
> > Commit 71994620bb25 ("virtio_balloon: replace oom notifier with shrinker")
> > changed the behavior when deflation happens automatically. Instead of
> > deflating when called by the OOM handler, the shrinker is used.
> > 
> > However, the balloon is not simply some slab cache that should be
> > shrunk when under memory pressure. The shrinker does not have a concept of
> > priorities, so this behavior cannot be configured.
> > 
> > There was a report that this results in undesired side effects when
> > inflating the balloon to shrink the page cache. [1]
> > 	"When inflating the balloon against page cache (i.e. no free memory
> > 	 remains) vmscan.c will both shrink page cache, but also invoke the
> > 	 shrinkers -- including the balloon's shrinker. So the balloon
> > 	 driver allocates memory which requires reclaim, vmscan gets this
> > 	 memory by shrinking the balloon, and then the driver adds the
> > 	 memory back to the balloon. Basically a busy no-op."
> > 
> > The name "deflate on OOM" makes it pretty clear when deflation should
> > happen - after other approaches to reclaim memory failed, not while
> > reclaiming. This allows to minimize the footprint of a guest - memory
> > will only be taken out of the balloon when really needed.
> > 
> > Especially, a drop_slab() will result in the whole balloon getting
> > deflated - undesired. While handling it via the OOM handler might not be
> > perfect, it keeps existing behavior. If we want a different behavior, then
> > we need a new feature bit and document it properly (although, there should
> > be a clear use case and the intended effects should be well described).
> > 
> > Keep using the shrinker for VIRTIO_BALLOON_F_FREE_PAGE_HINT, because
> > this has no such side effects. Always register the shrinker with
> > VIRTIO_BALLOON_F_FREE_PAGE_HINT now. We are always allowed to reuse free
> > pages that are still to be processed by the guest. The hypervisor takes
> > care of identifying and resolving possible races between processing a
> > hinting request and the guest reusing a page.
> > 
> > In contrast to pre commit 71994620bb25 ("virtio_balloon: replace oom
> > notifier with shrinker"), don't add a moodule parameter to configure the
> > number of pages to deflate on OOM. Can be re-added if really needed.
> > Also, pay attention that leak_balloon() returns the number of 4k pages -
> > convert it properly in virtio_balloon_oom_notify().
> > 
> > Note1: using the OOM handler is frowned upon, but it really is what we
> >        need for this feature.
> > 
> > Note2: without VIRTIO_BALLOON_F_MUST_TELL_HOST (iow, always with QEMU) we
> >        could actually skip sending deflation requests to our hypervisor,
> >        making the OOM path *very* simple. Besically freeing pages and
> >        updating the balloon. If the communication with the host ever
> >        becomes a problem on this call path.
> > 
> 
> @Michael, how to proceed with this?
> 

I'd like to see some reports that this helps people.
e.g. a tested-by tag.

> -- 
> Thanks,
> 
> David / dhildenb