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


--
Thanks,

David / dhildenb

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