On Friday, February 14, 2020 5:52 PM, David Hildenbrand wrote:

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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).

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Keep using the shrinker for VIRTIO_BALLOON_F_FREE_PAGE_HINT,

because

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this has no such side effects. Always register the shrinker with
VIRTIO_BALLOON_F_FREE_PAGE_HINT now. We are always allowed to

reuse

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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

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       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 vote for not going back. When there are solid request and strong reasons in the future, we could reopen this discussion.

Best,
Wei