On Thursday, February 6, 2020 12:34 AM, 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."

Not sure if we need to go back to OOM, which has many drawbacks as we discussed.
Just posted out another approach, which is simple.

Best,
Wei