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

quoted

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.

Could you explain why some more? drop_caches shouldn't be really used in
any production workloads and if somebody really wants all the cache to
be dropped then why is balloon any different?

Deflation should happen when the guest is out of memory, not when
somebody thinks it's time to reclaim some memory. That's what the
feature promised from the beginning: Only give the guest more memory in
case it *really* needs more memory.

Deflate on oom, not deflate on reclaim/memory pressure. (that's what the
report was all about)

A priority for shrinkers might be a step into the right direction.

-- 
Thanks,

David / dhildenb