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

[David Hildenbrand]

On 06.02.20 10:12, Michael S. Tsirkin wrote:
> On Wed, Feb 05, 2020 at 05:34:02PM +0100, 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.
>>
>> [1] https://www.spinics.net/lists/linux-virtualization/msg40863.html
>>
>> Reported-by: Tyler Sanderson <tysand@xxxxxxxxxx>
>> Cc: Michael S. Tsirkin <mst@xxxxxxxxxx>
>> Cc: Wei Wang <wei.w.wang@xxxxxxxxx>
>> Cc: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>
>> Cc: David Rientjes <rientjes@xxxxxxxxxx>
>> Cc: Nadav Amit <namit@xxxxxxxxxx>
>> Cc: Michal Hocko <mhocko@xxxxxxxxxx>
>> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> 
> 
> I guess we should add a Fixes tag to the patch it's reverting,
> this way it's backported and hypervisors will be able to rely on OOM
> behaviour.

Makes sense,

Fixes: 71994620bb25 ("virtio_balloon: replace oom notifier with shrinker")

-- 
Thanks,

David / dhildenb