Re: [PATCH v36 2/5] virtio_balloon: replace oom notifier with shrinker

On Mon, Jul 23, 2018 at 06:30:46PM +0800, Wei Wang wrote:

On 07/22/2018 10:48 PM, Michael S. Tsirkin wrote:

quoted

On Fri, Jul 20, 2018 at 04:33:02PM +0800, Wei Wang wrote:

quoted

+static unsigned long virtio_balloon_shrinker_scan(struct shrinker *shrinker,
+						  struct shrink_control *sc)
+{
+	unsigned long pages_to_free = balloon_pages_to_shrink,
+		      pages_freed = 0;
+	struct virtio_balloon *vb = container_of(shrinker,
+					struct virtio_balloon, shrinker);
+
+	/*
+	 * One invocation of leak_balloon can deflate at most
+	 * VIRTIO_BALLOON_ARRAY_PFNS_MAX balloon pages, so we call it
+	 * multiple times to deflate pages till reaching
+	 * balloon_pages_to_shrink pages.
+	 */
+	while (vb->num_pages && pages_to_free) {
+		pages_to_free = balloon_pages_to_shrink - pages_freed;
+		pages_freed += leak_balloon(vb, pages_to_free);
+	}
+	update_balloon_size(vb);

Are you sure that this is never called if count returned 0?

Yes. Please see do_shrink_slab, it just returns if count is 0.

quoted

quoted

+
+	return pages_freed / VIRTIO_BALLOON_PAGES_PER_PAGE;
+}
+
+static unsigned long virtio_balloon_shrinker_count(struct shrinker *shrinker,
+						   struct shrink_control *sc)
+{
+	struct virtio_balloon *vb = container_of(shrinker,
+					struct virtio_balloon, shrinker);
+
+	/*
+	 * We continue to use VIRTIO_BALLOON_F_DEFLATE_ON_OOM to handle the
+	 * case when shrinker needs to be invoked to relieve memory pressure.
+	 */
+	if (!virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_DEFLATE_ON_OOM))
+		return 0;

So why not skip notifier registration when deflate on oom
is clear?

Sounds good, thanks.

quoted

  	vb->vb_dev_info.inode->i_mapping->a_ops = &balloon_aops;
  #endif
+	err = virtio_balloon_register_shrinker(vb);
+	if (err)
+		goto out_del_vqs;
So we can get scans before device is ready. Leak will fail
then. Why not register later after device is ready?

Probably no.

- it would be better not to set device ready when register_shrinker failed.

That's very rare so I won't be too worried.

- When the device isn't ready, ballooning won't happen, that is,
vb->num_pages will be 0, which results in shrinker_count=0 and shrinker_scan
won't be called.

So I think it would be better to have shrinker registered before
device_ready.

Best,
Wei