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

[Wei Wang]

On 07/22/2018 10:48 PM, Michael S. Tsirkin wrote:
> On Fri, Jul 20, 2018 at 04:33:02PM +0800, Wei Wang wrote:
> >   
> > +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.

> > +
> > +	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.


>   	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.
- 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