Re: [PATCH v11 5/7] virtio_balloon: introduce migration primitives to balloon pages

[Rusty Russell <rusty@rustcorp.com.au>]

Rafael Aquini <aquini@redhat.com> writes:
> + * virtballoon_migratepage - perform the balloon page migration on behalf of
> + *			     a compation thread.     (called under page lock)

> +	if (!mutex_trylock(&vb->balloon_lock))
> +		return -EAGAIN;

Erk, OK...

> +	/* balloon's page migration 1st step  -- inflate "newpage" */
> +	spin_lock_irqsave(&vb_dev_info->pages_lock, flags);
> +	balloon_page_insert(newpage, mapping, &vb_dev_info->pages);
> +	vb_dev_info->isolated_pages--;
> +	spin_unlock_irqrestore(&vb_dev_info->pages_lock, flags);
> +	vb->num_pfns = VIRTIO_BALLOON_PAGES_PER_PAGE;
> +	set_page_pfns(vb->pfns, newpage);
> +	tell_host(vb, vb->inflate_vq);

tell_host does wait_event(), so you can't call it under the page_lock.
Right?

You probably get away with it because current qemu will service you
immediately.  You could spin here in this case for the moment.

There's a second call to tell_host():

> +	/*
> +	 * balloon's page migration 2nd step -- deflate "page"
> +	 *
> +	 * It's safe to delete page->lru here because this page is at
> +	 * an isolated migration list, and this step is expected to happen here
> +	 */
> +	balloon_page_delete(page);
> +	vb->num_pfns = VIRTIO_BALLOON_PAGES_PER_PAGE;
> +	set_page_pfns(vb->pfns, page);
> +	tell_host(vb, vb->deflate_vq);

The first one can be delayed, the second one can be delayed if the host
didn't ask for VIRTIO_BALLOON_F_MUST_TELL_HOST (qemu doesn't).

We could implement a proper request queue for these, and return -EAGAIN
if the queue fills.  Though in practice, it's not important (it might
help performance).

Cheers,
Rusty.