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SubjectRe: [PATCH v11 5/7] virtio_balloon: introduce migration primitives to balloon pages
Date
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.


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