+static const struct address_space_operations virtio_balloon_aops;
+#ifdef CONFIG_BALLOON_COMPACTION
+/*
+ * virtballoon_migratepage - perform the balloon page migration on behalf of
+ * a compation thread. (called under page lock)
+ * @mapping: the page->mapping which will be assigned to the new migrated page.
+ * @newpage: page that will replace the isolated page after migration finishes.
+ * @page : the isolated (old) page that is about to be migrated to newpage.
+ * @mode : compaction mode -- not used for balloon page migration.
+ *
+ * After a ballooned page gets isolated by compaction procedures, this is the
+ * function that performs the page migration on behalf of a compaction thread
+ * The page migration for virtio balloon is done in a simple swap fashion which
+ * follows these two macro steps:
+ * 1) insert newpage into vb->pages list and update the host about it;
+ * 2) update the host about the old page removed from vb->pages list;
+ *
+ * This function preforms the balloon page migration task.
+ * Called through balloon_mapping->a_ops->migratepage
+ */
+int virtballoon_migratepage(struct address_space *mapping,
+ struct page *newpage, struct page *page, enum migrate_mode mode)
+{
+ struct balloon_dev_info *vb_dev_info = balloon_page_device(page);
+ struct virtio_balloon *vb;
+ unsigned long flags;
+
+ BUG_ON(!vb_dev_info);
+
+ vb = vb_dev_info->balloon_device;
+
+ if (!mutex_trylock(&vb->balloon_lock))
+ return -EAGAIN;
+
+ /* 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);
+
+ /*
+ * 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);
+
+ mutex_unlock(&vb->balloon_lock);
+
+ return MIGRATEPAGE_BALLOON_SUCCESS;
+}
+
+/* define the balloon_mapping->a_ops callback to allow balloon page migration */
+static const struct address_space_operations virtio_balloon_aops = {
+ .migratepage = virtballoon_migratepage,
+};
+#endif /* CONFIG_BALLOON_COMPACTION */
+