Based on the provided code slice and the specified vulnerability (CWE-401: ) on line 4, here are five possible patches to address the issue:

Patch 1: Add a null check before calling free_unref_page
```
if (page) {
    free_unref_page(page, order);
    page = NULL;
}
```

Patch 2: Use a reference counting mechanism
```
if (atomic_dec_and_test(&page->refcount)) {
    free_unref_page(page, order);
}
```

Patch 3: Implement a deferred freeing mechanism
```
queue_delayed_work(system_wq, &page_free_work, msecs_to_jiffies(100));
```

Patch 4: Use a lock to ensure exclusive access
```
spin_lock(&page_lock);
free_unref_page(page, order);
spin_unlock(&page_lock);
```

Patch 5: Implement a two-phase free approach
```
mark_page_for_free(page);
if (is_safe_to_free(page)) {
    free_unref_page(page, order);
}
```

Note: These patches are hypothetical and may need to be adjusted based on the broader context of the codebase and the specific requirements of the system.