Based on the provided code snippet and the identified CWE-457  vulnerability at line 4, here are five possible patches to address the issue:

Patch 1: Add a null check before calling free_unref_page
```c
if (page) {
    free_unref_page(page, order);
    page = NULL;  // Set to NULL after freeing
}
```

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

Patch 3: Implement a deferred freeing mechanism
```c
queue_delayed_work(system_wq, &page_free_work, 0);
// Define a work function that calls free_unref_page after a delay
```

Patch 4: Use a flag to mark the page as freed
```c
if (!test_and_set_bit(PG_freed, &page->flags)) {
    free_unref_page(page, order);
}
```

Patch 5: Implement a two-phase free approach
```c
mark_page_for_free(page);
// ... other operations ...
if (is_page_marked_for_free(page)) {
    free_unref_page(page, order);
}
```

Note: The effectiveness of these patches depends on the broader context of the code and the specific requirements of the system. It's crucial to thoroughly test any changes and ensure they don't introduce new issues or conflicts with existing functionality.