a question about slub in function __slab_free()

[Xishi Qiu]

___slab_alloc()
        deactivate_slab()
                add_full(s, n, page);
The page will be added to full list and the frozen is 0, right?

__slab_free()
        prior = page->freelist;  // prior is NULL
        was_frozen = new.frozen;  // was_frozen is 0
        ...
                /*
                 * Slab was on no list before and will be
                 * partially empty
                 * We can defer the list move and instead
                 * freeze it.
                 */
                new.frozen = 1;
        ...

I don't understand why "Slab was on no list before"?

Thanks,
Xishi Qiu