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