Memory allocation in zswap

I have a question regarding allocation of pool in zswap. It seems that zswap allocates memory for caching on-the-fly whenever it needs to store new compressed pages. But swapping will happen and there will be a page_io request to zswap only when physical memory is full. At that time if zswap is trying to allocate from physical memory (which is already full) will it not get a -ENOMEM error always? Also will it not create more pressure on physical memory that is already trying to swap out pages?