There is no arbitrary limit on the number of slabs that a kernel routine (whether or not it is a module) can use, but no computer has an infinite amount of memory either. Zones are different than slabs. Memory locations have an address and not all addresses are identical in nature. This stuff varies greatly depending on the cpu. As an example, on a typical Pentium, ZONE_DMA memory has an addresss below 16 MB and is usable for DMA transfers. There are only a very few zones. This stuff is documented in "Understanding the Linux Kernel", by Bovet and Cesati, ISBN 0-596-00565-2.