get_user_pages and pinning

Folks Since Linux uses lazy evaluation with respect to allocating memory what happens if a user process allocates memory using malloc in excess of say 8 or nine pages and calls a device ioctl and passes the user space pointer to it to be used to pin pages using get_user_pages(). This buffer is to be used for a dma read from a device. What happens if the memory isn't actually allocated (no initialization with memset or calloc etc. and all PTEs are pointing to some zero filled page) and there is no IOMMU? Thanks S