DMA is more like a bulk memory transfer operation usually facilitated by specific hardware that generally is asynchronous and requires manual synchronization.

DMA is more like a bulk memory transfer operation usually facilitated by specific hardware that generally is asynchronous and requires manual synchronization. Usually hardware devices perform DMAs of memory regions, like a memcpy() but between physical memories. Memory mappings established via mmap() more so set up the kernel to map in pages when faults on accesses occur. In this case you're not calling into the kernel, the TLB is generating an interrupt when you go to read an address referring to memory that's not yet paged in, which the kernel than handles and restores control flow to userspace without userspace being any wiser (unless userspace is keeping track of time). Handling page faults is faster than the syscalls involved in read() calls, it would seem.