MmAllocatePagesForMdl(Ex) & use of physical memory

Hello everyone,

I was planning to use MmAllocatePagesForMdl(Ex) in my driver to reserve a range of physical memory.
What are the things that govern the maximum size of physical memory that can be described by an MDL returned by MmAllocatePagesForMdl(Ex)?
For instance can i pass a huge no. say 512 MB in the “Total Bytes” parameter?

Thank you.
Tushar.