How does mmap work?

I am working on programs in Linux which needs mmap file from harddrive, but i have a question, what can make it fail. Like if all the memories are fragmented, which has only 200M each, but i want to mmap a file to a memory of 1000M, will it succeed??

And another question, are there any tools in linux for recollect memory like some tools in Windows, e.g. the built-in tool for xp.

Thanks.