doubt regarding mmap/mmap2 system call and implementation

[Himanshu Aggarwal <lkml.himanshu@xxxxxxxxx>]

I have read that mmap/mmap2 system calls are more efficient than a read and a write system call. I am unable to understand why is this so?

mmap/mmap2 always create a vm_area_struct which is inserted into the process's userspace region. Given that this is the case, how is mmap/mmap2 more efficient than the read/write system call? Shouldn't it be less efficient, because each access to mmapped region might be access this vm_area_struct mapping?

I have another doubt as well : if mmap/mmap2 are more efficient than a write call, then why don't all drivers just implement a mmap file operation. Why do they implement a write file-operation which is supposedly less efficient. I have found only a few drivers implementing the mmap file operation. Is there any reason for this? 

Also, i found that LDD3 book also gives a very "secondary" treatment to this topic. Shouldn't mmap be used more extensively in device driver development? Is there any reason for this?

I hope my doubt will be cleared. Thanks to everyone for their replies and suggestions.

~Himanshu Aggarwal