mmap(void * start, size_t length, int prot, int flags, int sg_fd, off_t offset). This system call returns a pointer to the beginning of the reserved buffer associated with the sg file descriptor 'sg_fd'. The 'start' argument is a hint to the kernel and is ignored by this driver; best set it to 0. The 'length' argument should be less than or equal to the size of the reserved buffer associated with 'sg_fd'. If it exceeds the reserved buffer size (after 'length' has been rounded up to a page size multiple) then MAP_FAILED is returned and ENOMEM is placed in errno. The 'prot' argument should either be PROT_READ or (PROT_READ | PROT_WRITE). The 'flags' argument should contain MAP_SHARED. In a sense, the user application is "sharing" data with the sg driver. The MAP_PRIVATE flag does not play well with compiler optimization flags such as '-O2'. The 'offset' argument must be set to 0 (or NULL).
The mmap() system call can be made multiple times on the same sg_fd. The munmap() system call is not required if close() is called on sg_fd. Mmap-ed IO is well-behaved when a process is fork()-ed (or the equivalent finer grained clone() system call is made). In the case of a fork(), 2 processes will be sharing the same memory mapped area together with the sg driver for a sg_fd and the last one to close the sg_fd (or exit) will cause the shared memory to be freed.
It is assumed that if the default reserved buffer size of 32 KB is not sufficient then a ioctl(SG_SET_RESERVED_SIZE) call is made prior to any calls to mmap(). If the required size is not a multiple of the kernel's page size (returned by getpagesize() system call) then the size passed to ioctl(SG_SET_RESERVED_SIZE) should be rounded up to the next page size multiple.
Mmap-ed IO is requested by setting (or or-ing in) the SG_FLAG_MMAP_IO constant into the flag member of the the sg_io_hdr structure prior to a call to write() or ioctl(SG_IO). The logic to do mmap-ed IO _assumes_ that an appropriate mmap() call has been made by the application. In other words it does not check. 
The sg driver does record that the mmap() system call has been invoked at least once on a file descriptor. This is not sufficient because the given 'length' may be too short for the current IO. Also the driver is unaware of munmap() calls so it could easily be tricked.