Because there is no toggling, there is no performance degradation. When mapping entire files. Many files are mapped simultaneously. Only a portion of a file needs to be mapped. Page-level protection needs to be set on the mapping.
Private mapping is required.
The segments can be from 1 byte to 256M bytes in size. The process can attach these segments into the address space for the size of the segment. Another segment can be attached at the end of the first one in the same 256M byte region. Some restrictions exist on the use of the extended shmat feature. The mmap services are specified by various standards and commonly used as the file-mapping interface of choice in other operating system implementations.