Understanding the Linux Kernel, 3rd Edition

Memory Mapping

As already mentioned in the section "Memory Regions" in Chapter 9, a memory region can be associated with some portion of either a regular file in a disk-based filesystem or a block device file. This means that an access to a byte within a page of the memory region is translated by the kernel into an operation on the corresponding byte of the file. This technique is called memory mapping.

Two kinds of memory mapping exist:

Shared

Each write operation on the pages of the memory region changes the file on disk; moreover, if a process writes into a page of a shared memory mapping, the changes are visible to all other processes that map the same file.

Private

Meant to be used when the process creates the mapping just to read the file, not to write it. For this purpose, private mapping is more efficient than shared mapping. But each write operation on a privately mapped page will cause it to stop mapping the page in the file. Thus, a write does not change the file on disk, nor is the change visible to any other processes that access the same file. However, pages of a private memory mapping that have not been modified by the process are affected by file updates performed by other processes.

A process can create a new memory mapping by issuing an mmap( ) system call (see the section "Creating a Memory Mapping" later in this chapter). Programmers must specify either the MAP_SHARED flag or the MAP_PRIVATE flag as a parameter of the system call; as you can easily guess, in the ...