Understanding the Linux Kernel, Second Edition

Memory Mapping

As already mentioned in Section 8.3, 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

Any 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 any 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.

A process can create a new memory mapping by issuing an mmap( ) system call (see Section 15.2.2 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 former case the mapping is shared, while in the latter it is private. Once the mapping is created, the process can read the data stored in the file by simply reading from the memory locations of ...