Understanding the Linux Kernel

15.1. Reading and Writing a Regular File

Chapter 13's Section 13.5.4 described how the read( ) and write( ) system calls are implemented. The corresponding service routines end up invoking the file object's read and write methods, which may be filesystem-dependent. For disk-based filesystems, these methods locate the physical blocks containing the data being accessed and activate the block device driver to start the data transfer. However, reading and writing are performed differently in Linux.

Reading a regular file is page-based: the kernel always transfers whole pages of data at once. If a process issues a read( ) system call in order to get a few bytes, and that data is not already in RAM, the kernel allocates a new page frame, fills the page with the suitable portion of the regular file, adds the page to the page cache, and finally copies the requested bytes into the process address space. For most filesystems, reading a page of data from a regular file is just a matter of finding what blocks on disk contain the requested data. Once this is done, the kernel can use one or more page I/O operations to fill the pages.

Write operations for disk-based filesystems are much more complicated to handle, since the file size could change, and therefore the kernel might allocate or release some physical blocks on the disk. Of course, how this is precisely done depends on the filesystem type.

As a matter of fact, the read method of most filesystems is implemented by a common function ...