Understanding the Linux Kernel

Chapter 15. Accessing Regular Files

Accessing a regular file is a complex activity that involves the VFS abstraction (Chapter 12), the handling of block devices (Chapter 13), and the use of disk caches (Chapter 14). This chapter shows how the kernel builds on all those facilities to accomplish file reads and writes. The topics covered in this chapter apply to regular files stored either in disk-based filesystems or to network filesystems such as NFS or Samba.

The stage we are working at in this chapter starts after the proper read or write method of a particular file has been called (as described in Chapter 12). We show here how each read ends with the desired data delivered to a User Mode process and how each write ends with data marked ready for transfer to disk. The rest of the transfer is handled by the facilities in Chapter 13 and Chapter 14.

In particular, in Section 15.1 we describe how regular files are accessed by means of the read( ) and write( ) system calls. When a process reads from a regular file, data is first moved from the disk itself to a set of buffers in the kernel's address space. This set of buffers is included in a set of pages in the page cache (see Section 13.6 in Chapter 13). Next, the pages are copied into the process's user address space. This chapter deals only with the move from the kernel to the user address space. A write is basically the opposite, although some stages are different from reads in important ways.

We also discuss in Section 15.2 ...