Understanding the Linux Kernel, 3rd Edition

Chapter 16. Accessing Files

Accessing a disk-based file is a complex activity that involves the VFS abstraction layer (Chapter 12), handling block devices (Chapter 14), and the use of the page cache (Chapter 15). This chapter shows how the kernel builds on all those facilities to carry out file reads and writes. The topics covered in this chapter apply both to regular files stored in disk-based filesystems and to block device files; these two kinds of files will be referred to simply as “files.”

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 described in Chapter 14 and Chapter 15.

There are many different ways to access a file. In this chapter we will consider the following cases:

Canonical mode

The file is opened with the O_SYNC and O_DIRECT flags cleared, and its content is accessed by means of the read( ) and write( ) system calls. In this case, the read( ) system call blocks the calling process until the data is copied into the User Mode address space (however, the kernel is always allowed to return fewer bytes than requested!). The write( ) system call is different, because it terminates as soon as the data is copied into the page cache (deferred write). This case ...