Understanding the Linux Kernel, 3rd Edition

Chapter 15. The Page Cache

As already mentioned in the section "The Common File Model" in Chapter 12, a disk cache is a software mechanism that allows the system to keep in RAM some data that is normally stored on a disk, so that further accesses to that data can be satisfied quickly without accessing the disk.

Disk caches are crucial for system performance, because repeated accesses to the same disk data are quite common. A User Mode process that interacts with a disk is entitled to ask repeatedly to read or write the same disk data. Moreover, different processes may also need to address the same disk data at different times. As an example, you may use the cp command to copy a text file and then invoke your favorite editor to modify it. To satisfy your requests, the command shell will create two different processes that access the same file at different times.

We have already encountered other disk caches in Chapter 12: the dentry cache , which stores dentry objects representing filesystem pathnames, and the inode cache , which stores inode objects representing disk inodes. Notice, however, that dentry objects and inode objects are not mere buffers storing the contents of some disk blocks; thus, the dentry cache and the inode cache are rather peculiar as disk caches.

This chapter deals with the page cache , which is a disk cache working on whole pages of data. We introduce the page cache in the first section. Then, we discuss in the section "Storing Blocks in the Page Cache" how the ...