The page cache is the main disk cache used by the Linux kernel. In most cases, the kernel refers to the page cache when reading from or writing to disk. New pages are added to the page cache to satisfy User Mode processes's read requests. If the page is not already in the cache, a new entry is added to the cache and filled with the data read from the disk. If there is enough free memory, the page is kept in the cache for an indefinite period of time and can then be reused by other processes without accessing the disk.
Similarly, before writing a page of data to a block device, the kernel verifies whether the corresponding page is already included in the cache; if not, a new entry is added to the cache and filled with the data to be written on disk. The I/O data transfer does not start immediately: the disk update is delayed for a few seconds, thus giving a chance to the processes to further modify the data to be written (in other words, the kernel implements deferred write operations).
Kernel code and kernel data structures don't need to be read from or written to disk.[*] Thus, the pages included in the page cache can be of the following types:
Pages containing data of regular files; in Chapter 16, we describe how the kernel handles read, write, and memory mapping operations on them.
Pages containing directories; as we'll see in Chapter 18, Linux handles the directories much like regular files.
Pages containing data directly read from block device files (skipping the ...