UNIX Filesystems: Evolution, Design, and Implementation

File I/O in uxfs

File I/O is typically one of the most difficult areas of a filesystem to implement. To increase filesystem performance, this is one area where a considerable amount of time is spent. In Linux, it is very easy to provide a fully working filesytem while spending a minimal amount of time of the I/O paths. There are many generic functions in Linux that the filesystem can call to handle all the interactions with the page cache and buffer cache.

The section File I/O in the 2.4 Linux Kernel in Chapter 8 describes some of the interactions with the page cache. Because this chapter presents a simplified view of filesystem activity, the page cache internals won't be described. Instead, the following sections show how the kernel interacts with the ux_get_block() function exported by uxfs. This function can be used to read data from a file or allocate new data blocks and write data.

First of all, consider the main entry points into the filesystem for file I/O. These are exported through the file_operations structure as follows:

struct file_operations ux_file_operations = {
        llseek:    generic_file_llseek,
        read:      generic_file_read,
        write:     generic_file_write,
        mmap:      generic_file_mmap,
};

So for all of the main file I/O related operations, the filesystem defers to the Linux generic file I/O routines. The same is true for operations on any of the mapped file interactions, whether for user-level mappings or for handling operation within the page cache. The address space related operations ...