Understanding the Linux Kernel, 3rd Edition

Reading and Writing a File

The section "The read( ) and write( ) System Calls" in Chapter 12 described how the read( ) and write( ) system calls are implemented. The corresponding service routines end up invoking the file object’s read and write methods, which may be filesystem-dependent. For disk-based filesystems, these methods locate the physical blocks that contain the data being accessed and activate the block device driver to start the data transfer.

Reading a file is page-based: the kernel always transfers whole pages of data at once. If a process issues a read( ) system call to get a few bytes, and that data is not already in RAM, the kernel allocates a new page frame, fills the page with the suitable portion of the file, adds the page to the page cache, and finally copies the requested bytes into the process address space. For most filesystems, reading a page of data from a file is just a matter of finding what blocks on disk contain the requested data. Once this is done, the kernel fills the pages by submitting the proper I/O operations to the generic block layer. In practice, the read method of all disk-based filesystems is implemented by a common function named generic_file_read( ).

Write operations on disk-based files are slightly more complicated to handle, because the file size could increase, and therefore the kernel might allocate some physical blocks on the disk. Of course, how this is precisely done depends on the filesystem type. However, many disk-based filesystems ...