Linux Kernel Architecture

Chapter 8: The Virtual Filesystem

address_

space_

operations

dentry_

operations

s_files

super_block

host

page_tree

a_ops

address_space

f_op

f_dentry

f_op

f_dentry

f_op

f_dentry

file instances of the superblock

d_inode

d_sb

d_ops

NAME

task_struct

files files_struct

inode_

operations

file_

operations

i_op

i_dentry

i_fop

i_sb

i_mapping

inode

struct page

Doubly chained

list (schematically)

Figure 8-3: Interplay of the VFS components.

Because opened files are always assigned to a specific system process, the kernel must store the connec-

tion between the file and the process in its data structures. As discussed briefly in Chapter 2, the task

structure includes an element in which all opened files are held (via a roundabout route). This element is

an array that is accessed using the file descriptor as an index. The objects it contains are not only linked

with the inode of the corresponding file, but also have a pointer to an element of thedentry cacheused to

speed lookup operations.

The individual filesystem implementations are also able to store their own data (that is not manipulated

by the VFS layer) in the VFS inode.

Filesystemand SuperblockInformation

The supported filesystem types are linked by means of a special kernel object that features a method of

reading thesuperblock. As well as key information on the filesystem (block size, maximum file size, etc.),

the superblock contains function pointersto read, write, and manipulate inodes.

The kernel also creates a list of the superblock instances of all active filesystems. I use the termactive

instead ofmountedbecause, in certain circumstances, it is possible to use a single superblock for several

mount points.^4

(^4) When a filesystem of a block device is mounted at several points in the directory hierarchy.