Bern University of Applied Sciences
Engineering and Information Technology
Module I-7410
Advanced Linux
Virtual
File
System
By Franz Meyer
Version 1.4
April 2012
Berne University of Applied Sciences
School of Information Technology HTI
Module:
Advanced Linux
Processes - Topics
Layering Model
Kernel Subsystems
Virtual File System in Linux Kernel (version 2.6.35.9)
Operation on Processes: Creation and Termination
Introduction to Virtual File System
V1.0 © 2012 by franz.meyer@bfh.ch
Slide-2
Berne University of Applied Sciences
School of Information Technology HTI
Module:
Advanced Linux
The Linux Kernel 2.6 Layering Model
User1
User2
User3
UserN
Layer
User
System Call Interface
File
Process
Memory
Management Management Systems
Device
Control
Networking
Scheduling
Multitasking
Virtual
Memory
Block
Devices
Character
Devices
Network
Drivers
CPUs
Memory
Disks
CDs
Consoles
Network
interfaces
Introduction to Virtual File System
V1.0 © 2012 by franz.meyer@bfh.ch
Kernel
Subsystem
Kernel
Kernel
Features
Hardware
Slide-3
Berne University of Applied Sciences
School of Information Technology HTI
Module:
Advanced Linux
Kernel Subsystem Linux Kernel 2.6 (1)
Process Management
Creating and destroying processes
Inter process communication and synchronization
Processor scheduling
Memory Management
Provides virtual memory (virtual address space)
Allocation/Liberation of memory blocks
File Systems
Most important view of the kernel by users/devices
Structuring data on unstructured carrier devices
Multiples files system types supported (ext2/3,reiser,fat32,etc.)
Introduction to Virtual File System
V1.0 © 2012 by franz.meyer@bfh.ch
Slide-4
Berne University of Applied Sciences
School of Information Technology HTI
Module:
Advanced Linux
Kernel Subsystem Linux Kernel 2.6 (2)
File Subsystem more detailed
The implementation consists of three major layers
File System Interface: contains file system relevant system calls like open,
close, read, write, etc.
The Virtual File System:
Separates generic functions from their implementation by defining a clean
virtual interface. This allows several concrete file system implementations to
coexist on the same machine.
Provides a mechanism for representing a file across a network (NFS).
The concrete file system implementation or the remote file system protocol.
File System Interface
Virtual File System Interface
Local File System Type 1
Local File System Type 2
Carrier Device
Kernel RAM for pseudo FS
Introduction to Virtual File System
V1.0 © 2012 by franz.meyer@bfh.ch
Slide-5
Berne University of Applied Sciences
School of Information Technology HTI
Module:
Advanced Linux
Kernel Subsystem Linux Kernel 2.6 (3)
File System Organization
Directory: can contain files and directories
Directory Entry: a component in the path to a file or directory
File: A collection of data that is described by meta-data
Directories are files of type directory
Meta Data: information about file systems and files
Inode: meta data of a particular file or directory
Superblock: contains information about a file system on a disk or disk
partition
Introduction to Virtual File System
V1.0 © 2012 by franz.meyer@bfh.ch
Slide-6
Berne University of Applied Sciences
School of Information Technology HTI
Module:
Advanced Linux
Kernel Subsystem Linux Kernel 2.6 (4)
Virtual File System “Objects”
Although the kernel is written in C object-oriented techniques can be used
Data structures that contain data fields and function pointers to operate on
these data fields.
There are four primary object types in the VFS
Superblock: represents a specific mounted file system
super_operations: the superblock's methods
Inode: represents a specific file
inode_operations: the inode's methods
Dentry: represents a directory entry, a single component in the path. Note that a
directory is a file.
dentry_operations: the dentry's methods
File: represents an open file associated with a process
file_operations: the methods that a process can invoke on an open file in the
form of a file system call
Introduction to Virtual File System
V1.0 © 2012 by franz.meyer@bfh.ch
Slide-7
Berne University of Applied Sciences
School of Information Technology HTI
Module:
Advanced Linux
Kernel Subsystem Linux Kernel 2.6 (5)
Virtual File System “Objects”, cont.
In the driver part of the course you will see inode objects and file objects.
Of course, the glue between those objects is the dentry object.
The inode object
Recall that an inode in a Unix file system represents all information about a file
or directory
When a process opens a file the kernel finds the corresponding inode on the
disk an loads it into the inode cache and adds some control information
struct inode { from <linux/fs.h>, some fields deleted
struct hlist_node
i_hash;
/* hash list
struct list_head
i_list;
/* inode list */
struct list_head
i_dentry; /* directroy entry list */
unsigned long
i_ino;
/* i­number */
atomic_t
i_count; /* reference counter */
unsigned int
i_nlink; /* numer of hard links */
uid_t
i_uid;
/* user id of owner */
gid_t
i_gid;
/* group id of owner */
......
Introduction to Virtual File System
V1.0 © 2012 by franz.meyer@bfh.ch
Slide-8
Berne University of Applied Sciences
School of Information Technology HTI
Module:
Advanced Linux
Kernel Subsystem Linux Kernel 2.6 (6)
Virtual File System “Objects”, inode object, cont.
struct inode { cont
....
const struct inode_operations i_op; /*file operations*/
const struct file_operations *i_fop;/*default file ops*/
....
union { /* link to device type */ struct pipe_inode_info *i_pipe; /*pipe special file*/
struct block_device
*i_bdev; /*block special file*/ struct cdev
*i_cdev; /*character spec file*/
};
....
void *i_private; /*fs private pointer*/ };
Virtual File System “Objects”, inode operations see
struct inode_operation {...}; Introduction to Virtual File System
V1.0 © 2012 by franz.meyer@bfh.ch
Slide-9
Berne University of Applied Sciences
School of Information Technology HTI
Module:
Advanced Linux
Kernel Subsystem Linux Kernel 2.6 (7)
Virtual File System “Objects”, cont.
The file object
Recall that a file object in a Unix file system is the in-memory representation of
a file opened by a process
The file object is created in response to the open() system call and destroyed in
the close() system call.
If multiple processes open and manipulate a file the kernel will create an
instance of a file object for each process
The file object points back to the dentry object that in turn points back to the
inode object.
struct file { from <linux/fs.h>, some fields deleted
struct path
f_path;
/*contains dentry*/
struct file_operation *f_op; /*file operation pnters*/
atomic_t
f_count; /*reference counter*/
unsigned int
f_flags; /*specified open flags*/
loff_t
i_pos;
/*file position*/
void
*private_data; /*driver hook*/
...
Introduction to Virtual File System
V1.0 © 2012 by franz.meyer@bfh.ch
Slide-10
Berne University of Applied Sciences
School of Information Technology HTI
Module:
Advanced Linux
Kernel Subsystem Linux Kernel 2.6 (8)
Virtual File System “Objects”, cont.
The file operations
struct file_operations { from <linux/fs.h>, some fields del
struct module
*owner
loff_t (*llseek) (struct file *, loff_t, int);
ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
ssize_t (*write) (struct file *, const char __user *, size_t,
loff_t *);
...
int (*ioctl) (struct inode *, struct file *, unsigned int,
unsigned long);
int (*mmap) (struct file *, struct vm_area_struct *);
int (*open) (struct inode *, struct file *);
int (*flush) (struct file *, fl_owner_t id);
... /* more file operations */
};
Introduction to Virtual File System
V1.0 © 2012 by franz.meyer@bfh.ch
Slide-11
Berne University of Applied Sciences
School of Information Technology HTI
Module:
Advanced Linux
Kernel Subsystem Linux Kernel 2.6 (8)
Virtual File System “Objects”, cont.
The dentry object is a specific component in a path: a file, a directory, a
mount point, etc.
struct dentry { from <linux/dcache.h>, some fields del
atomic_t d_count;
/*reference counter*/
unsigned int d_flags;
/*dentry flags*/
spinlock_t d_lock;
/*per dentry lock*/
int d_mounted;
/*is it a mount point?*/
struct inode *d_inode;
/*associated inode*/
...
struct qstr d_name;
struct list_head d_subdirs;
/*our children */
struct list_head d_alias;
/*inode alias list */
unsigned long d_time;
/*used by d_revalidate */
const struct dentry_operations *d_op;
struct super_block *d_sb; /*root of the dentry tree */
unsigned char d_iname[DNAME_INLINE_LEN_MIN]; /*small names*/
};
Introduction to Virtual File System
V1.0 © 2012 by franz.meyer@bfh.ch
Slide-12
Berne University of Applied Sciences
School of Information Technology HTI
Module:
Advanced Linux
Kernel Subsystem Linux Kernel 2.6 (9)
Virtual File System “Objects”, cont.
The dentry cache
File to Inode: Find the inode instance (struct inode) that corresponds with a
given instance of an open file (stuct file). Recall that an inode object is a
system wide representation of an open file, while the file object is the per
process representation of an open file, and the glue between these objects
is a dentry (directory entry) object.
struct dentry *dentry;
struct file *file
struct inode *inode
inode = file­>f_path.dentry­>d_inode;
Introduction to Virtual File System
V1.0 © 2012 by franz.meyer@bfh.ch
Slide-13