CSC 660: Advanced OS

CSC 660: Advanced Operating Systems Slide #1

CSC 660: Advanced OS

Virtual Filesystem


Topics

1. Filesystems

2. Filenames and Pathnames

3. File Attributes

4. File Operations

5. Virtual Filesystem

6. VFS Objects

7. Processes and Files


Why filesystems?

Physical Disk Filesystem

Access by Blocks Bytes

Addressing Block # Pathname

Security None ACLs

Reliability Corruption on crash Robust response to system failures.


Tree Structure

/

bin boot tmp usr var

ls grub bin lib X11R6

vmlinuzmenu.lst

less

zip bin lib

xclock xterm


Filenames and Pathnames

• Filenames– Human-readable identifier for data.– File suffixes have special meanings in Windows.

• Directories– Store lists of filename/location mappings.

• Pathnames– Identify file in directory hierarchy.– Ex: /usr/X11R6/bin/xclock


File Attributes• Filename and Data• Location• File Type

– Regular files– Directories– Device (block + char) files– IPC files

• Ownership• Access Control List• Locking• Size• Timestamps


File Operations

• Create• Delete• Open• Close• Read• Write

• Append• Seek• Get Attributes• Set Attributes• Rename


File Descriptors

User process reference to a file.A non-negative integer unique to process.

Returned by open or creat system calls.

Used as argument to other file system calls.

View with: ls –l /proc/self/fd

Common file descriptors:0 STDIN

1 STDOUT

2 STDERR


Process File Context

• Root directory– Usually /, but can change for security.

• Current working directory– Default location for file commands.– Relative pathnames are relative to CWD.

• Open file table– Maps file descriptors to kernel file objects.– files_struct member of task_struct


Open File Tablestruct files_struct {

atomic_t count;

spinlock_t file_lock;

int max_fds;

int max_fdset;

int next_fd;

struct file ** fd;

fd_set *close_on_exec;

fd_set *open_fds;

fd_set close_on_exec_init;

fd_set open_fds_init;

struct file * fd_array[NR_OPEN_DEFAULT];

};


Opening and Closing#include <fcntl.h>#include <unistd.h>

int open(char *path, int oflag);/*Common flags: O_RDONLY, O_WRONLY, O_CREAT Returns FD on success, -1 on failure

*/

int close(int filedes);/* Returns 0 on success, -1 on failure */


Reading and Writing#include <unistd.h>

ssize_t read(int fd, void *buf, size_t nbytes);/* Returns # bytes read, 0 if EOF, -1 error */

ssize_t write(int fd, void *buf, size_t nbytes);/* Returns # bytes written, -1 on error */

off_t lseek(int fd, off_t offset, int whence)/*

Whence: SEEK_SET (f/ 0) or SEEK_CUR (f/ current)Returns new file offset, -1 on error

*/


Virtual Filesystem


Classes of Filesystems

• Disk-based filesystems– Linux (ext2, ext3, reiserfs)– UNIX (ufs, minix, JFS, XFS)– MS (FAT, VFAT, NTFS) and other proprietary– ISO9660 CD-ROM, UDF DVD

• Network filesystems– NFS, AFS, CIFS

• Special filesystems– Procfs, sysfs


VFS Objects

• superblock– Represents a mounted filesystem.

• inode– Represents a specific file.

• dentry– Represents a directory entry, a single path comp.

• file– Represents an open file associated w/ a process.


VFS Objects

• Directories are files– Directories are handled by file objects.– dentry objects are path components, not dirs.

• Each object contains an operations object– Define methods kernel invokes on object.– Defined as struct of function pointers.

• What if a fs doesn’t have a type of object?– Objects of that type made on the fly for VFS.


VFS Objects


struct super_blockType Field Description

struct list_head s_list Superblock linked list

dev_t s_dev Device identifier

u_long s_blocksize Block size in bytes

u_char s_dirt Dirty (modified) flag

struct

super_operations *

s_op Superblock methods

struct semaphore s_lock Superblock semaphore

struct list_head s_inodes List of all inodes

struct list_head s_io Inodes waiting for write

struct list_head s_files List of file objects


struct inodeType Field Description

struct list_head i_list Inode linked list

struct list_head i_dentry List of dentries to this inode

inode_operations *i_op Inode methods

u_long i_ino Inode number

atomic_t i_count Reference count

umode_t i_mode ACL for file

u_long i_nlink Number of hard links

uid_t,gid_t i_{uid,gid} UID and GID of owner

loff_t i_size File size in bytes

struct timespec i_[amc]time Last access, modify, change


Inode Operationsint create(struct inode *dir, struct dentry *dentry, int mode)struct dentry *lookup(struct inode *dir, struct dentry *dentry)int link(struct dentry *old, struct inode *dir, struct dentry

*dentry)int unlink(struct inode *dir, struct dentry *dentry)int symlink(struct inode *dir, struct dentry *dentry, const char

*symname)int mkdir(struct inode *dir, struct dentry *dentry, int mode)int rmdir(struct inode *dir, struct dentry *dentry)int rename(struct inode *old_dir, struct dentry *old, struct

inode *new_dir, struct dentry *new) int readlink(struct dentry *dentry, char *buffer, int buflen)


Dentry Objects

• Path components– Ex: /bin/vi has 3 dentries: /, etc, and vi

• Not an on-disk data structure– Constructed on fly from pathname string.– Cached by the kernel to avoid re-construction.

• States– Used: corresponds to valid inode in use.– Unused: valid inode not currently used (cached).– Negative: invalid path, no corresponding inode


struct dentryType Field Description

atomic_t d_count Usage count

spinlock_t d_lock Lock for this dentry

struct inode *d_inode Corresponding inode

struct

dentry_operations

*d_op Dentry methods

struct list_head d_lru List of unused dentries.

struct list_head d_child Dir dentries of same parent.

struct list_head d_subdirs Subdirectory dentries.

struct list_head d_alias Dentries w/ same inode.


struct fileType Field Description

struct list_head f_list Linked list of file objects

struct dentry *f_dentry Associated dentry object

atomic_t f_count Usage count

u_int f_flags Flags specified on open()

struct

file_operations *

f_op File methods (open, read, write, readdir, ioctl)

mode_t f_mode File access mode

loff_t f_pos File offset

u_int f_{uid,gid} User’s UID and GID

int f_error Error code


References1. Daniel P. Bovet and Marco Cesati, Understanding the

Linux Kernel, 3rd edition, O’Reilly, 2005.2. Robert Love, Linux Kernel Development, 2nd edition,

Prentice-Hall, 2005.3. Claudia Rodriguez et al, The Linux Kernel Primer,

Prentice-Hall, 2005.4. Peter Salzman et. al., Linux Kernel Module Programming

Guide, version 2.6.1, 2005.5. Avi Silberchatz et. al., Operating System Concepts, 7th

edition, 2004.6. Andrew S. Tanenbaum, Modern Operating Systems, 3rd

edition, Prentice-Hall, 2005.

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