378-LKP: Lecture 14

Linux File System

Mar 8, 2002
Yongguang Zhang

Mid-Term Surveys

Text book, project
- 2.2 vs 2.4
- Linux vs UML
Use more life example in class
- Point to the code itself
Teach with real examples
- For example, do a real device driver in class

Kernel Helper

Kernel source code cross-reference
- http://lxr.linux.no/source/
- Any volunteer to install one (2.4.9-5um) locally for our class?

Linux File System

VFS: a layer on top of all file systems
- So that they have the same interface to rest of kernel
Real device-based filesystems
- Build on top of a block device (disk, storage, ...)
- Example: EXT2, VFAT, ...
Network filesystems
- Special module to deal with network protocols
- Example: NFS, SMB, ...
Virtual/special filesystems
- /proc, /dev (devfs)

Using File Systems in UML (1)

You need a block device
- In UML: a block device can be emulated by a file
- Create a file (say, 4M in size)
```
dd if=/dev/zero of=new_fs bs=1M count=4
```

Run UML with the new block device

With command line argument ubd1=
```
./linux ubd0=... ubd1=new_fs ...
```

Look at /dev/ubd/:

usermode:~# ls -l /dev/ubd
total 0
brw-rw----    1 root     root      98,   0 Dec 31  1969 0
brw-rw----    1 root     root      98,   1 Dec 31  1969 1

Using File Systems in UML (2)

Create a new file system on the new block device
- Make a MSDOS file system:
```
/projects/cs378.ygz/bin/mkdosfs -S 1024 -v /dev/ubd/1 
```
- Make a MINIX file system
```
mkfs.minix /dev/ubd/1
```
- Make a EXT2 file system
```
mkfs.ext2 /dev/ubd/1
```
- Beware of disaster: don't ever mistype as /dev/ubd/0!

Using File Systems in UML (3)

You need a mount point
- Usually under /mnt
```
mkdir /mnt/test
```
Mount the new block device (with filesystem)
- Give the block device file, and the mount point
```
mount /dev/ubd/1 /mnt/test
```
Sysadmin tools for file systems
- mount
- umount /mnt/test

VFS Common File Model

Four basic object types
- superblock: about the whole file system
- inode: about a particular file
- dentry: about a directory (name to inode mapping)
- file: about an open file handle
Service all filesystem-related system calls

VFS Data Structures

In include/linux/fs.h
- Superblock: struct super_block
- Inode: struct inode
- File: struct file
In include/linux/dcache.h
- Dentry: struct dentry
Each object type
- Include a set of operations on the object
```
struct xxxxx_operations
```

To Start

Fields in task_struct
struct fs_struct *fs
- Defined in include/linux/fs_struct.h
- Has the following fields:
```
struct dentry * root, * pwd
```
struct files_struct *files
- Defined in include/linux/fs_struct.h
- Has the following fields:
```
struct file ** fd
```
To get to an open file in a process: ->files->fd[i]

To follow a system call

sys_open
sys_read

Next Lecture

Week after Spring Break: Networking
- Review: "Linux Networking-concepts" (URL: http://netfilter.samba.org/documentation/HOWTO/networking-concepts-HOWTO.html)

© 2002 Yongguang Zhang