1. Linux Virtual Filesystem
Chris Gill and James Orr
CSE 422S - Operating Systems Organization
Washington University in St. Louis
St. Louis, MO 63130

2. CSE 422S – Operating Systems Organization
Linux VFS Layer
Kernel-wide filesystem view
Superblock (struct super_block) describes a filesystem, provides operations the OS can use
Each superblock has a list (in its s_list field) of superblocks for all available file systems
It also keeps lists of index nodes, directory entries, files
Process-specific filesystem view
Process’ task_struct points to 3 filesystem structs
Field fs points to a filesystem structure (struct fs_struct)
Field files points to open files table (struct files_struct)
Field nsproxy points to a wrapper for its namespace, etc.
Each of these has further structure we will look at next
CSE 422S – Operating Systems Organization

3. Linux Process’ Views of Filesystem
Fields in the process’ task_struct point to key filesystem structs
The new nsproxy field points to a nsproxy struct that wraps a pointer to the filesystem’s namespace (mnt_namespace)
The files field points to a table of process’ open files
The fs field points to a filesystem structure that points to paths for the process’ root and current working directories
current
task_struct
nsproxy fs
files
nsproxy
files_struct
mnt_namespace
fs_struct
root
pwd
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4. Paths and Directory Entries
Each path points to a VFS mount structure (vfs_mount) and a directory entry (dentry) structure (filesystem+directory)
A VFS mount structure describes a specific filesystem instance (a “mount point”)
A directory entry structure describes any element in the filesystem (directory or file, though in a path it’s a directory)
current
task_struct fs
fs_struct
root
pwd
path
mnt
dentry
Vfs_mount
dentry
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5. Directory Entries and Index Nodes
Each directory entry points to an associated index node (inode) that contains metadata about the directory entry
Last access time, permissions, etc.
E.g., what you see with ls command
Index nodes also implement operations the superblock uses
Each directory entry also has a character array field (d_iname) that contains its name
dentry
‘/’ \0
d_iname
...
d_inode
inode
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6. Directory Entry Hierarchy
A directory’s dentry lists other directory entries that it contains
Stored in its d_subdirs field
List iteration requires special operations (see LKD Chapter 6)
E.g., list_empty(), list_for_each(), list_for_each_entry(), etc.
Can use those operations to traverse a path (or an entire filesystem) recursively via the directory entry hierarchy
dentry
d_iname
‘/’ \0
...
d_subdirs
...
dentry
d_iname
‘b’
‘i’
‘n’ \0
...
d_subdirs
...
...
dentry
d_iname
‘l’
‘s’ \0
...
d_subdirs (empty)
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7. Studio Exercises Today
Create a kernel module with a single thread
Runs in kernel space, in process context
Has access to filesystem, but details differ a bit from the view a user-space process has
Kernel thread can get PID for a user-space process, get its task_struct and see its filesystem view
Explore different file system data structures
Working from process to filesystem structure to root and working directory paths, to dentrys, etc.
Looking at directory entries specifically as you go
CSE 422S – Operating Systems Organization