Linux Virtual Filesystem презентация

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

Linux Process’ Views of Filesystem
CSE 422S – Operating Systems Organization
files_struct
task_struct
nsproxy
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
fs_struct
nsproxy
mnt_namespace
current
fs
files
root
pwd

Paths and Directory Entries
CSE 422S – Operating Systems Organization
task_struct
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)
fs_struct
current
fs
root
pwd
path
dentry
mnt
dentry
Vfs_mount

Directory Entries and Index Nodes
CSE 422S – Operating Systems Organization
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
inode
d_inode
dentry
d_iname
‘/’
\0
...

Directory Entry Hierarchy
CSE 422S – Operating Systems Organization
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’
...
d_subdirs
...
‘n’
\0
...
dentry
d_iname
‘l’
‘s’
...
d_subdirs (empty)
\0