Data Structures Associated with Filesystems

In addition to the fundamental VFS objects, the kernel uses other standard data structures to manage data related to filesystems. The first object is used to describe a specific variant of a filesystem, such as ext3 or XFS. The second data structure is used to describe a mounted instance of a filesystem.

Because Linux supports so many different filesystems, the kernel must have a special structure for describing the abilities and behavior of each filesystem.

struct file_system_type {
        const char              *name;     /* filesystem's name */
        struct subsystem        subsys;    /* sysfs subsystem object */
        int                     fs_flags;  /* filesystem type flags */

        /* the following is used to read the superblock off the disk */
        struct super_block      *(*get_sb) (struct file_system_type *, int,
                                            char *, void *);

        /* the following is used to terminate access to the superblock */
        void                    (*kill_sb) (struct super_block *);

        struct module           *owner;    /* module owning the filesystem */
        struct file_system_type *next;     /* next file_system_type in list */
        struct list_head        fs_supers; /* list of superblock objects */
};

The get_sb() function is used to read the superblock from the disk and populate the superblock object when the filesystem is loaded. The remaining functions describe the filesystem's properties.

There is only one file_system_type per filesystem, regardless of how many instances of the filesystem are mounted on the system, or whether the filesystem is even mounted at all.

Things get more interesting when the filesystem is actually mounted, at which point the vfsmount structure is created. This structure is used to represent a specific instance of a filesystemin other words, a mount point.

The vfsmount structure is defined in <linux/mount.h>. Here it is:

struct vfsmount {
        struct list_head   mnt_hash;        /* hash table list */
        struct vfsmount    *mnt_parent;     /* parent filesystem */
        struct dentry      *mnt_mountpoint; /* dentry of this mount point */
        struct dentry      *mnt_root;       /* dentry of root of this fs */
        struct super_block *mnt_sb;         /* superblock of this filesystem */
        struct list_head   mnt_mounts;      /* list of children */
        struct list_head   mnt_child;       /* list of children */
        atomic_t           mnt_count;       /* usage count */
        int                mnt_flags;       /* mount flags */
        char               *mnt_devname;    /* device file name */
        struct list_head   mnt_list;        /* list of descriptors */
        struct list_head   mnt_fslink;      /* fs-specific expiry list */
        struct namespace   *mnt_namespace   /* associated namespace */
};

The complicated part of maintaining the list of all mount points is the relation between the filesystem and all the other mount points. The various linked lists in vfsmount keep track of this information.

The vfsmount structure also stores the flags, if any, specified on mount in the mnt_flags field. Table 12.1 is a list of the standard mount flags.

Table 12.1. Listing of Standard Mount Flags

Flag	Description
MNT_NOSUID	Forbids setuid and setgid flags on binaries on this filesystem
MNT_NODEV	Forbids access to device files on this filesystem
MNT_NOEXEC	Forbids execution of binaries on this filesystem

These flags are most useful on removable devices that the administrator does not trust. They are defined in <linux/mount.h>.