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update LKD 13.md
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firmianay committed May 27, 2017
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Expand Up @@ -157,3 +157,139 @@ The following are some of the superblock operations that are specified by `super


## The Inode Object
The inode Object represents all the information needed by the kernel to manipulate a file or directory. For Unix-style filesystems, this information is simply read from the on-disk inode.

The inode object is represented by `struct inode` and is defined in `<linux/fs.h>`.
```
struct inode {
struct hlist_node i_hash; /* hash list */
struct list_head i_list; /* list of inodes */
struct list_head i_sb_list; /* list of superblocks */
struct list_head i_dentry; /* list of dentries */
unsigned long i_ino; /* inode number */
atomic_t i_count; /* reference counter */
unsigned int i_nlink; /* number of hard links */
uid_t i_uid; /* user id of owner */
gid_t i_gid; /* group id of owner */
kdev_t i_rdev; /* real device node */
u64 i_version /* versioning number */
loff_t i_size /* file size in bytes */
seqcount_t i_size_seqcount; /* serializer for i_size */
struct timespec i_atime; /* last access time */
struct timespec i_mtime; /* last modify time */
struct timespec i_ctime; /* last change time */
unsigned int i_blkbits; /* block size in bits */
blkcnt_t i_blocks; /* block size in bits */
unsigned short i_bytes; /* bytes consumed */
umode_t i_mode; /* access permissions */
spinlock_t i_lock; /* spinlock */
struct rw_semaphore i_alloc_sem; /* nests inside of i_sem */
struct semaphore i_sem; /* inode semaphore */
struct inode_operations *i_op; /* inode ops table */
struct file_operations *i_fop; /* default inode ops */
struct super_block *i_sb; /* associated superblock */
struct file_lock *i_flock; /* file lock list */
struct address_space *i_mapping; /* associated mapping */
struct address_space i_data; /* mapping for device */
struct dquot *i_dquot[MAXQUOTAS]; /* disk quotas for inode */
struct list_head i_devices; /* list of block devices */
union {
struct pipe_inode_info *i_pipe; /* pipe information */
struct block_device *i_bdev; /* block device driver */
struct cdev *i_cdev; /* character device driver */
};
unsigned long i_dnotify_mask; /* directory notify mask */
struct dnotify_struct *i_dnotify; /* dnotify */
struct list_head inotify_watches; /* inotify watches */
struct mutex inotify_mutex; /* protects inotify_watches */
unsigned long i_state; /* state flags */
unsigned long dirtied_when; /* first dirtying time */
unsigned int i_flags; /* filesystem flags */
atomic_t i_writecount; /* count of writers */
void *i_security; /* security module */
void *i_private; /* fs private pointer */
};
```
An inode represents each file on a filesystem, but the inode object is constructed in memory only as file are accessed.


## Inode Operations
Inode Operations are invoked via:
```
i->i_op->truncate(i)
```
`i` is a reference to a particular inode, the `truncate()` operation defined by the filesystem on which `i` exists is called on the given inode.

The `inode_operations` structure is defined in `<linux/fs.h>`:
```
struct inode_operations {
int (*create) (struct inode *, struct dentry *, int, struct nameidata *);
struct dentry * (*lookup) (struct inode *, struct dentry *, struct nameidata *);
int (*link) (struct dentry *, struct inode *, struct dentry *);
int (*unlink) (struct inode *, struct dentry *);
int (*symlink) (struct inode *, struct dentry *, const char *);
int (*mkdir) (struct inode *, struct dentry *, int);
int (*rmdir) (struct inode *, struct dentry *);
int (*mknod) (struct inode *, struct dentry *, int, dev_t);
int (*rename) (struct inode *, struct dentry *, struct inode *, struct dentry *);
int (*readlink) (struct dentry *, char __user *, int);
void * (*follow_link) (struct dentry *, struct nameidata *);
void (*put_link) (struct dentry *, struct nameidata *, void *);
void (*truncate) (struct inode *);
int (*permission) (struct inode *, int);
int (*setattr) (struct dentry *, struct iattr *);
int (*getattr) (struct vfsmount *mnt, struct dentry *, struct kstat *);
int (*setxattr) (struct dentry *, const char *, const void *, size_t, int);
ssize_t (*getxattr) (struct dentry *, const char *, void *, size_t);
ssize_t (*listxattr) (struct dentry *, const char *);
int (*removexattr) (struct dentry *, const char *);
void (*truncate_range) (struct inode *, loff_t, loff_t);
long (*fallocate) (struct inode *inode, int mode, loff_t offset, loff_t len);
int (*fiemap) (struct inode *, struct fiemap_extent_info *, u64 start, u64 len);
};
```

The following interfaces constitute the various functions that the VFS may perform, or ask a specific filesystem to perform, on a given inode:
- `int create(struct inode *dir, struct dentry *dentry, int mode)`
- TheVFS calls this function from the `creat()` and `open()` system calls to create a new inode associated with the given dentry object with the specified initial access mode.
- `struct dentry * lookup(struct inode *dir, struct dentry *dentry)`
- This function searches a directory for an inode corresponding to a filename specified in the given dentry.
- `int link(struct dentry *old_dentry, struct inode *dir, struct dentry *dentry)`
- Invoked by the link() system call to create a hard link of the file `old_dentry` in the directory dir with the new filename `dentry`.
- `int unlink(struct inode *dir, struct dentry *dentry)`
- Called from the `unlink()` system call to remove the inode specified by the directory entry dentry from the directory `dir`.
- `int symlink(struct inode *dir, struct dentry *dentry, const char *symname)`
- Called from the `symlink()` system call to create a symbolic link named symname to the file represented by dentry in the directory `dir`.
- `int mkdir(struct inode *dir, struct dentry *dentry, int mode)`
- Called from the `mkdir()` system call to create a new directory with the given initial mode.
- `int rmdir(struct inode *dir, struct dentry *dentry)`
- Called by the `rmdir()` system call to remove the directory referenced by dentry from the directory `dir`.
- `int mknod(struct inode *dir, struct dentry *dentry, int mode, dev_t rdev)`
- Called by the `mknod()` system call to create a special file (device file, named pipe, or socket). The file is referenced by the device rdev and the directory entry dentry in the directory `dir`. The initial permissions are given via `mode`.
- `int rename(struct inode *old_dir, struct dentry *old_dentry, struct inode *new_dir, struct dentry *new_dentry)`
- Called by the VFS to move the file specified by `old_dentry` from the `old_dir` directory to the directory `new_dir`, with the filename specified by `new_dentry`.
- `int readlink(struct dentry *dentry, char *buffer, int buflen)`
- Called by the `readlink()` system call to copy at most `buflen` bytes of the full path associated with the symbolic link specified by `dentry` into the specified buffer.
- `int follow_link(struct dentry *dentry, struct nameidata *nd)`
- Called by the VFS to translate a symbolic link to the inode to which it points. The link pointed at by `dentry` is translated, and the result is stored in the `nameidata` structure pointed at by `nd`.
- `int put_link(struct dentry *dentry, struct nameidata *nd)`
- Called by the VFS to clean up after a call to `follow_link()`.
- `void truncate(struct inode *inode)`
- Called by the VFS to modify the size of the given file. Before invocation, the inode’s `i_size` field must be set to the desired new size.
- `int permission(struct inode *inode, int mask)`
- Checks whether the specified access mode is allowed for the file referenced by `inode`. This function returns zero if the access is allowed and a negative error code otherwise. Most filesystems set this field to `NULL` and use the generic VFS method, which simply compares the mode bits in the inode’s objects to the given mask.
- `int setattr(struct dentry *dentry, struct iattr *attr)`
- Called from `notify_change()` to notify a “change event” after an inode has been modified.
- `int getattr(struct vfsmount *mnt, struct dentry *dentry, struct kstat *stat)`
- Invoked by the VFS upon noticing that an inode needs to be refreshed from disk. Extended attributes allow the association of key/values pairs with files.
- `int setxattr(struct dentry *dentry, const char *name, const void *value, size_t size, int flags)`
- Used by the VFS to set the extended attribute name to the value value on the file referenced by dentry .
- `ssize_t getxattr(struct dentry *dentry, const char *name, void *value, size_t size)`
- Used by the VFS to copy into value the value of the extended attribute `name` for the specified file.
- `ssize_t listxattr(struct dentry *dentry, char *list, size_t size)`
- Copies the list of all attributes for the specified file into the buffer `list`.
- `int removexattr(struct dentry *dentry, const char *name)`
- Removes the given attribute from the given file.


## The Dentry Object

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