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iakovlev.org

Виртуальная файловая система в Линукс

Neil Brown neilb@cse.unsw.edu.au and others.

29 December 1999 - v1.6


The Linux operating system supports multiple different file-systems, including ext2 (the Second Extended file-system), nfs (the Network File-system), FAT (The MS-DOS File Allocation Table file system), and others. To enable the upper levels of the kernel to deal equally with all of these and other file-systems, Linux defines an abstract layer, known as the Virtual File-system, or vfs. Each lower level file-system must present an interface which conforms to this Virtual file-system. This document describes the vfs interface (as present in Linux 2.3.29). NOTE this document is incomplete.

2. Обьекты и методы

The Virtual File-system interface is structured around a number of generic object types, and a number of methods which can be called on these objects.

The basic objects known to the VFS layer are files, file-systems, inodes, and names for inodes.

2.1 Файлы

Files are things that can be read from or written to. They can also be mapped into memory and sometimes a list of file names can be read from them. They map very closely to the file descriptor concept that unix has. Files are represented within Linux by a struct file which has a number of methods stored in a struct file_operations.

2.2 Ноды

An inode represents a basic object within a file-system. It can be a regular file, a directory, a symbolic link, or a few other things. The VFS does not make a strong distinction between different sorts of objects, but leaves it to the actual file-system implementation to provide appropriate behaviours, and to the higher levels of the kernel to treat different objects differently.

Each inode is represented by a struct inode which has a number of methods stored in a struct inode_operations.

It may seem that Files and Inodes are very similar. They are but there are some important differences. One thing to note is that there are some things that have inodes but never have files. A good example of this is a symbolic link. Conversely there are files which do not have inodes, particularly pipes (though not named pipes) and sockets (though not UNIX domain sockets).

Also, a File has state information that an inode does not have, particularly a position, which indicates where in the file the next read or write will be performed.

2.3 Файловые системы

A file-system is a collection of inodes with one distinguished inode known as the root. Other inodes are accessed by starting at the root and looking up a file name to get to another inode.

A file-system has a number of characteristics which apply uniformly to all inodes within the file-system. Some of these are flags such as the READ-ONLY flag. Another important one is the blocksize. I'm not entirely sure why this is needed globallly.

Each file-system is represented by a struct super_block, and has a number of methods stored in a struct super_operations.

There is a strong correlation within Linux between super-blocks (and hence file-systems) and device numbers. Each file-system must (appear to) have a unique device on which the file-system resides. Some file-systems (such as nfs and proc) are marked as not needing a real device. For these, an anonymous device, with a major number of 0, is automatically assigned.

As well as knowing about file-systems, Linux VFS knows about different file-system types. Each type of file-system is represented in Linux by a struct file_system_type. This contains just one method, read_super which instantiates a super_block to represent a given file-system.

2.4 Имена

All inodes within a file-system are accessed by name. As the name-to-inode lookup process may be expensive for some file-systems, Linux's VFS layer maintains a cache of currently active and recently used names. This cache is referred to as the dcache.

The dcache is structured in memory as a tree. Each node in the tree corresponds to an inode in a given directory with a given name. An inode can be associated with more than one node in the tree.

While the dcache is not a complete copy of the file tree, it is a proper prefix of that tree (if that is a correct usage of the term). This means that if any node of the file tree is in the cache, then every ancestor of that node is also in the cache.

Each node in the tree is represented by a struct dentry which has a number of methods stored in a struct dentry_operations.

The dentries act as an intermediary between Files and Inodes. Each file points to the dentry that it has open. Each dentry points to the inode that it references. This implies that for every open file, the dentry of that file, and of all the parents of that file are cached in memory. This allows a full path name of every open file to be easily determined, as can be seen from doing:


# ls -l /proc/self/fd
 total 0
 lrwx------   1 root     root           64 Nov 23 07:51 0 -> /dev/pts/2
 lrwx------   1 root     root           64 Nov 23 07:51 1 -> /dev/pts/2
 lrwx------   1 root     root           64 Nov 23 07:51 2 -> /dev/pts/2
 lr-x------   1 root     root           64 Nov 23 07:51 3 -> /proc/15588/fd/
 

3. Регистрация и монтирование файловой системы

It is probably worth starting by observing that there is possible ambiguity in our use of the word file-system. It can be used to mean a particular type, or class, of file-system, such as ext2 or nfs or coda, or it can be used to mean a particular instance of a file-system, such as /usr or /home or The file-system on /dev/hda4.

The first usage is implied when registering a file-system, the second is implied while mounting a file-system. I will continue to use this ambiguous language as most people are familiar with it and nothing better is obvious.

Linux finds out about new file-system types by calls register_filesystem (and forgets about them by the calls to its counterpart unregister_filesystem). The formal declarations are:

#include <linux/fs.h>
 
 int register_filesystem(struct file_system_type * fs);
 int unregister_filesystem(struct file_system_type * fs);
 
The function register_filesystem returns 0 on success and -EINVAL if fs==NULL. It returns -EBUSY if either fs->next != NULL or there is already a file-system registered under the same name. It should be called (directly or indirectly) from init_module for file-systems which are being loaded as modules, or from filesystem_setup in fs/filesystems.c. The function unregister_filesystem should only be called from the cleanup_module routine of a module. It returns 0 on success and -EINVAL if the argument is not a pointer to a registered file-system. (In particular, unregister_filesystem(NULL) may Oops).

An example of file-system registration and unregistration can be seen in fs/ext2/super.c:

static struct file_system_type ext2_fs_type = {
         "ext2",
         FS_REQUIRES_DEV /* | FS_IBASKET */,     /* ibaskets have unresolved bugs */
         ext2_read_super,
         NULL
 };
 
 int __init init_ext2_fs(void)
 {
         return register_filesystem(&ext2_fs_type);
 }
 
 #ifdef MODULE
 EXPORT_NO_SYMBOLS;
 
 int init_module(void)
 {
         return init_ext2_fs();
 }
 
 void cleanup_module(void)
 {
         unregister_filesystem(&ext2_fs_type);
 }
 
 #endif
 

A struct file_system_type is defined in linux/fs.h and has the following format:


struct file_system_type {
         const char *name;
         int fs_flags;
         struct super_block *(*read_super) (struct super_block *, void *, int);
         struct file_system_type * next;
 };
 

name

The name field simply gives the name of the file-system type, such as ext2 or iso9660 or msdos. This field is used as a key, and it is not possible to register a file-system with a name that is already in use. It is also used for the /proc/filesystems file which lists all file-system types currently registered with the kernel. When a file-system is implemented as a module, the name points to the module's address space (mapped to a vmalloc'd area) which means that if you forget to unregister_filesystem in cleanup_module and try to cat /proc/filesystems/ you will get an Oops trying to dereference name - a common mistake made by file-system writers at the first stages of development..

fs_flags

A number of adhoc flags which record features of the file-system.

FS_REQUIRES_DEV

As mentioned above, every mounted file-system is connected to some device, or at least some device number. If a file-system type has FS_REQUIRES_DEV, then a real device must be given when mounting the file-system, otherwise an anonymous device is allocated.

nfs and procfs are examples of file-systems that don't require a device. ext2 and msdos do.

FS_NO_DCACHE

This flag is declared but not used at all. From the comment in fs.h the intent is that for file-systems marked this way, the dcache only keeps entries for files that are actually in use.

FS_NO_PRELIM

Like FS_NO_DCACHE, this flag is never used. The intent appears to be that the dcache will have entries that are in use or have been used, but will not speculatively cache anything else.

FS_IBASKET

Another vapour-flag. See section on ibaskets below, which may be a vapour-section.

next

next is simply a pointer for chaining all file_system_types together. It should be initialised to NULL (register_filesystem does not set it for you and will return -EBUSY if you don't set next to NULL).

read_super

The read_super method is called when a file-system (instance) is being mounted.

The struct super_block is clean (all fields zero) except for the s_dev and s_flags fields. The void * pointer points to the data what has been passed down from the mount system call. The trailing int field tells whether read_super should be silent about errors. It is set only when mounting the root file-system. When mounting root, every possible file-system is tried in turn until one succeeds. Printing errors in this case would be untidy.

read_super must determine whether the device given in s_dev together with the data from mount define a valid file-system of this type. If they do, then it should fill out the rest of the struct super_block and return the pointer. If not, it should return NULL.

4. Super-Block и его функции

Each mounted file-system is represented by the super_block structure. The fact that it is mounted is stored in a struct vfsmount, the declaration of which can be found in linux/mount.h:


struct vfsmount
 {
   kdev_t mnt_dev;                       /* Device this applies to */
   char *mnt_devname;                    /* Name of device e.g. /dev/dsk/hda1 */
   char *mnt_dirname;                    /* Name of directory mounted on */
   unsigned int mnt_flags;               /* Flags of this device */
   struct super_block *mnt_sb;           /* pointer to superblock */
   struct quota_mount_options mnt_dquot; /* Diskquota specific mount options */
   struct vfsmount *mnt_next;            /* pointer to next in linkedlist */
 };
 

These vfsmount structures are linked together in a simple linked list starting from vfsmntlist in fs/super.c. This list is mainly used for finding mounted file-system information given a device, particularly be the disc quota code.

The reason why vfsmount is kept separate from the list of super blocks super_blocks is because if the super-block already exists then fs/super.c:read_super() is satisfied by fs/super.c:get_super() instead of going through the read_super file-system-specific method. But the entry in vfsmntlist is unlinked as soon as the file-system is unmounted.

Each mount is also recorded in the dcache which will be described later, and this is the source of mount information used when traversing path names.

4.1 Структура суперблока

A somewhat reduced description of the super-block structure is:


struct super_block {
         struct list_head        s_list;         /* Keep this first */
         kdev_t                  s_dev;
         unsigned long           s_blocksize;
         unsigned char           s_blocksize_bits;
         unsigned char           s_lock;
         unsigned char           s_dirt;
         struct file_system_type *s_type;
         struct super_operations *s_op;
         struct dquot_operations *dq_op;
         unsigned long           s_flags;
         unsigned long           s_magic;
         struct dentry           *s_root;
         wait_queue_head_t       s_wait;
 
         struct inode            *s_ibasket;
         short int               s_ibasket_count;
         short int               s_ibasket_max;
         struct list_head        s_dirty;        /* dirty inodes */
         struct list_head        s_files;
 
         union {
                 /* Configured-in filesystems get entries here */
                 void                    *generic_sbp;
         } u;
         /*
          * The next field is for VFS *only*. No filesystems have any business
          * even looking at it. You had been warned.
          */
         struct semaphore s_vfs_rename_sem;      /* Kludge */
 };
 

See linux/fs.h for a complete declaration which includes all file-system-specific components of the union u which were suppressed above. The various fields in the super-block are:

s_list

A doubly linked list of all mounted file-systems (see linux/list.h).

s_dev

The device (possibly anonymous) that this file-system is mounted on.

s_blocksize

The basic blocksize of the file-system. I'm not sure exactly how this is used yet. It must be a power of 2.

s_blocksize_bits

The power of 2 that s_blocksize is (i.e. log2(s_blocksize)).

s_lock

This indicates whether the super-block is currently locked. It is managed by lock_super and unlock_super.

lock_kernel.

s_wait

This is a queue of processes that are waiting for the s_lock lock on the super-block.

s_dirt

This is a flag which gets set when a super-block is changed, and is cleared whenever the super-block is written to the device. This happens when a filesystem is unmounted, or in response to a sync system call.

s_type

This is simply a pointer to the struct file_system_type structure discussed above.

s_op

This is a pointer to the struct super_operations which will be described next.

dq_op

This is a pointer to Disc Quota operations which will be described later.

s_flags

This is a list of flags which are logically ored with the flags in each inode to determine certain behaviours. There is one flag which applies only to the whole file-system, and so will be described here. The others are described under the discussion on inodes.

MS_RDONLY

A file-system with the flag set has been mounted read-only. No writing will be permitted, and no indirect modification, such as mount times in the super-block or access times on files, will be made.

s_magic

This records an identification number that has been read from the device to confirm that the data on the device corresponds to the file-system in question. It seems to be used by the Minix file-system to distinguish between various flavours of that file-system. It is not clear why this is in the generic part of the structure, and not confined to the file-system specific part for those file-systems which need it. Maybe this is historical.

The one interesting usage of the field is in fs/nfsd/vfs.c:nfsd_lookup() where it is used to make sure that a proc or nfs type file-system is never accessed via NFS.

s_root

This is a stuct dentry which refers to the root of the file-system. It is normally created by loading the root inode from the file-system, and passing it to d_alloc_root. This dentry will get spliced into the dcache by the mount command (do_mount calls d_mount).

s_ibasket, s_ibasket_count, s_ibasket_max

These three refer to a basket of inodes I guess, but there is no such thing in current versions.

s_dirty

A list of dirty inodes linked on the i_list field.

When an inode is marked as dirty with mark_inode_dirty it gets put on this list. When sync_inodes is called, any inode in this list gets passed to the file-system's write_inode method.

s_files

This is a list of files (linked on f_list) of open files on this file-system. It is used, for example, to check if there are any files open for write before remounting the file-system as read-only.

u.generic_sbp

The u union contains one file-system-specific super-block information structure for each file-system known about at compile time. Any file-system loaded as a module must allocate a separate structure and place a pointer in u.generic_sbp.

s_vfs_rename_sem

This semaphore is used as a file-system wide lock while renaming a directory. This appears to be to guard against possible races which may end up renaming a directory to be a child of itself. This semaphore is not needed or used when renaming things that are not directories.

4.2 Методы суперблока

The methods defined in the struct super_operations are:


struct super_operations {
         void (*read_inode) (struct inode *);
         void (*write_inode) (struct inode *);
         void (*put_inode) (struct inode *);
         void (*delete_inode) (struct inode *);
         int (*notify_change) (struct dentry *, struct iattr *);
         void (*put_super) (struct super_block *);
         void (*write_super) (struct super_block *);
         int (*statfs) (struct super_block *, struct statfs *, int);
         int (*remount_fs) (struct super_block *, int *, char *);
         void (*clear_inode) (struct inode *);
         void (*umount_begin) (struct super_block *);
 };
 

All of these methods get called with only the kernel lock held. This means that they can safely block, but are responsible from guarding against concurrent access themselves. All are called from a process context, not from interrupt handlers or the bottom half.

read_inode

This method is called to read a specific inode from a mounted file-system. It is only called from get_new_inode out of iget in fs/inode.c.

In the struct inode * argument passed to this method the fields i_sb, i_dev and particularly i_ino will be initialised to indicate which inode should be read from which file-system. It must set (among other things) the i_op field of struct inode to point to the relevant struct inode_operations so that VFS can call the methods on this inode as needed.

iget is mostly called from within particular file-systems to read inodes for that file-system. One notable exception is in fs/nfsd/nfsfh.h where it is used to get an inode based on information in the nfs file handle.

It is not clear that this method needs to be exported as (with the exception of nfsd) it is only (indirectly) used by the file-system which provides it. Avoiding it would allow more flexibility than a simple 32bit inode number to identify a particular inode.

The nfsd usage could better be replaced by an interface that takes a file handle (or part there-of) and returns an inode.

write_inode

This method gets called on inodes which have been marked dirty with mark_inode_dirty. It is called when a sync request is made on the file, or on the file-system. It should make sure that any information in the inode is safe on the device.

put_inode

If defined, this method is called whenever the reference count on an inode is decreased. Note that this does not mean that the inode is not in use any more, just that it has one fewer users.

put_inode is called before the i_count field is decreased, so if put_inode wants to check if this is the last reference, it should check if i_count is 1 or not.

Almost all file-systems that define this method use it to do some special handling when the last reference to the inode is release. i.e. when i_count is 1 and is about to be come zero.

delete_inode

If defined, delete_inode is called whenever the reference count on an inode reaches 0, and it is found that the link count (i_nlink) is also zero. It is presumed that the file-system will deal with this situation be invalidating the inode in the file-system and freeing up any resourses used.

It could be argued that this and the previous methods should be replaced by one method that is called whenever the i_count field reaches 0, and then the file-system gets to decide if it should do something special with i_nlink being 0. The only difficulty that this might cause with current file-systems is that ext2 calls ext2_discard_prealloc when put_inode is called, independently of i_count. This would no longer be possible. But is this even desirable? Would it not make more sense to do this only in ext2_release_file (which does it as well).

notify_change

This is called when inode attributes are changed, the argument struct iattr * pointing to the new set of attributes. If the file-system does not define this method (i.e. it is NULL) then VFS uses the routine fs/iattr.c:inode_change_ok which implements POSIX standard attributes verification. Then VFS marks the inode as dirty. If the file-system implements its own notify_change then it should call mark_inode_dirty(inode) after it has set the attributes. An example of how to implement this method can be seen in fs/ext2/inode.c:ext2_notify_change().

put_super

This is called at the last stages of umount(2) system call, before removing the entry from vfsmntlist. This method is called with super-block lock held. A typical implementation would free file-system-private resources specific for this mount instance, such as inode bitmaps, block bitmaps, a buffer header containing super-block and decrement module hold count if the file-system is implemented as a dynamically loadable module. For example, fs/bfs/inode.c:bfs_put_super() looks very simple:


static void bfs_put_super(struct super_block *s)
 {
         brelse(s->su_sbh);
         kfree(s->su_imap);
         kfree(s->su_bmap);
         MOD_DEC_USE_COUNT;
 }
 

write_super

Called when VFS decides that the super-block needs to be written to disk. Called from fs/buffer.c:file_fsync, fs/super.c:sync_supers and fs/super.c:do_umount. Obviously not needed for a read-only file-system.

statfs

This method is needed to implement statfs(2) system call and is called from fs/open.c:sys_statfs if implemented, otherwise statfs(2) will fail with errno set to ENODEV.

remount_fs

Called when file-system is being remounted, i.e. if the MS_REMOUNT flag is specified with the mount(2) system call. This can be used to change various mount options without unmounting the file-system. A common usage is to change a readonly file-system into a writable file-system.

clear_inode

Optional method, called when VFS clears the inode. This is needed (at least) by any file-system which attaches kmalloced data to the inode structure, as particularly might be the case for file-systems using the generic_ip field in struct inode.

It is currently used by ntfs which does attach kalloced data to an inode, and by fat which does interesting things to present a pretense of stable inode numbers on a file-system which does not support inode numbers.

umount_begin

This method is called early in the unmounting process if the MNT_FORCE flag was given to umount. The intentions is that it should cause any incomplete transaction on the file-system to fail quickly rather than block waiting on some external event such as a remote server responding.

Note that calling umount_begin will probably not make an active file-system become unmountable, but it should allow any processes using that file-system to be killable, rather than being in an uninterruptible wait.

Currently, NFS is the only file-system which provides umount_begin.

5. Файл

A file object is used where-ever there is a need to read from or write to something. This includes accessing objects within file-system, communicating through a pipe, or over a network. Files are accessible to processes through their file descriptors.

5.1 Структура файла

The file structure is defined in linux/fs.h to be:


struct fown_struct {
         int pid;                /* pid or -pgrp where SIGIO should be sent */
         uid_t uid, euid;        /* uid/euid of process setting the owner */
         int signum;             /* posix.1b rt signal to be delivered on IO */
 };
 
 struct file {
         struct list_head        f_list;
         struct dentry           *f_dentry;
         struct file_operations  *f_op;
         atomic_t                f_count;
         unsigned int            f_flags;
         mode_t                  f_mode;
         loff_t                  f_pos;
         unsigned long           f_reada, f_ramax, f_raend, f_ralen, f_rawin;
         struct fown_struct      f_owner;
         unsigned int            f_uid, f_gid;
         int                     f_error;
 
         unsigned long           f_version;
 
         /* needed for tty driver, and maybe others */
         void                    *private_data;
 };
 

The fields have the following meaning:

f_list

This field links files together into one of a number of lists. There is one list for each active file-system, starting at the s_files pointer in the super-block. There is one for free file structures (free_list in fs/file_table.c). And there is one for anonymous files (anon_list in fs/file_table.c) such as pipes.

f_dentry

This field records the dcache entry that points to the inode for this file. If the inode refers to an object, such as a pipe, which isn't in a regular file-system, the dentry is a root dentry created with d_alloc_root.

f_op

This field points to the methods to use on this file.

f_count

The number of references to this file. One for each different user-process file descriptor, plus one for each internal usage.

f_flags

This field stores the flags for this file such as access type (read/write), nonblocking, appendonly etc. These are defined in the per-architecture include file asm/fcntl.h. Some of these flags are only relevant at the time of opening, and are not stored in f_flags. These excluded flags are O_CREAT, O_EXCL, O_NOCTTY, O_TRUNC. This list is from filp_open in fs/open.c.

f_mode

The bottom two bits of f_flags encode read and write access in a way that it is not easy to extract the individual read and write access information. f_mode stores the read and write access as two separate bits.

f_pos

This records the current file position which will be the address used for the next read request, and for the next write request if the file does NOT have the O_APPEND flag.

f_reada, f_remax, f_raend, f_ralen, f_rawin

These five fields are used to keeping track of sequential access patterns on the file, and determining how much read-ahead to do. There may be a separate section on read-ahead.

f_owner

This structure stores a process id and a signal to send to the process when certain events happen with the file, such as new data being available. Currently, keyboards, mice, serial ports and network sockes seem to be the only files which is this feature (via kill_fasync).

f_uid, f_gid

These fields get set to the owner and group of the process which opened the file. They don't seem to be used at all.

f_error

This is used by the NFS client file-system code to return write errors. It is set in fs/nfs/write.c and checked in fs/nfs/file.c, and used in mm/filemap.c:generic_file_write

f_version

This field is available to be used by the underlying file-system to help cache state, and check for the cache being invalid. It is changed whenever the file has its f_pos value changed.

For example, the ext2 file-system uses it in conjuction with the i_version field in the inode to detect when a directory may have changed. If neither the directory nor the file position has changed, then ext2 can be sure that the current file position is the start of a valid directory entry, otherwise it much re-check from the start of the block.

private_data

This is used by many device drivers, and even a few file-systems, to store extra per-open-file information (such as credentials in coda).

5.2 Функции файла

The list of file methods are defined in linux/fs.h to be:


typedef int (*filldir_t)(void *, const char *, int, off_t, ino_t);
 
 struct file_operations {
         loff_t (*llseek) (struct file *, loff_t, int);
         ssize_t (*read) (struct file *, char *, size_t, loff_t *);
         ssize_t (*write) (struct file *, const char *, size_t, loff_t *);
         int (*readdir) (struct file *, void *, filldir_t);
         unsigned int (*poll) (struct file *, struct poll_table_struct *);
         int (*ioctl) (struct inode *, struct file *, unsigned int, unsigned long);
         int (*mmap) (struct file *, struct vm_area_struct *);
         int (*open) (struct inode *, struct file *);
         int (*flush) (struct file *);
         int (*release) (struct inode *, struct file *);
         int (*fsync) (struct file *, struct dentry *);
         int (*fasync) (int, struct file *, int);
         int (*check_media_change) (kdev_t dev);
         int (*revalidate) (kdev_t dev);
         int (*lock) (struct file *, int, struct file_lock *);
 };
 

llseek

This implements the lseek system call. If it is left undefined, then default_llseek from fs/read_write.c is used instead. This updates the f_pos field as expected, and also may change the f_reada field and f_version field.

read

This is used to implement the read system call and to support other occasions for reading files such a loading executables and reading the quotas file. It is expected to update the offset value (last argument) which is usually a pointer to the f_pos field in the file structure, except for the pread and pwrite system calls.

For file-systems on block devices, there is a routine generic_file_read in mm/filemap.c which can be used for this method providing that the inode has a readpage method defined.

write

This method allows writing to a file such as when using the write system call. This method does not necessarily make sure that the data has reached the device, but may only queue it ready for writing when convenient, depending on the semantics of the file type.

For file-systems on block devices, generic_file_write may be used in conjunction with block_write_partial_page from fs/buffer.c to implement this method.

readdir

readdir should read directory entries from the file, which would presumably be a directory, and return them using the filldir_t callback function. This function takes the void * handle that was passed along with a pointer to a name, the length of the name, the postion in the file where this name was found, and the inode number associated with the name.

If the filldir call-back returns non-zero, then readdir should assume that it has had enough, and should return as well.

When readdir reaches the end of the directory, it should return with the value 0. Otherwise it may return after just some of the entres have been given to filldir. In this case is should return a non-zero value. It should return a negative number on error.

poll

poll is use to implement the select and poll system calls. It should add a poll_table_entry to the poll_table_struct that it is passed, and do some other stuff.... I haven't looked into this much yet.

ioctl

This implements ad hoc ioctl functionality. If an ioctl request is not one of a set of known requests (FIBMAP, FIGETBSZ, FIONREAD), then the request is passed on the underlying file implementation.

mmap

This routine implements memory mapping of files. It can often be implemented using generic_file_mmap. Its task seems to be to validate that the mapping is allowed, and to set up the vm_ops field of the vm_area_struct to point to something appropriate.

open

This method, if defined, is called when a new file has been opened in an inode. It can do any setup that may be needed on open. This is not used with many file-systems. One exception is coda which tries to get the file cached locally at open.

flush

flush is called when a file descriptor is closed. There may be other file descriptors open on this file, so it isn't necessarily a final close of the file, just an interim one. The only file-system that currently defines this method is the NFS client, which flushes out any write-behind requests that are pending.

Flush can return an error status back through the close system call, and so needs to be used if errors need to be checked for. Unfortunately, there is no way that flush can reliably determine if it is the last call to flush.

release

release is called when the last handle on a file is closed. It should do any special cleanup that is needed.

release cannot return any error status to anyone, and so should really be of type void rather than int.

fsync

This method implements the fsync and fdatasync system calls (they are currently identical). It should not return until all pending writes for the file have successfully reached the device.

fsync may be partially implemented using generic_buffer_fdatasync which will write out all dirty buffers on all mapped pages of the inode.

fasync

This method is called when the FIOASYNC flag of the file changes. The int parameter contains the new value of this flag. No file-systems currently use this method.

check_media_change

This method should check if the underlying media has changed, and should return true if it has. The only place out-side of disc drivers where it is called is in read_super when a file-system is about to be mounted. If it returns true at this point, all buffers associated with the device are invalidated.

revalidate

Revalidate is called after buffers have been invalidated after a media change, as reported by check_media_change. So it is only meaningful if check_media_change is defined. This shouldn't be confused with the inode:revalidate method which is quite different.

lock

This method allows a file service to provide extra handling of POSIX locks. It is not used for FLOCK style locks. This is useful particularly for network file-systems where other locks might be held in ways only noticeable by the file-system.

When locks are being set or removed, a lock is obtained firstly with this method, and then also with the standard posix lock code. If this method succeeds in getting a lock, but the local code fails, then the lock will never be released...

When a process is trying to find what locks are present, information returned by this method is used, the local locks are not checked.

6. Имена-dentrys

The VFS layer does all management of path names of files, and converts them into entries in the dcache before passing allowing the underlying file-system to see them. The one exception to this is the target of an symbolic link, which is passed untouched to the underlying file-system. The underlying file-system is then expected to interpret it. This seems like a slightly blurred module boundry.

The dcache is made up of lots of struct dentrys. Each dentry corresponds to one filename component in the file-system and the object associated with that name (if there is one). Each dentry references its parent which must exist in the dcache. dentrys also record file-system mounting relationships.

The dcache is a master of the inode cache. Whenever a dcache entry exists, the inode will also exist in the inode cache. Conversely whenever there is an inode in the inode cache, it will reference a dentry in the dcache.

6.1 Структура Dentry

The dentry structure is defined in linux/dcache.h.


struct qstr {
         const unsigned char * name;
         unsigned int len;
         unsigned int hash;
 };
 
 #define DNAME_INLINE_LEN 16
 
 struct dentry {
         int d_count;
         unsigned int d_flags;
         struct inode  * d_inode;        /* Where the name belongs to - NULL is negative */
         struct dentry * d_parent;       /* parent directory */
         struct dentry * d_mounts;       /* mount information */
         struct dentry * d_covers;
         struct list_head d_hash;        /* lookup hash list */
         struct list_head d_lru;         /* d_count = 0 LRU list */
         struct list_head d_child;       /* child of parent list */
         struct list_head d_subdirs;     /* our children */
         struct list_head d_alias;       /* inode alias list */
         struct qstr d_name;
         unsigned long d_time;           /* used by d_revalidate */
         struct dentry_operations  *d_op;
         struct super_block * d_sb;      /* The root of the dentry tree */
         unsigned long d_reftime;        /* last time referenced */
         void * d_fsdata;                /* fs-specific data */
         unsigned char d_iname[DNAME_INLINE_LEN]; /* small names */
 };
 

d_count

This is a simple reference count.

The count does NOT include the reference from the parent through the d_subdirs list, but does include the d_parent references from children. This implies that only leaf nodes in the cache may have a d_count of 0. These entries are linked together by the d_lru list as will be seen.

d_flags

There are currently two possible flags, both for use by specific file-system implementations (so why are they exposed?), and so will not be documented here. They are DCACHE_AUTOFS_PENDING and DCACHE_NFSFS_RENAMED.

d_inode

Simply a pointer to the inode related to this name. This field may be NULL, which indicates a negative entry, implying that the name is known not to exist.

d_parent

This will point to the parent dentry. For the root of a file-system, or for an anonymous entry like that for a file, this points back to the containing dentry itself.

d_mounts

For a directory that has had a file-system mounted on it, this points to the root dentry of that file-system. For other dentries, this points back to the dentry itself.

It is not possible to mount a file-system on a mountpoint, so there will never be a chain of d_mount entries longer than one.

d_covers

This is the inverse of d_mounts. For the root of a mounted file-system, this points to the dentry of the directory that it is mounted on. For other dentrys, this points to the dentry itself.

d_hash

This doubly linked list chains together the entries in one hash bucket.

d_lru

This provides a doubly linked list of unreferenced leaf nodes in the cache. The head of the list is the dentry_unused global variable. It is stored in Least Recently Used order.

When other parts of the kernel need to reclaim memory or inodes, which may be locked up in unused entries in the dcache, they can call select_dcache which finds removable entries in the d_lru and prepares them to be removed by prune_dcache.

d_child

This list_head is used to link together all the children of the d_parent of this dentry. One might think that d_sibling might be a better name.

d_subdirs

This is the head of the d_child list that links all the children of this dentry. Ofcourse, elements may refer to file and not just sub-directories, so d_child may be a better name, but that is already in use:-).

d_alias

As files (and some other file-system objects) may have multiple names in the file-system through multiple hard links, it is possible that multiple dentrys refer to the same inode. When this happens, the dentrys are linked on the d_alias field. The inode's i_dentry field is the head of this list.

d_name

The d_name field contains the name of this entry, together with its hash value. The name subfield may point to the d_iname field of the dentry or, if that isn't long enough, it will point to a separately allocated string.

d_time

This field is only used by underlying file-systems, which can presumably do whatever they want. The intention is to use it to record something about when this entry was last known to be valid to get some idea about when its validity might need to be checked again.

d_op

This points to the struct dentry_operations with specifics for how to handle this dentry.

d_sb

This points to the super-block of the file-system on which the object refered to by the dentry resides. It is not clear why this is needed rather than using d_inode->i_sb.

d_reftime

This is set to the current time in jiffies whenever the d_count reaches zero, but it is never used.

d_fsdata

This is available for specific file-systems to use as they wish. This is currently only used by nfs to store a file handle. (Odd that, I would have thought that the filehandle is per-inode, not per-name, but I gather some nfs servers don't agree).

d_iname

This stores the first 16 characters of the name of the file for easy reference. If the name fits completely, then d_name.name points here, otherwise it points to separately allocated memory.

6.2 Методы Dentry

Most handling of dentries is common across all file-systems, so most operations that you would expect to do on dentries do not have methods in the dentry_operations list. Rather, it provides for a few operations which may be handled in a non-obvious way by some file-system implementations. A file-system can choose to leave all of the methods as NULL, in which case the default operation will apply.

The structure definition from linclude/linux/dcache.h is:


struct dentry_operations {
         int (*d_revalidate)(struct dentry *, int);
         int (*d_hash) (struct dentry *, struct qstr *);
         int (*d_compare) (struct dentry *, struct qstr *, struct qstr *);
         void (*d_delete)(struct dentry *);
         void (*d_release)(struct dentry *);
         void (*d_iput)(struct dentry *, struct inode *);
 };
 

d_revalidate

This method is called whenever a path lookup uses an entry in the dcache, in order to see if the entry is still valid. It should return 1 if it can still be trusted, else 0. The default is to assume a return value of 1.

The int argument gives the flags relevant to this lookup, and can include any of LOOKUP_FOLLOW, LOOKUP_DIRECTORY, LOOKUP_SLASHOK, LOOKUP_CONTINUE. These will be described (if at all) under the section on namei.

This method is only needed if the file-system is likely to change without the VFS layer doing anything, as may happen with shared file systems.

If d_revalidate returns 0, the VFS layer will attempt to prune the dentry from the dcache. This is done by d_invalidate which removes any children which are not in active use and, if that was successful, unhashes the dentry.

d_hash

If the file-system has non-standard rules about valid names or name equivalence, then this routine should be provided to check for validity and return a canonical hash.

If the name is valid, a hash should be calculated (which should be the same for all equivalent names) and stored in the qstr argument. If the name is not valid, an appropriate (negative) error code should be returned.

The dentry argument is the dentry of the parent of the name in question (which is found in the qstr), as the dentry of the name will not be complete yet.

d_compare

This should compare the two qstrs (again in the context of the dentry being their parent) to see if they are equivalent. It should return 0 only if they are the same. Ordering is not important.

d_delete

This is called when the reference count reaches zero, before the dentry is placed on the dentry_unused list.

d_release

This is called just before a dentry is finally freed up. It can be used to release the d_fsdata if any.

d_iput

If defined, this is called instead of iput to release the inode when the dentry is being discarded. It should do the equivalent of iput plus anything else that it wants.

7. Ноды

Linux keeps a cache of active and recently used inodes. There are two paths by which these inodes can be accessed.

The first is through the dcache described above. Each dentry in the dcache refers to an inode, and thereby keeps that inode in the cache.

The second path is through the inode hash table. Each inode is hashed (to an 8 bit number) based on the address of the file-system's super-block and the inode number. Inodes with the same hash value are then chained together in a doubly linked list.

Access though the hash table is achieved using the iget function. iget is only called by individual file-system implementations when looking up an inode (which wasn't found in the dcache), and by nfsd.

Basing the hash on the inode number is a bit restrictive as it assumes that every file-system can uniquely identify a file in 32 bits. This is a problem at least of the NFS file-system, which would prefer to use the 256 bit file handle as the unique identifier in the hash.

The nfsd usage might be better served by having the file-system provide a filehandle-to-inode mapping function which has interpret the filehandle however is most appropriate.

7.1 Структура ноды


struct inode {
         struct list_head        i_hash;
         struct list_head        i_list;
         struct list_head        i_dentry;
 
         unsigned long           i_ino;
         unsigned int            i_count;
         kdev_t                  i_dev;
         umode_t                 i_mode;
         nlink_t                 i_nlink;
         uid_t                   i_uid;
         gid_t                   i_gid;
         kdev_t                  i_rdev;
         off_t                   i_size;
         time_t                  i_atime;
         time_t                  i_mtime;
         time_t                  i_ctime;
         unsigned long           i_blksize;
         unsigned long           i_blocks;
         unsigned long           i_version;
         unsigned long           i_nrpages;
         struct semaphore        i_sem;
         struct inode_operations *i_op;
         struct super_block      *i_sb;
         wait_queue_head_t       i_wait;
         struct file_lock        *i_flock;
         struct vm_area_struct   *i_mmap;
         struct page             *i_pages;
         spinlock_t              i_shared_lock;
         struct dquot            *i_dquot[MAXQUOTAS];
         struct pipe_inode_info  *i_pipe;
 
         
         unsigned long           i_state;
 
         unsigned int            i_flags;
         unsigned char           i_sock;
 
         atomic_t                i_writecount;
         unsigned int            i_attr_flags;
         __u32                   i_generation;
         union {
                 ....
                 struct ext2_inode_info          ext2_i;
                 ....
                 struct socket                   socket_i;
                 void                            *generic_ip;
         } u;
 };
 

Many fields in the inode structure will have an obvious meaning to anyone familiar with Unix file-systems, so they will be skipped. Here I will only deal with those specific to Linux or which have interesting usage.

i_hash

The i_hash linked list links together all inodes which hash to the same hash bucket. Hash values are based on the address of the super-block structure, and the inode number of the inode.

i_list

The i_list linked list links inodes in various states. There is the inode_in_use list which lists unchanged inodes that are in active use, inode_unused which lists unused inodes, and superblock->s_dirty which holds all the dirty inodes on the given file system.

i_dentry

The i_dentry list is a list of all struct dentrys that refer to this inode. They are linked together with the d_alias field of the dentry.

i_version

The i_version field is available for file-systems to use to record that a change has been made since some previous time. Typically the i_version is set to the current value of the event global variable which is then incremented. The file-system code will sometimes assign the current value of i_version to the f_version field of an associated file structure. On a subsequent use of the file structure, it is then possible to tell if the inode has been changed, and if necessary, data cached in the file structure can be refreshed.

i_nrpages

This field records the number of pages, linked at i_pages which are currently cached for this inode. It is incremented by add_page_to_inode_queue and decremented by remove_page_from_inode_queue.

i_sem

This semaphore guards changes to the inode. Any code that wants to make non-atomic access to the inode (i.e. two related accesses with the possibility of sleeping inbetween) must first claim this semaphore. This includes such things as allocating and deallocating blocks and searching through directories.

It appears that it is not possible to claim a shared lock for read-only operations.

i_flock

This points to the list of struct file_lock structures that impose locks in this inode.

i_mmap

All of the vm_area_struct structures that describe mapping of an inode are linked together with the vm_next_share and vm_pprev_share pointers. This i_mmap pointer points into that list.

i_pages

This is the list of all pages in the page cache that refer to this inode. They are linked together on the next and prev links in the page structure.

i_shared_lock

This spin lock guards the vm_next_share and vm_prev_share pointers in the i_mmap list.

i_state

There are three possible inode state bits: I_DIRTY, I_LOCK, I_FREEING.

I_DIRTY

Dirty inodes are on the per-super-block s_dirty list, and will be written next time a sync is requested.

I_LOCK

Inodes are locked while they are being created, read or written.

I_FREEING

An inode is has this state when the reference count and link count have both reached zero. This seems to be only used by igrab called from the fat file-system. fat does funny things with inodes.

i_flags

The i_flags field correspond to the s_flags field in the super block. Many of the flags can be set system wide or per inode. The per-inode flags are:

MS_NOSUID

Setuid/setgid is not permitted in this file.

MS_NODEV

If this inode is a device special file, it cannot be opened.

MS_NOEXEC

This file cannot be executed.

MS_SYNCHRONOUS

All write should be synchronous.

MS_MANDLOCK

Mandatory locking is honoured.

S_QUOTA

Quotas have been initialised.

S_APPEND

The file can only be appended to.

S_IMMUTABLE

The file may not be changed, even by root.

MS_NOATIME

Do not update access time on the inode when the file is accessed.

MS_NODIRATIME

Do not update access time on directories (but still do so on files unless MS_NOATIME).

MS_ODD_RENAME

Wierd nfs thing.

i_writecount

If this is positive, it counts the number of clients (files or memory maps) which have write access. If negative, then the absolute value ofthis number counts the number of VM_DENYWRITE mappings that are current. Otherwise it is 0, and nobody is trying to write or trying to stop others from writing.

i_attr_flags

This is never used, and is only set by ext2_read_inode to be some combination of ATTR_FLAG_SYNCRONOUS, ATTR_FLAG_APPEND, ATTR_FLAG_IMMUTABLE and ATTR_FLAG_NOATIME.

i_generation

The intent of i_generation is to be able to distinguish between an inode before and after a delete/reuse cycle. This is important for NFS. Currently, only ext2 and nfsd maintain this field.

It is not clear that this could be exported to the VFS layer at all as it's use is so specific. Rather each file-system should have the opportunity to provide a unique file handle for a given inode, and each can then do whatever seems best to guarantee uniqueness.

7.2 Функции ноды


struct inode_operations {
         struct file_operations * default_file_ops;
         int (*create) (struct inode *,struct dentry *,int);
         struct dentry * (*lookup) (struct inode *,struct dentry *);
         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,int);
         int (*rename) (struct inode *, struct dentry *,
                         struct inode *, struct dentry *);
         int (*readlink) (struct dentry *, char *,int);
         struct dentry * (*follow_link) (struct dentry *, struct dentry *, unsigned int);
 
         int (*get_block) (struct inode *, long, struct buffer_head *, int);
 
         int (*readpage) (struct file *, struct page *);
         int (*writepage) (struct file *, struct page *);
         int (*flushpage) (struct inode *, struct page *, unsigned long);
 
         void (*truncate) (struct inode *);
         int (*permission) (struct inode *, int);
         int (*smap) (struct inode *,int);
         int (*revalidate) (struct dentry *);
 };
 

default_file_ops

This points to the default table of file operations for files opened on this inode. When a file is opened, the f_op field in the file structure is initialised from this, and then the open method in the file_operations table is called. That method may choose to change the f_op to a different (non-default) method table. This is done, for example, when a device special file is opened.

create

This, and the next 8 methods are only meaningful on directory inodes.

create is called when the VFS wants to create a file with the given name (in the dentry) in the given directory. The VFS will have already checked that the name doesn't exist, and the dentry passed will be a negative dentry meaning that the inode pointer will be NULL.

Create should, if successful, get a new empty inode from the cache with get_empty_inode, fill in the fields and insert it into the hash table with insert_inode_hash, mark it dirty with mark_inode_dirty, and instantiate it into the dcache with d_instantiate.

The int argument contains the mode of the file which should indicate that it is S_IFREG and specify the required permission bits.

lookup

lookup should check if that name (given by the dentry) exists in the directory (given by the inode) and should update the dentry using d_add if it does. This involves finding and loading the inode.

If the lookup failed to find anything, this is indicated by returning a negative dentry, with an inode pointer of NULL.

As well as returning an error or NULL, indicating that the dentry was correctly updated, lookup can return an alternate dentry, in which case the passed dentry will be released. I don't know if this possibility is actually used.

link

The link method should make a hard link from the name refered to by the first dentry to the name referred to by the second dentry, which is in the directory refered to by the inode.

If successful, it should call d_instantiate to link the inode of the linked file to the new dentry (which was a negative dentry).

unlink

This should remove the name refered to by the dentry from the directory referred to by the inode. It should d_delete the dentry on success.

symlink

This should create a symbolic link in the given directory with the given name having the given value. It should d_instantiate the new inode into the dentry on success.

mkdir

Create a directory with the given parent, name, and mode.

rmdir

Remove the named directory (if empty) and d_delete the dentry.

mknod

Create a device special file with the given parent, name, mode, and device number. Then d_instantiate the new inode into the dentry.

rename

The first inode and entry refer to a directory and name that exist. rename should rename the object to have the parent and name given by the second inode and dentry. All generic checks, including that the new parent isn't a child of the old name, have already been done.

readlink

The symbolic link referred to by the dentry is read and the value is copied into the user buffer (with copy_to_user) with a maximum length given by the int.

follow_link

If we have a directory (the first dentry) and a name within that directory (the second dentry) then the obvious result of following the name from the directory would arrive at the second dentry. If an inode requires some other, non-obvious, result -- as do symbolic links -- the inode should provide a follow_link method to return the appropriate new dentry. The int argument contains a number of LOOKUP flags which are described in the section on namei lookups.

get_block

This method is used to find the device block that holds a given block of a file. The inode and long indicate the file and block number being sought (the block number is the file offset divided by the file-system block size). get_block should initialise the b_dev and b_blocknr fields of the buffer_head, and should possibly modify the b_state flags.

If the int argument is non-zero then a new block should be allocated if one does not already exist.

readpage

Readpage is only called by mm/filemap.c It is called by:

  • try_to_read_ahead from generic_file_readahead and filemap_nopage
  • do_generic_file_read
  • sys_sendfile
  • filemap_nopage
  • generic_file_mmap requires it to be non-null.

Thus it is needed for memory mapping of files (as you would expect), for using the sendfile system call, or if the generic_read_file is to be used for the file:read method.

readpage is not expected to actually read in the page. It must arrange for the read to happen. Clients wait for the page to be unlocked before using the data.

readpage can be implemented using block_read_full_page which is defined in fs/buffer.c. This routine assumes that inode:get_block has been defined and sets up a buffer_heads to access the block in question. These buffer_heads will be set to call 'end_buffer_io_async' on completion, which will unlock the page when all buffers on the page complete.

writepage

Writepage is called from linux/mm/filemap.c too.

it is called by do_write_page from filemap_write_page, from filemap_swapout, filemap_sync_pte, and from generic_file_mmap.

Writepage can be implemented using block_write_full_page from fs/buffer.c. It is a close twin of block_read_fullpage. The important differences being:

  • block_read_fullpage initiates a read with ll_rw_block, while block_write_fullpage only sets up the buffers, but doesn't initiate the write.
  • block_read_fullpage calls inode:get_block with the create flags set to zero, while block_write_fullpage sets it to one, and
  • block_read_fullpage calls init_buffer to get end_buffer_io_async called on completion.

These two routines could be cleaned up a bit so that the similarity and differences stand out more.

flushpage

flushpage is called from mm/filemap.c and mm/swap_state.c.

In mm/filemap.c is called by truncate_inode_pages to make sure no I/O is pending on a page before the page is released. mm/swap_state.c similarly calls it when a page is being removed from the swap cache -- all I/O must be finished.

HEREish

truncate

TODO

permission

TODO

smap

TODO

revalidate

TODO

8. Блокировка

All file-system operations are still protected by the big kernel lock. The moves to make file-system code SMP safe seem to be progressing from the bottom up, with the buffer cache and page cache essentially SMP safe, the inode cache probably SMP safe (there is spin lock called inode_lock which must be held during inode operations) and the dcache totally SMP-unsafe.

As file-system operations are mostly done at the dcache level, file system operations are all under the kernel lock.

The main (only?) non-SMP locking issues that file-systems need to deal with are consistancy of the hierarchical structure in the dcache, and consistancy of any internal structure a individual files (or file-system objects).

8.1 Dcache consistancy

Changes to the dache involve adding and deleting dentries as children of pre-existing dentries.

Deleting entries in performed in a lazy fashion. Entires that are not wanted any longer are unhash so that they will not be found by future lookups. Once the last reference to the unwanted dentry is removed, the dentry will be pruned by dput.

Adding entries is done by first adding a 'negative' entry which has a NULL pointer for the d_inode, and then instantiating that entry by filling in the d_inode pointer appropriately.

Any operation which might change the dcache structure must hold a lock while making the change. The protocol used in the VFS layer that the i_sem semaphore on the parent inode must be held when adding a dentry as a child of that inode, or when changing the d_inode pointer in any child of the inode. Note that unhashing or pruning entries do not require the semaphore to be held as these can be done atomically under the kernel lock.

The situations which require i_sem to be help down include:

  • performing a lookup operation in the file-system which will add a new child dentry - possibly a negative one.
  • creating a new file to instantiate a negative dentry.
  • Unlinking a file, and hence changing a dentry into a negative dentry.

Many operations require a two step processes. The first step does a lookup of some name in a directory. The second step performs some operation on the name that was found, such as to instantiate it or is some other way change the d_inode pointer. This requires the i_sem semaphore two be taken and released twice, once of the lookup and once for the other step. In order the ensure that no incompatible operations has occurred between the two holds on the semaphore, the VFS locking protocol requires that after the second down(&inode->i_sem, the operation must check that the parent dentry really is still the parent of the child dentry. This can be done using code similar to the check_parent macro in fs/namei.c.

Rename

A particularly interesting case for dcache locking involves the rename operation, as this changes two entries in the one operation.

When renaming a file (or other non-directory object) it is sufficient to lock both parent directories. If order to avoid deadlocks, the convention is to HERE
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