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docs: filesystems: vfs: Convert vfs.txt to RST

Browse Source 
vfs.txt is currently stale.  If we convert it to RST this is a good
first step in the process of getting the VFS documentation up to date.

This patch does the following (all as a single patch so as not to
introduce any new SPHINX build warnings)

 - Use '.. code-block:: c' for C code blocks and indent the code blocks.
 - Use double backticks for struct member descriptions.
 - Fix a couple of build warnings by guarding pointers (*) with double
   backticks .e.g  ``*ptr``.
 - Add vfs to Documentation/filesystems/index.rst

The member descriptions paragraph indentation was not touched.  It is
not pretty but these do not cause build warnings.  These descriptions
all need updating anyways so leave it as it is for now.

Signed-off-by: Tobin C. Harding <tobin@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
This commit is contained in:
Tobin C. Harding 2019-05-15 10:29:13 +10:00 committed by Jonathan Corbet
parent 1b44ae63de
commit af96c1e304
2 changed files with 292 additions and 274 deletions
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1
Documentation/filesystems/index.rst
View File

@ -16,6 +16,7 @@ algorithms work.
.. toctree:: .. toctree::
:maxdepth: 2 :maxdepth: 2
vfs
path-lookup.rst path-lookup.rst
api-summary api-summary
splice splice

303
Documentation/filesystems/vfs.txt → Documentation/filesystems/vfs.rst
View File

@ -85,6 +85,8 @@ Registering and Mounting a Filesystem
To register and unregister a filesystem, use the following API To register and unregister a filesystem, use the following API
functions: functions:
.. code-block:: c
#include <linux/fs.h> #include <linux/fs.h>
extern int register_filesystem(struct file_system_type *); extern int register_filesystem(struct file_system_type *);
@ -108,7 +110,9 @@ struct file_system_type
This describes the filesystem. As of kernel 2.6.39, the following This describes the filesystem. As of kernel 2.6.39, the following
members are defined: members are defined:
struct file_system_type { .. code-block:: c
struct file_system_operations {
const char *name; const char *name;
int fs_flags; int fs_flags;
struct dentry *(*mount) (struct file_system_type *, int, struct dentry *(*mount) (struct file_system_type *, int,
@ -119,36 +123,36 @@ struct file_system_type {
struct list_head fs_supers; struct list_head fs_supers;
struct lock_class_key s_lock_key; struct lock_class_key s_lock_key;
struct lock_class_key s_umount_key; struct lock_class_key s_umount_key;
}; };
name: the name of the filesystem type, such as "ext2", "iso9660", ``name``: the name of the filesystem type, such as "ext2", "iso9660",
"msdos" and so on "msdos" and so on
fs_flags: various flags (i.e. FS_REQUIRES_DEV, FS_NO_DCACHE, etc.) ``fs_flags``: various flags (i.e. FS_REQUIRES_DEV, FS_NO_DCACHE, etc.)
mount: the method to call when a new instance of this ``mount``: the method to call when a new instance of this filesystem should
filesystem should be mounted be mounted
kill_sb: the method to call when an instance of this filesystem ``kill_sb``: the method to call when an instance of this filesystem
should be shut down should be shut down
owner: for internal VFS use: you should initialize this to THIS_MODULE in ``owner``: for internal VFS use: you should initialize this to THIS_MODULE in
most cases. most cases.
next: for internal VFS use: you should initialize this to NULL ``next``: for internal VFS use: you should initialize this to NULL
s_lock_key, s_umount_key: lockdep-specific s_lock_key, s_umount_key: lockdep-specific
The mount() method has the following arguments: The mount() method has the following arguments:
struct file_system_type *fs_type: describes the filesystem, partly initialized ``struct file_system_type *fs_type``: describes the filesystem, partly initialized
by the specific filesystem code by the specific filesystem code
int flags: mount flags ``int flags``: mount flags
const char *dev_name: the device name we are mounting. ``const char *dev_name``: the device name we are mounting.
void *data: arbitrary mount options, usually comes as an ASCII ``void *data``: arbitrary mount options, usually comes as an ASCII
string (see "Mount Options" section) string (see "Mount Options" section)
The mount() method must return the root dentry of the tree requested by The mount() method must return the root dentry of the tree requested by
@ -174,22 +178,22 @@ implementation.
Usually, a filesystem uses one of the generic mount() implementations Usually, a filesystem uses one of the generic mount() implementations
and provides a fill_super() callback instead. The generic variants are: and provides a fill_super() callback instead. The generic variants are:
mount_bdev: mount a filesystem residing on a block device ``mount_bdev``: mount a filesystem residing on a block device
mount_nodev: mount a filesystem that is not backed by a device ``mount_nodev``: mount a filesystem that is not backed by a device
mount_single: mount a filesystem which shares the instance between ``mount_single``: mount a filesystem which shares the instance between
all mounts all mounts
A fill_super() callback implementation has the following arguments: A fill_super() callback implementation has the following arguments:
struct super_block *sb: the superblock structure. The callback ``struct super_block *sb``: the superblock structure. The callback
must initialize this properly. must initialize this properly.
void *data: arbitrary mount options, usually comes as an ASCII ``void *data``: arbitrary mount options, usually comes as an ASCII
string (see "Mount Options" section) string (see "Mount Options" section)
int silent: whether or not to be silent on error ``int silent``: whether or not to be silent on error
The Superblock Object The Superblock Object
@ -204,7 +208,9 @@ struct super_operations
This describes how the VFS can manipulate the superblock of your This describes how the VFS can manipulate the superblock of your
filesystem. As of kernel 2.6.22, the following members are defined: filesystem. As of kernel 2.6.22, the following members are defined:
struct super_operations { .. code-block:: c
struct super_operations {
struct inode *(*alloc_inode)(struct super_block *sb); struct inode *(*alloc_inode)(struct super_block *sb);
void (*destroy_inode)(struct inode *); void (*destroy_inode)(struct inode *);
@ -227,31 +233,31 @@ struct super_operations {
ssize_t (*quota_write)(struct super_block *, int, const char *, size_t, loff_t); ssize_t (*quota_write)(struct super_block *, int, const char *, size_t, loff_t);
int (*nr_cached_objects)(struct super_block *); int (*nr_cached_objects)(struct super_block *);
void (*free_cached_objects)(struct super_block *, int); void (*free_cached_objects)(struct super_block *, int);
}; };
All methods are called without any locks being held, unless otherwise All methods are called without any locks being held, unless otherwise
noted. This means that most methods can block safely. All methods are noted. This means that most methods can block safely. All methods are
only called from a process context (i.e. not from an interrupt handler only called from a process context (i.e. not from an interrupt handler
or bottom half). or bottom half).
alloc_inode: this method is called by alloc_inode() to allocate memory ``alloc_inode``: this method is called by alloc_inode() to allocate memory
for struct inode and initialize it. If this function is not for struct inode and initialize it. If this function is not
defined, a simple 'struct inode' is allocated. Normally defined, a simple 'struct inode' is allocated. Normally
alloc_inode will be used to allocate a larger structure which alloc_inode will be used to allocate a larger structure which
contains a 'struct inode' embedded within it. contains a 'struct inode' embedded within it.
destroy_inode: this method is called by destroy_inode() to release ``destroy_inode``: this method is called by destroy_inode() to release
resources allocated for struct inode. It is only required if resources allocated for struct inode. It is only required if
->alloc_inode was defined and simply undoes anything done by ->alloc_inode was defined and simply undoes anything done by
->alloc_inode. ->alloc_inode.
dirty_inode: this method is called by the VFS to mark an inode dirty. ``dirty_inode``: this method is called by the VFS to mark an inode dirty.
write_inode: this method is called when the VFS needs to write an ``write_inode``: this method is called when the VFS needs to write an
inode to disc. The second parameter indicates whether the write inode to disc. The second parameter indicates whether the write
should be synchronous or not, not all filesystems check this flag. should be synchronous or not, not all filesystems check this flag.
drop_inode: called when the last access to the inode is dropped, ``drop_inode``: called when the last access to the inode is dropped,
with the inode->i_lock spinlock held. with the inode->i_lock spinlock held.
This method should be either NULL (normal UNIX filesystem This method should be either NULL (normal UNIX filesystem
@ -264,43 +270,43 @@ or bottom half).
but does not have the races that the "force_delete()" approach but does not have the races that the "force_delete()" approach
had. had.
delete_inode: called when the VFS wants to delete an inode ``delete_inode``: called when the VFS wants to delete an inode
put_super: called when the VFS wishes to free the superblock ``put_super``: called when the VFS wishes to free the superblock
(i.e. unmount). This is called with the superblock lock held (i.e. unmount). This is called with the superblock lock held
sync_fs: called when VFS is writing out all dirty data associated with ``sync_fs``: called when VFS is writing out all dirty data associated with
a superblock. The second parameter indicates whether the method a superblock. The second parameter indicates whether the method
should wait until the write out has been completed. Optional. should wait until the write out has been completed. Optional.
freeze_fs: called when VFS is locking a filesystem and ``freeze_fs``: called when VFS is locking a filesystem and
forcing it into a consistent state. This method is currently forcing it into a consistent state. This method is currently
used by the Logical Volume Manager (LVM). used by the Logical Volume Manager (LVM).
unfreeze_fs: called when VFS is unlocking a filesystem and making it writable ``unfreeze_fs``: called when VFS is unlocking a filesystem and making it writable
again. again.
statfs: called when the VFS needs to get filesystem statistics. ``statfs``: called when the VFS needs to get filesystem statistics.
remount_fs: called when the filesystem is remounted. This is called ``remount_fs``: called when the filesystem is remounted. This is called
with the kernel lock held with the kernel lock held
clear_inode: called then the VFS clears the inode. Optional ``clear_inode``: called then the VFS clears the inode. Optional
umount_begin: called when the VFS is unmounting a filesystem. ``umount_begin``: called when the VFS is unmounting a filesystem.
show_options: called by the VFS to show mount options for ``show_options``: called by the VFS to show mount options for
/proc/<pid>/mounts. (see "Mount Options" section) /proc/<pid>/mounts. (see "Mount Options" section)
quota_read: called by the VFS to read from filesystem quota file. ``quota_read``: called by the VFS to read from filesystem quota file.
quota_write: called by the VFS to write to filesystem quota file. ``quota_write``: called by the VFS to write to filesystem quota file.
nr_cached_objects: called by the sb cache shrinking function for the ``nr_cached_objects``: called by the sb cache shrinking function for the
filesystem to return the number of freeable cached objects it contains. filesystem to return the number of freeable cached objects it contains.
Optional. Optional.
free_cache_objects: called by the sb cache shrinking function for the ``free_cache_objects``: called by the sb cache shrinking function for the
filesystem to scan the number of objects indicated to try to free them. filesystem to scan the number of objects indicated to try to free them.
Optional, but any filesystem implementing this method needs to also Optional, but any filesystem implementing this method needs to also
implement ->nr_cached_objects for it to be called correctly. implement ->nr_cached_objects for it to be called correctly.
@ -328,27 +334,27 @@ On filesystems that support extended attributes (xattrs), the s_xattr
superblock field points to a NULL-terminated array of xattr handlers. superblock field points to a NULL-terminated array of xattr handlers.
Extended attributes are name:value pairs. Extended attributes are name:value pairs.
name: Indicates that the handler matches attributes with the specified name ``name``: Indicates that the handler matches attributes with the specified name
(such as "system.posix_acl_access"); the prefix field must be NULL. (such as "system.posix_acl_access"); the prefix field must be NULL.
prefix: Indicates that the handler matches all attributes with the specified ``prefix``: Indicates that the handler matches all attributes with the specified
name prefix (such as "user."); the name field must be NULL. name prefix (such as "user."); the name field must be NULL.
list: Determine if attributes matching this xattr handler should be listed ``list``: Determine if attributes matching this xattr handler should be listed
for a particular dentry. Used by some listxattr implementations like for a particular dentry. Used by some listxattr implementations like
generic_listxattr. generic_listxattr.
get: Called by the VFS to get the value of a particular extended attribute. ``get``: Called by the VFS to get the value of a particular extended attribute.
This method is called by the getxattr(2) system call. This method is called by the getxattr(2) system call.
set: Called by the VFS to set the value of a particular extended attribute. ``set``: Called by the VFS to set the value of a particular extended attribute.
When the new value is NULL, called to remove a particular extended When the new value is NULL, called to remove a particular extended
attribute. This method is called by the the setxattr(2) and attribute. This method is called by the the setxattr(2) and
removexattr(2) system calls. removexattr(2) system calls.
When none of the xattr handlers of a filesystem match the specified When none of the xattr handlers of a filesystem match the specified
attribute name or when a filesystem doesn't support extended attributes, attribute name or when a filesystem doesn't support extended attributes,
the various *xattr(2) system calls return -EOPNOTSUPP. the various ``*xattr(2)`` system calls return -EOPNOTSUPP.
The Inode Object The Inode Object
@ -363,7 +369,9 @@ struct inode_operations
This describes how the VFS can manipulate an inode in your filesystem. This describes how the VFS can manipulate an inode in your filesystem.
As of kernel 2.6.22, the following members are defined: As of kernel 2.6.22, the following members are defined:
struct inode_operations { .. code-block:: c
struct inode_operations {
int (*create) (struct inode *,struct dentry *, umode_t, bool); int (*create) (struct inode *,struct dentry *, umode_t, bool);
struct dentry * (*lookup) (struct inode *,struct dentry *, unsigned int); struct dentry * (*lookup) (struct inode *,struct dentry *, unsigned int);
int (*link) (struct dentry *,struct inode *,struct dentry *); int (*link) (struct dentry *,struct inode *,struct dentry *);
@ -386,18 +394,18 @@ struct inode_operations {
int (*atomic_open)(struct inode *, struct dentry *, struct file *, int (*atomic_open)(struct inode *, struct dentry *, struct file *,
unsigned open_flag, umode_t create_mode); unsigned open_flag, umode_t create_mode);
int (*tmpfile) (struct inode *, struct dentry *, umode_t); int (*tmpfile) (struct inode *, struct dentry *, umode_t);
}; };
Again, all methods are called without any locks being held, unless Again, all methods are called without any locks being held, unless
otherwise noted. otherwise noted.
create: called by the open(2) and creat(2) system calls. Only ``create``: called by the open(2) and creat(2) system calls. Only
required if you want to support regular files. The dentry you required if you want to support regular files. The dentry you
get should not have an inode (i.e. it should be a negative get should not have an inode (i.e. it should be a negative
dentry). Here you will probably call d_instantiate() with the dentry). Here you will probably call d_instantiate() with the
dentry and the newly created inode dentry and the newly created inode
lookup: called when the VFS needs to look up an inode in a parent ``lookup``: called when the VFS needs to look up an inode in a parent
directory. The name to look for is found in the dentry. This directory. The name to look for is found in the dentry. This
method must call d_add() to insert the found inode into the method must call d_add() to insert the found inode into the
dentry. The "i_count" field in the inode structure should be dentry. The "i_count" field in the inode structure should be
@ -411,31 +419,31 @@ otherwise noted.
to a struct "dentry_operations". to a struct "dentry_operations".
This method is called with the directory inode semaphore held This method is called with the directory inode semaphore held
link: called by the link(2) system call. Only required if you want ``link``: called by the link(2) system call. Only required if you want
to support hard links. You will probably need to call to support hard links. You will probably need to call
d_instantiate() just as you would in the create() method d_instantiate() just as you would in the create() method
unlink: called by the unlink(2) system call. Only required if you ``unlink``: called by the unlink(2) system call. Only required if you
want to support deleting inodes want to support deleting inodes
symlink: called by the symlink(2) system call. Only required if you ``symlink``: called by the symlink(2) system call. Only required if you
want to support symlinks. You will probably need to call want to support symlinks. You will probably need to call
d_instantiate() just as you would in the create() method d_instantiate() just as you would in the create() method
mkdir: called by the mkdir(2) system call. Only required if you want ``mkdir``: called by the mkdir(2) system call. Only required if you want
to support creating subdirectories. You will probably need to to support creating subdirectories. You will probably need to
call d_instantiate() just as you would in the create() method call d_instantiate() just as you would in the create() method
rmdir: called by the rmdir(2) system call. Only required if you want ``rmdir``: called by the rmdir(2) system call. Only required if you want
to support deleting subdirectories to support deleting subdirectories
mknod: called by the mknod(2) system call to create a device (char, ``mknod``: called by the mknod(2) system call to create a device (char,
block) inode or a named pipe (FIFO) or socket. Only required block) inode or a named pipe (FIFO) or socket. Only required
if you want to support creating these types of inodes. You if you want to support creating these types of inodes. You
will probably need to call d_instantiate() just as you would will probably need to call d_instantiate() just as you would
in the create() method in the create() method
rename: called by the rename(2) system call to rename the object to ``rename``: called by the rename(2) system call to rename the object to
have the parent and name given by the second inode and dentry. have the parent and name given by the second inode and dentry.
The filesystem must return -EINVAL for any unsupported or The filesystem must return -EINVAL for any unsupported or
@ -449,7 +457,7 @@ otherwise noted.
exist; this is checked by the VFS. Unlike plain rename, exist; this is checked by the VFS. Unlike plain rename,
source and target may be of different type. source and target may be of different type.
get_link: called by the VFS to follow a symbolic link to the ``get_link``: called by the VFS to follow a symbolic link to the
inode it points to. Only required if you want to support inode it points to. Only required if you want to support
symbolic links. This method returns the symlink body symbolic links. This method returns the symlink body
to traverse (and possibly resets the current position with to traverse (and possibly resets the current position with
@ -463,19 +471,20 @@ otherwise noted.
argument. If request can't be handled without leaving RCU mode, argument. If request can't be handled without leaving RCU mode,
have it return ERR_PTR(-ECHILD). have it return ERR_PTR(-ECHILD).
If the filesystem stores the symlink target in ->i_link, the If the filesystem stores the symlink target in ->i_link, the
VFS may use it directly without calling ->get_link(); however, VFS may use it directly without calling ->get_link(); however,
->get_link() must still be provided. ->i_link must not be ->get_link() must still be provided. ->i_link must not be
freed until after an RCU grace period. Writing to ->i_link freed until after an RCU grace period. Writing to ->i_link
post-iget() time requires a 'release' memory barrier. post-iget() time requires a 'release' memory barrier.
readlink: this is now just an override for use by readlink(2) for the ``readlink``: this is now just an override for use by readlink(2) for the
cases when ->get_link uses nd_jump_link() or object is not in cases when ->get_link uses nd_jump_link() or object is not in
fact a symlink. Normally filesystems should only implement fact a symlink. Normally filesystems should only implement
->get_link for symlinks and readlink(2) will automatically use ->get_link for symlinks and readlink(2) will automatically use
that. that.
permission: called by the VFS to check for access rights on a POSIX-like ``permission``: called by the VFS to check for access rights on a POSIX-like
filesystem. filesystem.
May be called in rcu-walk mode (mask & MAY_NOT_BLOCK). If in rcu-walk May be called in rcu-walk mode (mask & MAY_NOT_BLOCK). If in rcu-walk
@ -485,20 +494,20 @@ otherwise noted.
If a situation is encountered that rcu-walk cannot handle, return If a situation is encountered that rcu-walk cannot handle, return
-ECHILD and it will be called again in ref-walk mode. -ECHILD and it will be called again in ref-walk mode.
setattr: called by the VFS to set attributes for a file. This method ``setattr``: called by the VFS to set attributes for a file. This method
is called by chmod(2) and related system calls. is called by chmod(2) and related system calls.
getattr: called by the VFS to get attributes of a file. This method ``getattr``: called by the VFS to get attributes of a file. This method
is called by stat(2) and related system calls. is called by stat(2) and related system calls.
listxattr: called by the VFS to list all extended attributes for a ``listxattr``: called by the VFS to list all extended attributes for a
given file. This method is called by the listxattr(2) system call. given file. This method is called by the listxattr(2) system call.
update_time: called by the VFS to update a specific time or the i_version of ``update_time``: called by the VFS to update a specific time or the i_version of
an inode. If this is not defined the VFS will update the inode itself an inode. If this is not defined the VFS will update the inode itself
and call mark_inode_dirty_sync. and call mark_inode_dirty_sync.
atomic_open: called on the last component of an open. Using this optional ``atomic_open``: called on the last component of an open. Using this optional
method the filesystem can look up, possibly create and open the file in method the filesystem can look up, possibly create and open the file in
one atomic operation. If it wants to leave actual opening to the one atomic operation. If it wants to leave actual opening to the
caller (e.g. if the file turned out to be a symlink, device, or just caller (e.g. if the file turned out to be a symlink, device, or just
@ -510,7 +519,7 @@ otherwise noted.
the method must only succeed if the file didn't exist and hence FMODE_CREATED the method must only succeed if the file didn't exist and hence FMODE_CREATED
shall always be set on success. shall always be set on success.
tmpfile: called in the end of O_TMPFILE open(). Optional, equivalent to ``tmpfile``: called in the end of O_TMPFILE open(). Optional, equivalent to
atomically creating, opening and unlinking a file in given directory. atomically creating, opening and unlinking a file in given directory.
@ -628,7 +637,9 @@ struct address_space_operations
This describes how the VFS can manipulate mapping of a file to page This describes how the VFS can manipulate mapping of a file to page
cache in your filesystem. The following members are defined: cache in your filesystem. The following members are defined:
struct address_space_operations { .. code-block:: c
struct address_space_operations {
int (*writepage)(struct page *page, struct writeback_control *wbc); int (*writepage)(struct page *page, struct writeback_control *wbc);
int (*readpage)(struct file *, struct page *); int (*readpage)(struct file *, struct page *);
int (*writepages)(struct address_space *, struct writeback_control *); int (*writepages)(struct address_space *, struct writeback_control *);
@ -660,9 +671,9 @@ struct address_space_operations {
int (*error_remove_page) (struct mapping *mapping, struct page *page); int (*error_remove_page) (struct mapping *mapping, struct page *page);
int (*swap_activate)(struct file *); int (*swap_activate)(struct file *);
int (*swap_deactivate)(struct file *); int (*swap_deactivate)(struct file *);
}; };
writepage: called by the VM to write a dirty page to backing store. ``writepage``: called by the VM to write a dirty page to backing store.
This may happen for data integrity reasons (i.e. 'sync'), or This may happen for data integrity reasons (i.e. 'sync'), or
to free up memory (flush). The difference can be seen in to free up memory (flush). The difference can be seen in
wbc->sync_mode. wbc->sync_mode.
@ -680,7 +691,7 @@ struct address_space_operations {
See the file "Locking" for more details. See the file "Locking" for more details.
readpage: called by the VM to read a page from backing store. ``readpage``: called by the VM to read a page from backing store.
The page will be Locked when readpage is called, and should be The page will be Locked when readpage is called, and should be
unlocked and marked uptodate once the read completes. unlocked and marked uptodate once the read completes.
If ->readpage discovers that it needs to unlock the page for If ->readpage discovers that it needs to unlock the page for
@ -688,7 +699,7 @@ struct address_space_operations {
In this case, the page will be relocated, relocked and if In this case, the page will be relocated, relocked and if
that all succeeds, ->readpage will be called again. that all succeeds, ->readpage will be called again.
writepages: called by the VM to write out pages associated with the ``writepages``: called by the VM to write out pages associated with the
address_space object. If wbc->sync_mode is WBC_SYNC_ALL, then address_space object. If wbc->sync_mode is WBC_SYNC_ALL, then
the writeback_control will specify a range of pages that must be the writeback_control will specify a range of pages that must be
written out. If it is WBC_SYNC_NONE, then a nr_to_write is given written out. If it is WBC_SYNC_NONE, then a nr_to_write is given
@ -697,7 +708,7 @@ struct address_space_operations {
instead. This will choose pages from the address space that are instead. This will choose pages from the address space that are
tagged as DIRTY and will pass them to ->writepage. tagged as DIRTY and will pass them to ->writepage.
set_page_dirty: called by the VM to set a page dirty. ``set_page_dirty``: called by the VM to set a page dirty.
This is particularly needed if an address space attaches This is particularly needed if an address space attaches
private data to a page, and that data needs to be updated when private data to a page, and that data needs to be updated when
a page is dirtied. This is called, for example, when a memory a page is dirtied. This is called, for example, when a memory
@ -705,14 +716,14 @@ struct address_space_operations {
If defined, it should set the PageDirty flag, and the If defined, it should set the PageDirty flag, and the
PAGECACHE_TAG_DIRTY tag in the radix tree. PAGECACHE_TAG_DIRTY tag in the radix tree.
readpages: called by the VM to read pages associated with the address_space ``readpages``: called by the VM to read pages associated with the address_space
object. This is essentially just a vector version of object. This is essentially just a vector version of
readpage. Instead of just one page, several pages are readpage. Instead of just one page, several pages are
requested. requested.
readpages is only used for read-ahead, so read errors are readpages is only used for read-ahead, so read errors are
ignored. If anything goes wrong, feel free to give up. ignored. If anything goes wrong, feel free to give up.
write_begin: ``write_begin``:
Called by the generic buffered write code to ask the filesystem to Called by the generic buffered write code to ask the filesystem to
prepare to write len bytes at the given offset in the file. The prepare to write len bytes at the given offset in the file. The
address_space should check that the write will be able to complete, address_space should check that the write will be able to complete,
@ -722,7 +733,7 @@ struct address_space_operations {
read already) so that the updated blocks can be written out properly. read already) so that the updated blocks can be written out properly.
The filesystem must return the locked pagecache page for the specified The filesystem must return the locked pagecache page for the specified
offset, in *pagep, for the caller to write into. offset, in ``*pagep``, for the caller to write into.
It must be able to cope with short writes (where the length passed to It must be able to cope with short writes (where the length passed to
write_begin is greater than the number of bytes copied into the page). write_begin is greater than the number of bytes copied into the page).
@ -736,7 +747,7 @@ struct address_space_operations {
Returns 0 on success; < 0 on failure (which is the error code), in Returns 0 on success; < 0 on failure (which is the error code), in
which case write_end is not called. which case write_end is not called.
write_end: After a successful write_begin, and data copy, write_end must ``write_end``: After a successful write_begin, and data copy, write_end must
be called. len is the original len passed to write_begin, and copied be called. len is the original len passed to write_begin, and copied
is the amount that was able to be copied. is the amount that was able to be copied.
@ -746,7 +757,7 @@ struct address_space_operations {
Returns < 0 on failure, otherwise the number of bytes (<= 'copied') Returns < 0 on failure, otherwise the number of bytes (<= 'copied')
that were able to be copied into pagecache. that were able to be copied into pagecache.
bmap: called by the VFS to map a logical block offset within object to ``bmap``: called by the VFS to map a logical block offset within object to
physical block number. This method is used by the FIBMAP physical block number. This method is used by the FIBMAP
ioctl and for working with swap-files. To be able to swap to ioctl and for working with swap-files. To be able to swap to
a file, the file must have a stable mapping to a block a file, the file must have a stable mapping to a block
@ -754,7 +765,7 @@ struct address_space_operations {
but instead uses bmap to find out where the blocks in the file but instead uses bmap to find out where the blocks in the file
are and uses those addresses directly. are and uses those addresses directly.
invalidatepage: If a page has PagePrivate set, then invalidatepage ``invalidatepage``: If a page has PagePrivate set, then invalidatepage
will be called when part or all of the page is to be removed will be called when part or all of the page is to be removed
from the address space. This generally corresponds to either a from the address space. This generally corresponds to either a
truncation, punch hole or a complete invalidation of the address truncation, punch hole or a complete invalidation of the address
@ -766,7 +777,7 @@ struct address_space_operations {
be done by calling the ->releasepage function, but in this case the be done by calling the ->releasepage function, but in this case the
release MUST succeed. release MUST succeed.
releasepage: releasepage is called on PagePrivate pages to indicate ``releasepage``: releasepage is called on PagePrivate pages to indicate
that the page should be freed if possible. ->releasepage that the page should be freed if possible. ->releasepage
should remove any private data from the page and clear the should remove any private data from the page and clear the
PagePrivate flag. If releasepage() fails for some reason, it must PagePrivate flag. If releasepage() fails for some reason, it must
@ -787,40 +798,40 @@ struct address_space_operations {
need to ensure this. Possibly it can clear the PageUptodate need to ensure this. Possibly it can clear the PageUptodate
bit if it cannot free private data yet. bit if it cannot free private data yet.
freepage: freepage is called once the page is no longer visible in ``freepage``: freepage is called once the page is no longer visible in
the page cache in order to allow the cleanup of any private the page cache in order to allow the cleanup of any private
data. Since it may be called by the memory reclaimer, it data. Since it may be called by the memory reclaimer, it
should not assume that the original address_space mapping still should not assume that the original address_space mapping still
exists, and it should not block. exists, and it should not block.
direct_IO: called by the generic read/write routines to perform ``direct_IO``: called by the generic read/write routines to perform
direct_IO - that is IO requests which bypass the page cache direct_IO - that is IO requests which bypass the page cache
and transfer data directly between the storage and the and transfer data directly between the storage and the
application's address space. application's address space.
isolate_page: Called by the VM when isolating a movable non-lru page. ``isolate_page``: Called by the VM when isolating a movable non-lru page.
If page is successfully isolated, VM marks the page as PG_isolated If page is successfully isolated, VM marks the page as PG_isolated
via __SetPageIsolated. via __SetPageIsolated.
migrate_page: This is used to compact the physical memory usage. ``migrate_page``: This is used to compact the physical memory usage.
If the VM wants to relocate a page (maybe off a memory card If the VM wants to relocate a page (maybe off a memory card
that is signalling imminent failure) it will pass a new page that is signalling imminent failure) it will pass a new page
and an old page to this function. migrate_page should and an old page to this function. migrate_page should
transfer any private data across and update any references transfer any private data across and update any references
that it has to the page. that it has to the page.
putback_page: Called by the VM when isolated page's migration fails. ``putback_page``: Called by the VM when isolated page's migration fails.
launder_page: Called before freeing a page - it writes back the dirty page. To ``launder_page``: Called before freeing a page - it writes back the dirty page. To
prevent redirtying the page, it is kept locked during the whole prevent redirtying the page, it is kept locked during the whole
operation. operation.
is_partially_uptodate: Called by the VM when reading a file through the ``is_partially_uptodate``: Called by the VM when reading a file through the
pagecache when the underlying blocksize != pagesize. If the required pagecache when the underlying blocksize != pagesize. If the required
block is up to date then the read can complete without needing the IO block is up to date then the read can complete without needing the IO
to bring the whole page up to date. to bring the whole page up to date.
is_dirty_writeback: Called by the VM when attempting to reclaim a page. ``is_dirty_writeback``: Called by the VM when attempting to reclaim a page.
The VM uses dirty and writeback information to determine if it needs The VM uses dirty and writeback information to determine if it needs
to stall to allow flushers a chance to complete some IO. Ordinarily to stall to allow flushers a chance to complete some IO. Ordinarily
it can use PageDirty and PageWriteback but some filesystems have it can use PageDirty and PageWriteback but some filesystems have
@ -829,17 +840,17 @@ struct address_space_operations {
allows a filesystem to indicate to the VM if a page should be allows a filesystem to indicate to the VM if a page should be
treated as dirty or writeback for the purposes of stalling. treated as dirty or writeback for the purposes of stalling.
error_remove_page: normally set to generic_error_remove_page if truncation ``error_remove_page``: normally set to generic_error_remove_page if truncation
is ok for this address space. Used for memory failure handling. is ok for this address space. Used for memory failure handling.
Setting this implies you deal with pages going away under you, Setting this implies you deal with pages going away under you,
unless you have them locked or reference counts increased. unless you have them locked or reference counts increased.
swap_activate: Called when swapon is used on a file to allocate ``swap_activate``: Called when swapon is used on a file to allocate
space if necessary and pin the block lookup information in space if necessary and pin the block lookup information in
memory. A return value of zero indicates success, memory. A return value of zero indicates success,
in which case this file can be used to back swapspace. in which case this file can be used to back swapspace.
swap_deactivate: Called during swapoff on files where swap_activate ``swap_deactivate``: Called during swapoff on files where swap_activate
was successful. was successful.
@ -856,7 +867,9 @@ struct file_operations
This describes how the VFS can manipulate an open file. As of kernel This describes how the VFS can manipulate an open file. As of kernel
4.18, the following members are defined: 4.18, the following members are defined:
struct file_operations { .. code-block:: c
struct file_operations {
struct module *owner; struct module *owner;
loff_t (*llseek) (struct file *, loff_t, int); loff_t (*llseek) (struct file *, loff_t, int);
ssize_t (*read) (struct file *, char __user *, size_t, loff_t *); ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
@ -886,48 +899,48 @@ struct file_operations {
long (*fallocate)(struct file *file, int mode, loff_t offset, long (*fallocate)(struct file *file, int mode, loff_t offset,
loff_t len); loff_t len);
void (*show_fdinfo)(struct seq_file *m, struct file *f); void (*show_fdinfo)(struct seq_file *m, struct file *f);
#ifndef CONFIG_MMU #ifndef CONFIG_MMU
unsigned (*mmap_capabilities)(struct file *); unsigned (*mmap_capabilities)(struct file *);
#endif #endif
ssize_t (*copy_file_range)(struct file *, loff_t, struct file *, loff_t, size_t, unsigned int); ssize_t (*copy_file_range)(struct file *, loff_t, struct file *, loff_t, size_t, unsigned int);
loff_t (*remap_file_range)(struct file *file_in, loff_t pos_in, loff_t (*remap_file_range)(struct file *file_in, loff_t pos_in,
struct file *file_out, loff_t pos_out, struct file *file_out, loff_t pos_out,
loff_t len, unsigned int remap_flags); loff_t len, unsigned int remap_flags);
int (*fadvise)(struct file *, loff_t, loff_t, int); int (*fadvise)(struct file *, loff_t, loff_t, int);
}; };
Again, all methods are called without any locks being held, unless Again, all methods are called without any locks being held, unless
otherwise noted. otherwise noted.
llseek: called when the VFS needs to move the file position index ``llseek``: called when the VFS needs to move the file position index
read: called by read(2) and related system calls ``read``: called by read(2) and related system calls
read_iter: possibly asynchronous read with iov_iter as destination ``read_iter``: possibly asynchronous read with iov_iter as destination
write: called by write(2) and related system calls ``write``: called by write(2) and related system calls
write_iter: possibly asynchronous write with iov_iter as source ``write_iter``: possibly asynchronous write with iov_iter as source
iopoll: called when aio wants to poll for completions on HIPRI iocbs ``iopoll``: called when aio wants to poll for completions on HIPRI iocbs
iterate: called when the VFS needs to read the directory contents ``iterate``: called when the VFS needs to read the directory contents
iterate_shared: called when the VFS needs to read the directory contents ``iterate_shared``: called when the VFS needs to read the directory contents
when filesystem supports concurrent dir iterators when filesystem supports concurrent dir iterators
poll: called by the VFS when a process wants to check if there is ``poll``: called by the VFS when a process wants to check if there is
activity on this file and (optionally) go to sleep until there activity on this file and (optionally) go to sleep until there
is activity. Called by the select(2) and poll(2) system calls is activity. Called by the select(2) and poll(2) system calls
unlocked_ioctl: called by the ioctl(2) system call. ``unlocked_ioctl``: called by the ioctl(2) system call.
compat_ioctl: called by the ioctl(2) system call when 32 bit system calls ``compat_ioctl``: called by the ioctl(2) system call when 32 bit system calls
are used on 64 bit kernels. are used on 64 bit kernels.
mmap: called by the mmap(2) system call ``mmap``: called by the mmap(2) system call
open: called by the VFS when an inode should be opened. When the VFS ``open``: called by the VFS when an inode should be opened. When the VFS
opens a file, it creates a new "struct file". It then calls the opens a file, it creates a new "struct file". It then calls the
open method for the newly allocated file structure. You might open method for the newly allocated file structure. You might
think that the open method really belongs in think that the open method really belongs in
@ -937,40 +950,40 @@ otherwise noted.
"private_data" member in the file structure if you want to point "private_data" member in the file structure if you want to point
to a device structure to a device structure
flush: called by the close(2) system call to flush a file ``flush``: called by the close(2) system call to flush a file
release: called when the last reference to an open file is closed ``release``: called when the last reference to an open file is closed
fsync: called by the fsync(2) system call. Also see the section above ``fsync``: called by the fsync(2) system call. Also see the section above
entitled "Handling errors during writeback". entitled "Handling errors during writeback".
fasync: called by the fcntl(2) system call when asynchronous ``fasync``: called by the fcntl(2) system call when asynchronous
(non-blocking) mode is enabled for a file (non-blocking) mode is enabled for a file
lock: called by the fcntl(2) system call for F_GETLK, F_SETLK, and F_SETLKW ``lock``: called by the fcntl(2) system call for F_GETLK, F_SETLK, and F_SETLKW
commands commands
get_unmapped_area: called by the mmap(2) system call ``get_unmapped_area``: called by the mmap(2) system call
check_flags: called by the fcntl(2) system call for F_SETFL command ``check_flags``: called by the fcntl(2) system call for F_SETFL command
flock: called by the flock(2) system call ``flock``: called by the flock(2) system call
splice_write: called by the VFS to splice data from a pipe to a file. This ``splice_write``: called by the VFS to splice data from a pipe to a file. This
method is used by the splice(2) system call method is used by the splice(2) system call
splice_read: called by the VFS to splice data from file to a pipe. This ``splice_read``: called by the VFS to splice data from file to a pipe. This
method is used by the splice(2) system call method is used by the splice(2) system call
setlease: called by the VFS to set or release a file lock lease. setlease ``setlease``: called by the VFS to set or release a file lock lease. setlease
implementations should call generic_setlease to record or remove implementations should call generic_setlease to record or remove
the lease in the inode after setting it. the lease in the inode after setting it.
fallocate: called by the VFS to preallocate blocks or punch a hole. ``fallocate``: called by the VFS to preallocate blocks or punch a hole.
copy_file_range: called by the copy_file_range(2) system call. ``copy_file_range``: called by the copy_file_range(2) system call.
remap_file_range: called by the ioctl(2) system call for FICLONERANGE and ``remap_file_range``: called by the ioctl(2) system call for FICLONERANGE and
FICLONE and FIDEDUPERANGE commands to remap file ranges. An FICLONE and FIDEDUPERANGE commands to remap file ranges. An
implementation should remap len bytes at pos_in of the source file into implementation should remap len bytes at pos_in of the source file into
the dest file at pos_out. Implementations must handle callers passing the dest file at pos_out. Implementations must handle callers passing
@ -983,7 +996,7 @@ otherwise noted.
set, the caller is ok with the implementation shortening the request set, the caller is ok with the implementation shortening the request
length to satisfy alignment or EOF requirements (or any other reason). length to satisfy alignment or EOF requirements (or any other reason).
fadvise: possibly called by the fadvise64() system call. ``fadvise``: possibly called by the fadvise64() system call.
Note that the file operations are implemented by the specific Note that the file operations are implemented by the specific
filesystem in which the inode resides. When opening a device node filesystem in which the inode resides. When opening a device node
@ -1010,7 +1023,9 @@ here. These methods may be set to NULL, as they are either optional or
the VFS uses a default. As of kernel 2.6.22, the following members are the VFS uses a default. As of kernel 2.6.22, the following members are
defined: defined:
struct dentry_operations { .. code-block:: c
struct dentry_operations {
int (*d_revalidate)(struct dentry *, unsigned int); int (*d_revalidate)(struct dentry *, unsigned int);
int (*d_weak_revalidate)(struct dentry *, unsigned int); int (*d_weak_revalidate)(struct dentry *, unsigned int);
int (*d_hash)(const struct dentry *, struct qstr *); int (*d_hash)(const struct dentry *, struct qstr *);
@ -1024,9 +1039,9 @@ struct dentry_operations {
struct vfsmount *(*d_automount)(struct path *); struct vfsmount *(*d_automount)(struct path *);
int (*d_manage)(const struct path *, bool); int (*d_manage)(const struct path *, bool);
struct dentry *(*d_real)(struct dentry *, const struct inode *); struct dentry *(*d_real)(struct dentry *, const struct inode *);
}; };
d_revalidate: called when the VFS needs to revalidate a dentry. This ``d_revalidate``: called when the VFS needs to revalidate a dentry. This
is called whenever a name look-up finds a dentry in the is called whenever a name look-up finds a dentry in the
dcache. Most local filesystems leave this as NULL, because all their dcache. Most local filesystems leave this as NULL, because all their
dentries in the dcache are valid. Network filesystems are different dentries in the dcache are valid. Network filesystems are different
@ -1045,7 +1060,7 @@ struct dentry_operations {
If a situation is encountered that rcu-walk cannot handle, return If a situation is encountered that rcu-walk cannot handle, return
-ECHILD and it will be called again in ref-walk mode. -ECHILD and it will be called again in ref-walk mode.
d_weak_revalidate: called when the VFS needs to revalidate a "jumped" dentry. ``_weak_revalidate``: called when the VFS needs to revalidate a "jumped" dentry.
This is called when a path-walk ends at dentry that was not acquired by This is called when a path-walk ends at dentry that was not acquired by
doing a lookup in the parent directory. This includes "/", "." and "..", doing a lookup in the parent directory. This includes "/", "." and "..",
as well as procfs-style symlinks and mountpoint traversal. as well as procfs-style symlinks and mountpoint traversal.
@ -1059,14 +1074,14 @@ struct dentry_operations {
d_weak_revalidate is only called after leaving rcu-walk mode. d_weak_revalidate is only called after leaving rcu-walk mode.
d_hash: called when the VFS adds a dentry to the hash table. The first ``d_hash``: called when the VFS adds a dentry to the hash table. The first
dentry passed to d_hash is the parent directory that the name is dentry passed to d_hash is the parent directory that the name is
to be hashed into. to be hashed into.
Same locking and synchronisation rules as d_compare regarding Same locking and synchronisation rules as d_compare regarding
what is safe to dereference etc. what is safe to dereference etc.
d_compare: called to compare a dentry name with a given name. The first ``d_compare``: called to compare a dentry name with a given name. The first
dentry is the parent of the dentry to be compared, the second is dentry is the parent of the dentry to be compared, the second is
the child dentry. len and name string are properties of the dentry the child dentry. len and name string are properties of the dentry
to be compared. qstr is the name to compare it with. to be compared. qstr is the name to compare it with.
@ -1083,22 +1098,22 @@ struct dentry_operations {
It is a tricky calling convention because it needs to be called under It is a tricky calling convention because it needs to be called under
"rcu-walk", ie. without any locks or references on things. "rcu-walk", ie. without any locks or references on things.
d_delete: called when the last reference to a dentry is dropped and the ``d_delete``: called when the last reference to a dentry is dropped and the
dcache is deciding whether or not to cache it. Return 1 to delete dcache is deciding whether or not to cache it. Return 1 to delete
immediately, or 0 to cache the dentry. Default is NULL which means to immediately, or 0 to cache the dentry. Default is NULL which means to
always cache a reachable dentry. d_delete must be constant and always cache a reachable dentry. d_delete must be constant and
idempotent. idempotent.
d_init: called when a dentry is allocated ``d_init``: called when a dentry is allocated
d_release: called when a dentry is really deallocated ``d_release``: called when a dentry is really deallocated
d_iput: called when a dentry loses its inode (just prior to its ``d_iput``: called when a dentry loses its inode (just prior to its
being deallocated). The default when this is NULL is that the being deallocated). The default when this is NULL is that the
VFS calls iput(). If you define this method, you must call VFS calls iput(). If you define this method, you must call
iput() yourself iput() yourself
d_dname: called when the pathname of a dentry should be generated. ``d_dname``: called when the pathname of a dentry should be generated.
Useful for some pseudo filesystems (sockfs, pipefs, ...) to delay Useful for some pseudo filesystems (sockfs, pipefs, ...) to delay
pathname generation. (Instead of doing it when dentry is created, pathname generation. (Instead of doing it when dentry is created,
it's done only when the path is needed.). Real filesystems probably it's done only when the path is needed.). Real filesystems probably
@ -1112,13 +1127,15 @@ struct dentry_operations {
Example : Example :
.. code-block:: c
static char *pipefs_dname(struct dentry *dent, char *buffer, int buflen) static char *pipefs_dname(struct dentry *dent, char *buffer, int buflen)
{ {
return dynamic_dname(dentry, buffer, buflen, "pipe:[%lu]", return dynamic_dname(dentry, buffer, buflen, "pipe:[%lu]",
dentry->d_inode->i_ino); dentry->d_inode->i_ino);
} }
d_automount: called when an automount dentry is to be traversed (optional). ``d_automount``: called when an automount dentry is to be traversed (optional).
This should create a new VFS mount record and return the record to the This should create a new VFS mount record and return the record to the
caller. The caller is supplied with a path parameter giving the caller. The caller is supplied with a path parameter giving the
automount directory to describe the automount target and the parent automount directory to describe the automount target and the parent
@ -1138,7 +1155,7 @@ struct dentry_operations {
dentry. This is set by __d_instantiate() if S_AUTOMOUNT is set on the dentry. This is set by __d_instantiate() if S_AUTOMOUNT is set on the
inode being added. inode being added.
d_manage: called to allow the filesystem to manage the transition from a ``d_manage``: called to allow the filesystem to manage the transition from a
dentry (optional). This allows autofs, for example, to hold up clients dentry (optional). This allows autofs, for example, to hold up clients
waiting to explore behind a 'mountpoint' while letting the daemon go waiting to explore behind a 'mountpoint' while letting the daemon go
past and construct the subtree there. 0 should be returned to let the past and construct the subtree there. 0 should be returned to let the
@ -1156,7 +1173,7 @@ struct dentry_operations {
This function is only used if DCACHE_MANAGE_TRANSIT is set on the This function is only used if DCACHE_MANAGE_TRANSIT is set on the
dentry being transited from. dentry being transited from.
d_real: overlay/union type filesystems implement this method to return one of ``d_real``: overlay/union type filesystems implement this method to return one of
the underlying dentries hidden by the overlay. It is used in two the underlying dentries hidden by the overlay. It is used in two
different modes: different modes:
@ -1178,36 +1195,36 @@ Directory Entry Cache API
There are a number of functions defined which permit a filesystem to There are a number of functions defined which permit a filesystem to
manipulate dentries: manipulate dentries:
dget: open a new handle for an existing dentry (this just increments ``dget``: open a new handle for an existing dentry (this just increments
the usage count) the usage count)
dput: close a handle for a dentry (decrements the usage count). If ``dput``: close a handle for a dentry (decrements the usage count). If
the usage count drops to 0, and the dentry is still in its the usage count drops to 0, and the dentry is still in its
parent's hash, the "d_delete" method is called to check whether parent's hash, the "d_delete" method is called to check whether
it should be cached. If it should not be cached, or if the dentry it should be cached. If it should not be cached, or if the dentry
is not hashed, it is deleted. Otherwise cached dentries are put is not hashed, it is deleted. Otherwise cached dentries are put
into an LRU list to be reclaimed on memory shortage. into an LRU list to be reclaimed on memory shortage.
d_drop: this unhashes a dentry from its parents hash list. A ``d_drop``: this unhashes a dentry from its parents hash list. A
subsequent call to dput() will deallocate the dentry if its subsequent call to dput() will deallocate the dentry if its
usage count drops to 0 usage count drops to 0
d_delete: delete a dentry. If there are no other open references to ``d_delete``: delete a dentry. If there are no other open references to
the dentry then the dentry is turned into a negative dentry the dentry then the dentry is turned into a negative dentry
(the d_iput() method is called). If there are other (the d_iput() method is called). If there are other
references, then d_drop() is called instead references, then d_drop() is called instead
d_add: add a dentry to its parents hash list and then calls ``d_add``: add a dentry to its parents hash list and then calls
d_instantiate() d_instantiate()
d_instantiate: add a dentry to the alias hash list for the inode and ``d_instantiate``: add a dentry to the alias hash list for the inode and
updates the "d_inode" member. The "i_count" member in the updates the "d_inode" member. The "i_count" member in the
inode structure should be set/incremented. If the inode inode structure should be set/incremented. If the inode
pointer is NULL, the dentry is called a "negative pointer is NULL, the dentry is called a "negative
dentry". This function is commonly called when an inode is dentry". This function is commonly called when an inode is
created for an existing negative dentry created for an existing negative dentry
d_lookup: look up a dentry given its parent and path name component ``d_lookup``: look up a dentry given its parent and path name component
It looks up the child of that given name from the dcache It looks up the child of that given name from the dcache
hash table. If it is found, the reference count is incremented hash table. If it is found, the reference count is incremented
and the dentry is returned. The caller must use dput() and the dentry is returned. The caller must use dput()