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Subject[PATCH] docs: filesystems: vfs: Render method descriptions
Date
Currently vfs.rst does not render well into HTML the method descriptions
for VFS data structures. We can improve the HTML output by putting the
description string on a new line following the method name.

Suggested-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Tobin C. Harding <tobin@kernel.org>
---

Jon,

As discussed on LKML; this patch applies on top of the series

[PATCH v4 0/9] docs: Convert VFS doc to RST

If it does not apply cleanly to your branch please feel free to ask me
to fix it.

thanks,
Tobin.

Documentation/filesystems/vfs.rst | 1146 ++++++++++++++++-------------
1 file changed, 642 insertions(+), 504 deletions(-)

diff --git a/Documentation/filesystems/vfs.rst b/Documentation/filesystems/vfs.rst
index 3acb74bdddf6..4899085e98d9 100644
--- a/Documentation/filesystems/vfs.rst
+++ b/Documentation/filesystems/vfs.rst
@@ -127,35 +127,46 @@ members are defined:
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

-``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 filesystem should
-be mounted
+``mount``
+ the method to call when a new instance of this filesystem should
+ be mounted

-``kill_sb``: the method to call when an instance of this filesystem
- should be shut down
+``kill_sb``
+ the method to call when an instance of this filesystem should be
+ shut down

-``owner``: for internal VFS use: you should initialize this to THIS_MODULE in
- most cases.

-``next``: for internal VFS use: you should initialize this to NULL
+``owner``
+ for internal VFS use: you should initialize this to THIS_MODULE
+ in most cases.
+
+``next``
+ for internal VFS use: you should initialize this to NULL

s_lock_key, s_umount_key: lockdep-specific

The mount() method has the following arguments:

-``struct file_system_type *fs_type``: describes the filesystem, partly initialized
- by the specific filesystem code
+``struct file_system_type *fs_type``
+ describes the filesystem, partly initialized 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
- string (see "Mount Options" section)
+``void *data``
+ arbitrary mount options, usually comes as an ASCII string (see
+ "Mount Options" section)

The mount() method must return the root dentry of the tree requested by
caller. An active reference to its superblock must be grabbed and the
@@ -180,22 +191,27 @@ implementation.
Usually, a filesystem uses one of the generic mount() implementations
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
- all mounts
+``mount_single``
+ mount a filesystem which shares the instance between all mounts

A fill_super() callback implementation has the following arguments:

-``struct super_block *sb``: the superblock structure. The callback
- must initialize this properly.
+``struct super_block *sb``
+ the superblock structure. The callback must initialize this
+ properly.

-``void *data``: arbitrary mount options, usually comes as an ASCII
- string (see "Mount Options" section)
+``void *data``
+ arbitrary mount options, usually comes as an ASCII 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
@@ -242,87 +258,106 @@ 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
or bottom half).

-``alloc_inode``: this method is called by alloc_inode() to allocate memory
- for struct inode and initialize it. If this function is not
+``alloc_inode``
+ this method is called by alloc_inode() to allocate memory for
+ struct inode and initialize it. If this function is not
defined, a simple 'struct inode' is allocated. Normally
alloc_inode will be used to allocate a larger structure which
contains a 'struct inode' embedded within it.

-``destroy_inode``: this method is called by destroy_inode() to release
- resources allocated for struct inode. It is only required if
+``destroy_inode``
+ this method is called by destroy_inode() to release resources
+ allocated for struct inode. It is only required if
->alloc_inode was defined and simply undoes anything done by
->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
- inode to disc. The second parameter indicates whether the write
- should be synchronous or not, not all filesystems check this flag.
+``write_inode``
+ this method is called when the VFS needs to write an inode to
+ disc. The second parameter indicates whether the write should
+ be synchronous or not, not all filesystems check this flag.

-``drop_inode``: called when the last access to the inode is dropped,
- with the inode->i_lock spinlock held.
+``drop_inode``
+ called when the last access to the inode is dropped, with the
+ inode->i_lock spinlock held.

This method should be either NULL (normal UNIX filesystem
- semantics) or "generic_delete_inode" (for filesystems that do not
- want to cache inodes - causing "delete_inode" to always be
+ semantics) or "generic_delete_inode" (for filesystems that do
+ not want to cache inodes - causing "delete_inode" to always be
called regardless of the value of i_nlink)

- The "generic_delete_inode()" behavior is equivalent to the
- old practice of using "force_delete" in the put_inode() case,
- but does not have the races that the "force_delete()" approach
- had.
+ The "generic_delete_inode()" behavior is equivalent to the old
+ practice of using "force_delete" in the put_inode() case, but
+ does not have the races that the "force_delete()" approach 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

-``sync_fs``: called when VFS is writing out all dirty data associated with
- a superblock. The second parameter indicates whether the method
+``sync_fs``
+ called when VFS is writing out all dirty data associated with a
+ superblock. The second parameter indicates whether the method
should wait until the write out has been completed. Optional.

-``freeze_fs``: called when VFS is locking a filesystem and
- forcing it into a consistent state. This method is currently
- used by the Logical Volume Manager (LVM).
+``freeze_fs``
+ called when VFS is locking a filesystem and forcing it into a
+ consistent state. This method is currently 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.

-``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
- with the kernel lock held
+``remount_fs``
+ called when the filesystem is remounted. This is called 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
- /proc/<pid>/mounts. (see "Mount Options" section)
+``show_options``
+ called by the VFS to show mount options for /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
- filesystem to return the number of freeable cached objects it contains.
+``nr_cached_objects``
+ called by the sb cache shrinking function for the filesystem to
+ return the number of freeable cached objects it contains.
Optional.

-``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.
- Optional, but any filesystem implementing this method needs to also
- implement ->nr_cached_objects for it to be called correctly.
+``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.
+ Optional, but any filesystem implementing this method needs to
+ also implement ->nr_cached_objects for it to be called
+ correctly.

We can't do anything with any errors that the filesystem might
- encountered, hence the void return type. This will never be called if
- the VM is trying to reclaim under GFP_NOFS conditions, hence this
- method does not need to handle that situation itself.
+ encountered, hence the void return type. This will never be
+ called if the VM is trying to reclaim under GFP_NOFS conditions,
+ hence this method does not need to handle that situation itself.

- Implementations must include conditional reschedule calls inside any
- scanning loop that is done. This allows the VFS to determine
- appropriate scan batch sizes without having to worry about whether
- implementations will cause holdoff problems due to large scan batch
- sizes.
+ Implementations must include conditional reschedule calls inside
+ any scanning loop that is done. This allows the VFS to
+ determine appropriate scan batch sizes without having to worry
+ about whether implementations will cause holdoff problems due to
+ large scan batch sizes.

Whoever sets up the inode is responsible for filling in the "i_op"
field. This is a pointer to a "struct inode_operations" which describes
@@ -336,23 +371,31 @@ On filesystems that support extended attributes (xattrs), the s_xattr
superblock field points to a NULL-terminated array of xattr handlers.
Extended attributes are name:value pairs.

-``name``: Indicates that the handler matches attributes with the specified name
- (such as "system.posix_acl_access"); the prefix field must be NULL.
+``name``
+ Indicates that the handler matches attributes with the specified
+ name (such as "system.posix_acl_access"); the prefix field must
+ be NULL.

-``prefix``: Indicates that the handler matches all attributes with the specified
- name prefix (such as "user."); the name field must be NULL.
+``prefix``
+ Indicates that the handler matches all attributes with the
+ specified name prefix (such as "user."); the name field must be
+ NULL.

-``list``: Determine if attributes matching this xattr handler should be listed
- for a particular dentry. Used by some listxattr implementations like
- generic_listxattr.
+``list``
+ Determine if attributes matching this xattr handler should be
+ listed for a particular dentry. Used by some listxattr
+ implementations like generic_listxattr.

-``get``: Called by the VFS to get the value of a particular extended attribute.
- This method is called by the getxattr(2) system call.
+``get``
+ Called by the VFS to get the value of a particular extended
+ attribute. This method is called by the getxattr(2) system
+ call.

-``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
- attribute. This method is called by the the setxattr(2) and
- removexattr(2) system calls.
+``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 attribute. This method is called by the the
+ setxattr(2) and removexattr(2) system calls.

When none of the xattr handlers of a filesystem match the specified
attribute name or when a filesystem doesn't support extended attributes,
@@ -401,121 +444,141 @@ As of kernel 2.6.22, the following members are defined:
Again, all methods are called without any locks being held, unless
otherwise noted.

-``create``: called by the open(2) and creat(2) system calls. Only
- required if you want to support regular files. The dentry you
- get should not have an inode (i.e. it should be a negative
- dentry). Here you will probably call d_instantiate() with the
- dentry and the newly created inode
+``create``
+ called by the open(2) and creat(2) system calls. Only required
+ if you want to support regular files. The dentry you get should
+ not have an inode (i.e. it should be a negative dentry). Here
+ you will probably call d_instantiate() with the 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
method must call d_add() to insert the found inode into the
dentry. The "i_count" field in the inode structure should be
incremented. If the named inode does not exist a NULL inode
should be inserted into the dentry (this is called a negative
- dentry). Returning an error code from this routine must only
- be done on a real error, otherwise creating inodes with system
+ dentry). Returning an error code from this routine must only be
+ done on a real error, otherwise creating inodes with system
calls like create(2), mknod(2), mkdir(2) and so on will fail.
If you wish to overload the dentry methods then you should
- initialise the "d_dop" field in the dentry; this is a pointer
- to a struct "dentry_operations".
- This method is called with the directory inode semaphore held
+ initialise the "d_dop" field in the dentry; this is a pointer to
+ a struct "dentry_operations". This method is called with the
+ directory inode semaphore held

-``link``: called by the link(2) system call. Only required if you want
- to support hard links. You will probably need to call
+``link``
+ called by the link(2) system call. Only required if you want to
+ support hard links. You will probably need to call
d_instantiate() just as you would in the create() method

-``unlink``: called by the unlink(2) system call. Only required if you
- want to support deleting inodes
+``unlink``
+ called by the unlink(2) system call. Only required if you want
+ to support deleting inodes

-``symlink``: called by the symlink(2) system call. Only required if you
- want to support symlinks. You will probably need to call
+``symlink``
+ called by the symlink(2) system call. Only required if you want
+ to support symlinks. You will probably need to call
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
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

-``mknod``: called by the mknod(2) system call to create a device (char,
- block) inode or a named pipe (FIFO) or socket. Only required
- if you want to support creating these types of inodes. You
- will probably need to call d_instantiate() just as you would
- in the create() method
+``mknod``
+ called by the mknod(2) system call to create a device (char,
+ block) inode or a named pipe (FIFO) or socket. Only required if
+ you want to support creating these types of inodes. You will
+ probably need to call d_instantiate() just as you would in the
+ create() method

-``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.
+``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.

The filesystem must return -EINVAL for any unsupported or
- unknown flags. Currently the following flags are implemented:
- (1) RENAME_NOREPLACE: this flag indicates that if the target
- of the rename exists the rename should fail with -EEXIST
- instead of replacing the target. The VFS already checks for
- existence, so for local filesystems the RENAME_NOREPLACE
- implementation is equivalent to plain rename.
+ unknown flags. Currently the following flags are implemented:
+ (1) RENAME_NOREPLACE: this flag indicates that if the target of
+ the rename exists the rename should fail with -EEXIST instead of
+ replacing the target. The VFS already checks for existence, so
+ for local filesystems the RENAME_NOREPLACE implementation is
+ equivalent to plain rename.
(2) RENAME_EXCHANGE: exchange source and target. Both must
- exist; this is checked by the VFS. Unlike plain rename,
- source and target may be of different type.
-
-``get_link``: called by the VFS to follow a symbolic link to the
- inode it points to. Only required if you want to support
- symbolic links. This method returns the symlink body
- to traverse (and possibly resets the current position with
- nd_jump_link()). If the body won't go away until the inode
- is gone, nothing else is needed; if it needs to be otherwise
- pinned, arrange for its release by having get_link(..., ..., done)
- do set_delayed_call(done, destructor, argument).
- In that case destructor(argument) will be called once VFS is
- done with the body you've returned.
- May be called in RCU mode; that is indicated by NULL dentry
+ exist; this is checked by the VFS. Unlike plain rename, source
+ and target may be of different type.
+
+``get_link``
+ called by the VFS to follow a symbolic link to the inode it
+ points to. Only required if you want to support symbolic links.
+ This method returns the symlink body to traverse (and possibly
+ resets the current position with nd_jump_link()). If the body
+ won't go away until the inode is gone, nothing else is needed;
+ if it needs to be otherwise pinned, arrange for its release by
+ having get_link(..., ..., done) do set_delayed_call(done,
+ destructor, argument). In that case destructor(argument) will
+ be called once VFS is done with the body you've returned. May
+ be called in RCU mode; that is indicated by NULL dentry
argument. If request can't be handled without leaving RCU mode,
have it return ERR_PTR(-ECHILD).

-``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
fact a symlink. Normally filesystems should only implement
->get_link for symlinks and readlink(2) will automatically use
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.

- May be called in rcu-walk mode (mask & MAY_NOT_BLOCK). If in rcu-walk
- mode, the filesystem must check the permission without blocking or
- storing to the inode.
+ May be called in rcu-walk mode (mask & MAY_NOT_BLOCK). If in
+ rcu-walk mode, the filesystem must check the permission without
+ blocking or storing to the inode.

- 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.

-``setattr``: called by the VFS to set attributes for a file. This method
- is called by chmod(2) and related system calls.
-
-``getattr``: called by the VFS to get attributes of a file. This method
- is called by stat(2) and related system calls.
-
-``listxattr``: called by the VFS to list all extended attributes for a
- 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
- an inode. If this is not defined the VFS will update the inode itself
- and call mark_inode_dirty_sync.
-
-``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
- 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
- something filesystem won't do atomic open for), it may signal this by
- returning finish_no_open(file, dentry). This method is only called if
- the last component is negative or needs lookup. Cached positive dentries
- are still handled by f_op->open(). If the file was created,
- FMODE_CREATED flag should be set in file->f_mode. In case of O_EXCL
- the method must only succeed if the file didn't exist and hence FMODE_CREATED
- shall always be set on success.
-
-``tmpfile``: called in the end of O_TMPFILE open(). Optional, equivalent to
- atomically creating, opening and unlinking a file in given directory.
+``setattr``
+ called by the VFS to set attributes for a file. This method is
+ called by chmod(2) and related system calls.
+
+``getattr``
+ called by the VFS to get attributes of a file. This method is
+ called by stat(2) and related system calls.
+
+``listxattr``
+ called by the VFS to list all extended attributes for a 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
+ an inode. If this is not defined the VFS will update the inode
+ itself and call mark_inode_dirty_sync.
+
+``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 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 something filesystem won't do atomic
+ open for), it may signal this by returning finish_no_open(file,
+ dentry). This method is only called if the last component is
+ negative or needs lookup. Cached positive dentries are still
+ handled by f_op->open(). If the file was created, FMODE_CREATED
+ flag should be set in file->f_mode. In case of O_EXCL the
+ method must only succeed if the file didn't exist and hence
+ FMODE_CREATED shall always be set on success.
+
+``tmpfile``
+ called in the end of O_TMPFILE open(). Optional, equivalent to
+ atomically creating, opening and unlinking a file in given
+ directory.


The Address Space Object
@@ -668,70 +731,75 @@ cache in your filesystem. The following members are defined:
int (*swap_deactivate)(struct file *);
};

-``writepage``: called by the VM to write a dirty page to backing store.
- This may happen for data integrity reasons (i.e. 'sync'), or
- to free up memory (flush). The difference can be seen in
- wbc->sync_mode.
- The PG_Dirty flag has been cleared and PageLocked is true.
- writepage should start writeout, should set PG_Writeback,
- and should make sure the page is unlocked, either synchronously
- or asynchronously when the write operation completes.
-
- If wbc->sync_mode is WB_SYNC_NONE, ->writepage doesn't have to
- try too hard if there are problems, and may choose to write out
- other pages from the mapping if that is easier (e.g. due to
- internal dependencies). If it chooses not to start writeout, it
- should return AOP_WRITEPAGE_ACTIVATE so that the VM will not keep
- calling ->writepage on that page.
-
- See the file "Locking" for more details.
-
-``readpage``: called by the VM to read a page from backing store.
- The page will be Locked when readpage is called, and should be
- unlocked and marked uptodate once the read completes.
- If ->readpage discovers that it needs to unlock the page for
- some reason, it can do so, and then return AOP_TRUNCATED_PAGE.
- In this case, the page will be relocated, relocked and if
- that all succeeds, ->readpage will be called again.
-
-``writepages``: called by the VM to write out pages associated with the
+``writepage``
+ called by the VM to write a dirty page to backing store. This
+ may happen for data integrity reasons (i.e. 'sync'), or to free
+ up memory (flush). The difference can be seen in
+ wbc->sync_mode. The PG_Dirty flag has been cleared and
+ PageLocked is true. writepage should start writeout, should set
+ PG_Writeback, and should make sure the page is unlocked, either
+ synchronously or asynchronously when the write operation
+ completes.
+
+ If wbc->sync_mode is WB_SYNC_NONE, ->writepage doesn't have to
+ try too hard if there are problems, and may choose to write out
+ other pages from the mapping if that is easier (e.g. due to
+ internal dependencies). If it chooses not to start writeout, it
+ should return AOP_WRITEPAGE_ACTIVATE so that the VM will not
+ keep calling ->writepage on that page.
+
+ See the file "Locking" for more details.
+
+``readpage``
+ called by the VM to read a page from backing store. The page
+ will be Locked when readpage is called, and should be unlocked
+ and marked uptodate once the read completes. If ->readpage
+ discovers that it needs to unlock the page for some reason, it
+ can do so, and then return AOP_TRUNCATED_PAGE. In this case,
+ the page will be relocated, relocked and if that all succeeds,
+ ->readpage will be called again.
+
+``writepages``
+ called by the VM to write out pages associated with the
address_space object. If wbc->sync_mode is WBC_SYNC_ALL, then
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
- and that many pages should be written if possible.
- If no ->writepages is given, then mpage_writepages is used
- instead. This will choose pages from the address space that are
- tagged as DIRTY and will pass them to ->writepage.
-
-``set_page_dirty``: called by the VM to set a page dirty.
- This is particularly needed if an address space attaches
- 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
- mapped page gets modified.
+ written out. If it is WBC_SYNC_NONE, then a nr_to_write is
+ given and that many pages should be written if possible. If no
+ ->writepages is given, then mpage_writepages is used instead.
+ This will choose pages from the address space that are tagged as
+ DIRTY and will pass them to ->writepage.
+
+``set_page_dirty``
+ called by the VM to set a page dirty. This is particularly
+ needed if an address space attaches 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 mapped page gets modified.
If defined, it should set the PageDirty flag, and the
PAGECACHE_TAG_DIRTY tag in the radix tree.

-``readpages``: called by the VM to read pages associated with the address_space
- object. This is essentially just a vector version of
- readpage. Instead of just one page, several pages are
- requested.
+``readpages``
+ called by the VM to read pages associated with the address_space
+ object. This is essentially just a vector version of readpage.
+ Instead of just one page, several pages are requested.
readpages is only used for read-ahead, so read errors are
ignored. If anything goes wrong, feel free to give up.

-``write_begin``:
- 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
- address_space should check that the write will be able to complete,
- by allocating space if necessary and doing any other internal
- housekeeping. If the write will update parts of any basic-blocks on
- storage, then those blocks should be pre-read (if they haven't been
- read already) so that the updated blocks can be written out properly.
+``write_begin``
+ 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 address_space should check that the write will be able to
+ complete, by allocating space if necessary and doing any other
+ internal housekeeping. If the write will update parts of any
+ basic-blocks on storage, then those blocks should be pre-read
+ (if they haven't been read already) so that the updated blocks
+ can be written out properly.

- The filesystem must return the locked pagecache page for the specified
- offset, in ``*pagep``, for the caller to write into.
+ The filesystem must return the locked pagecache page for the
+ specified offset, in ``*pagep``, for the caller to write into.

- 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).
+ 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).

flags is a field for AOP_FLAG_xxx flags, described in
include/linux/fs.h.
@@ -739,114 +807,128 @@ cache in your filesystem. The following members are defined:
A void * may be returned in fsdata, which then gets passed into
write_end.

- Returns 0 on success; < 0 on failure (which is the error code), in
- which case write_end is not called.
-
-``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
- is the amount that was able to be copied.
-
- The filesystem must take care of unlocking the page and releasing it
- refcount, and updating i_size.
-
- Returns < 0 on failure, otherwise the number of bytes (<= 'copied')
- that were able to be copied into pagecache.
-
-``bmap``: called by the VFS to map a logical block offset within object to
- physical block number. This method is used by the FIBMAP
- 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
- device. The swap system does not go through the filesystem
- but instead uses bmap to find out where the blocks in the file
- are and uses those addresses directly.
-
-``invalidatepage``: If a page has PagePrivate set, then invalidatepage
- will be called when part or all of the page is to be removed
- from the address space. This generally corresponds to either a
- truncation, punch hole or a complete invalidation of the address
+ Returns 0 on success; < 0 on failure (which is the error code),
+ in which case write_end is not called.
+
+``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 is the amount that was able to be copied.
+
+ The filesystem must take care of unlocking the page and
+ releasing it refcount, and updating i_size.
+
+ Returns < 0 on failure, otherwise the number of bytes (<=
+ 'copied') that were able to be copied into pagecache.
+
+``bmap``
+ called by the VFS to map a logical block offset within object to
+ physical block number. This method is used by the FIBMAP 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 device. The swap
+ system does not go through the filesystem but instead uses bmap
+ to find out where the blocks in the file are and uses those
+ addresses directly.
+
+``invalidatepage``
+ If a page has PagePrivate set, then invalidatepage will be
+ called when part or all of the page is to be removed from the
+ address space. This generally corresponds to either a
+ truncation, punch hole or a complete invalidation of the address
space (in the latter case 'offset' will always be 0 and 'length'
will be PAGE_SIZE). Any private data associated with the page
- should be updated to reflect this truncation. If offset is 0 and
- length is PAGE_SIZE, then the private data should be released,
- because the page must be able to be completely discarded. This may
- be done by calling the ->releasepage function, but in this case the
- release MUST succeed.
-
-``releasepage``: releasepage is called on PagePrivate pages to indicate
- that the page should be freed if possible. ->releasepage
- should remove any private data from the page and clear the
- PagePrivate flag. If releasepage() fails for some reason, it must
- indicate failure with a 0 return value.
- releasepage() is used in two distinct though related cases. The
- first is when the VM finds a clean page with no active users and
- wants to make it a free page. If ->releasepage succeeds, the
- page will be removed from the address_space and become free.
+ should be updated to reflect this truncation. If offset is 0
+ and length is PAGE_SIZE, then the private data should be
+ released, because the page must be able to be completely
+ discarded. This may be done by calling the ->releasepage
+ function, but in this case the release MUST succeed.
+
+``releasepage``
+ releasepage is called on PagePrivate pages to indicate that the
+ page should be freed if possible. ->releasepage should remove
+ any private data from the page and clear the PagePrivate flag.
+ If releasepage() fails for some reason, it must indicate failure
+ with a 0 return value. releasepage() is used in two distinct
+ though related cases. The first is when the VM finds a clean
+ page with no active users and wants to make it a free page. If
+ ->releasepage succeeds, the page will be removed from the
+ address_space and become free.

The second case is when a request has been made to invalidate
- some or all pages in an address_space. This can happen
- through the fadvise(POSIX_FADV_DONTNEED) system call or by the
- filesystem explicitly requesting it as nfs and 9fs do (when
- they believe the cache may be out of date with storage) by
- calling invalidate_inode_pages2().
- If the filesystem makes such a call, and needs to be certain
- that all pages are invalidated, then its releasepage will
- need to ensure this. Possibly it can clear the PageUptodate
- bit if it cannot free private data yet.
-
-``freepage``: freepage is called once the page is no longer visible in
- the page cache in order to allow the cleanup of any private
- data. Since it may be called by the memory reclaimer, it
- should not assume that the original address_space mapping still
- exists, and it should not block.
-
-``direct_IO``: called by the generic read/write routines to perform
- direct_IO - that is IO requests which bypass the page cache
- and transfer data directly between the storage and the
- application's address space.
-
-``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
- via __SetPageIsolated.
-
-``migrate_page``: This is used to compact the physical memory usage.
- If the VM wants to relocate a page (maybe off a memory card
- that is signalling imminent failure) it will pass a new page
- and an old page to this function. migrate_page should
- transfer any private data across and update any references
- that it has to the page.
-
-``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
- prevent redirtying the page, it is kept locked during the whole
- operation.
-
-``is_partially_uptodate``: Called by the VM when reading a file through the
- pagecache when the underlying blocksize != pagesize. If the required
- block is up to date then the read can complete without needing the IO
- to bring the whole page up to date.
-
-``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
- to stall to allow flushers a chance to complete some IO. Ordinarily
- it can use PageDirty and PageWriteback but some filesystems have
- more complex state (unstable pages in NFS prevent reclaim) or
- do not set those flags due to locking problems. This callback
- allows a filesystem to indicate to the VM if a page should be
- treated as dirty or writeback for the purposes of stalling.
-
-``error_remove_page``: normally set to generic_error_remove_page if truncation
- is ok for this address space. Used for memory failure handling.
+ some or all pages in an address_space. This can happen through
+ the fadvise(POSIX_FADV_DONTNEED) system call or by the
+ filesystem explicitly requesting it as nfs and 9fs do (when they
+ believe the cache may be out of date with storage) by calling
+ invalidate_inode_pages2(). If the filesystem makes such a call,
+ and needs to be certain that all pages are invalidated, then its
+ releasepage will need to ensure this. Possibly it can clear the
+ PageUptodate bit if it cannot free private data yet.
+
+``freepage``
+ freepage is called once the page is no longer visible in the
+ page cache in order to allow the cleanup of any private data.
+ Since it may be called by the memory reclaimer, it should not
+ assume that the original address_space mapping still exists, and
+ it should not block.
+
+``direct_IO``
+ called by the generic read/write routines to perform direct_IO -
+ that is IO requests which bypass the page cache and transfer
+ data directly between the storage and the application's address
+ space.
+
+``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 via
+ __SetPageIsolated.
+
+``migrate_page``
+ This is used to compact the physical memory usage. If the VM
+ wants to relocate a page (maybe off a memory card that is
+ signalling imminent failure) it will pass a new page and an old
+ page to this function. migrate_page should transfer any private
+ data across and update any references that it has to the page.
+
+``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 prevent redirtying the page, it is kept locked during the
+ whole operation.
+
+``is_partially_uptodate``
+ Called by the VM when reading a file through the pagecache when
+ the underlying blocksize != pagesize. If the required block is
+ up to date then the read can complete without needing the IO to
+ bring the whole page up to date.
+
+``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 to
+ stall to allow flushers a chance to complete some IO.
+ Ordinarily it can use PageDirty and PageWriteback but some
+ filesystems have more complex state (unstable pages in NFS
+ prevent reclaim) or do not set those flags due to locking
+ problems. This callback allows a filesystem to indicate to the
+ VM if a page should be treated as dirty or writeback for the
+ purposes of stalling.
+
+``error_remove_page``
+ normally set to generic_error_remove_page if truncation is ok
+ for this address space. Used for memory failure handling.
Setting this implies you deal with pages going away under you,
unless you have them locked or reference counts increased.

-``swap_activate``: Called when swapon is used on a file to allocate
- space if necessary and pin the block lookup information in
- memory. A return value of zero indicates success,
- in which case this file can be used to back swapspace.
+``swap_activate``
+ Called when swapon is used on a file to allocate space if
+ necessary and pin the block lookup information in memory. A
+ return value of zero indicates success, in which case this file
+ can be used to back swapspace.

-``swap_deactivate``: Called during swapoff on files where swap_activate
- was successful.
+``swap_deactivate``
+ Called during swapoff on files where swap_activate was
+ successful.


The File Object
@@ -907,91 +989,120 @@ This describes how the VFS can manipulate an open file. As of kernel
Again, all methods are called without any locks being held, unless
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
- when filesystem supports concurrent dir iterators
+``iterate_shared``
+ called when the VFS needs to read the directory contents 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
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
- are used on 64 bit kernels.
+``compat_ioctl``
+ called by the ioctl(2) system call when 32 bit system calls 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
open method for the newly allocated file structure. You might
- think that the open method really belongs in
- "struct inode_operations", and you may be right. I think it's
- done the way it is because it makes filesystems simpler to
- implement. The open() method is a good place to initialize the
+ think that the open method really belongs in "struct
+ inode_operations", and you may be right. I think it's done the
+ way it is because it makes filesystems simpler to implement.
+ The open() method is a good place to initialize the
"private_data" member in the file structure if you want to point
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
- entitled "Handling errors during writeback".
+``fsync``
+ called by the fsync(2) system call. Also see the section above
+ 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

-``lock``: called by the fcntl(2) system call for F_GETLK, F_SETLK, and F_SETLKW
- commands
+``lock``
+ called by the fcntl(2) system call for F_GETLK, F_SETLK, and
+ F_SETLKW 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
- method is used by the splice(2) system call
+``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

-``splice_read``: called by the VFS to splice data from file to a pipe. This
- method is used by the splice(2) system call
+``splice_read``
+ called by the VFS to splice data from file to a pipe. This
+ method is used by the splice(2) system call

-``setlease``: called by the VFS to set or release a file lock lease. setlease
- implementations should call generic_setlease to record or remove
- the lease in the inode after setting it.
+``setlease``
+ called by the VFS to set or release a file lock lease. setlease
+ implementations should call generic_setlease to record or remove
+ 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
- FICLONE and FIDEDUPERANGE commands to remap file ranges. An
- implementation should remap len bytes at pos_in of the source file into
- the dest file at pos_out. Implementations must handle callers passing
- in len == 0; this means "remap to the end of the source file". The
- return value should the number of bytes remapped, or the usual
- negative error code if errors occurred before any bytes were remapped.
- The remap_flags parameter accepts REMAP_FILE_* flags. If
- REMAP_FILE_DEDUP is set then the implementation must only remap if the
- requested file ranges have identical contents. If REMAP_CAN_SHORTEN is
- set, the caller is ok with the implementation shortening the request
- length to satisfy alignment or EOF requirements (or any other reason).
+``remap_file_range``
+ called by the ioctl(2) system call for FICLONERANGE and FICLONE
+ and FIDEDUPERANGE commands to remap file ranges. An
+ implementation should remap len bytes at pos_in of the source
+ file into the dest file at pos_out. Implementations must handle
+ callers passing in len == 0; this means "remap to the end of the
+ source file". The return value should the number of bytes
+ remapped, or the usual negative error code if errors occurred
+ before any bytes were remapped. The remap_flags parameter
+ accepts REMAP_FILE_* flags. If REMAP_FILE_DEDUP is set then the
+ implementation must only remap if the requested file ranges have
+ identical contents. If REMAP_CAN_SHORTEN is set, the caller is
+ ok with the implementation shortening the request 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
filesystem in which the inode resides. When opening a device node
@@ -1036,89 +1147,104 @@ defined:
struct dentry *(*d_real)(struct dentry *, const struct inode *);
};

-``d_revalidate``: called when the VFS needs to revalidate a dentry. This
- is called whenever a name look-up finds a dentry in the
- dcache. Most local filesystems leave this as NULL, because all their
- dentries in the dcache are valid. Network filesystems are different
- since things can change on the server without the client necessarily
- being aware of it.
-
- This function should return a positive value if the dentry is still
- valid, and zero or a negative error code if it isn't.
-
- d_revalidate may be called in rcu-walk mode (flags & LOOKUP_RCU).
- If in rcu-walk mode, the filesystem must revalidate the dentry without
- blocking or storing to the dentry, d_parent and d_inode should not be
- used without care (because they can change and, in d_inode case, even
- become NULL under us).
-
- If a situation is encountered that rcu-walk cannot handle, return
+``d_revalidate``
+ called when the VFS needs to revalidate a dentry. This is
+ called whenever a name look-up finds a dentry in the dcache.
+ Most local filesystems leave this as NULL, because all their
+ dentries in the dcache are valid. Network filesystems are
+ different since things can change on the server without the
+ client necessarily being aware of it.
+
+ This function should return a positive value if the dentry is
+ still valid, and zero or a negative error code if it isn't.
+
+ d_revalidate may be called in rcu-walk mode (flags &
+ LOOKUP_RCU). If in rcu-walk mode, the filesystem must
+ revalidate the dentry without blocking or storing to the dentry,
+ d_parent and d_inode should not be used without care (because
+ they can change and, in d_inode case, even become NULL under
+ us).
+
+ If a situation is encountered that rcu-walk cannot handle,
+ return
-ECHILD and it will be called again in ref-walk mode.

-``_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
- doing a lookup in the parent directory. This includes "/", "." and "..",
- as well as procfs-style symlinks and mountpoint traversal.
+``_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 doing a lookup in the parent directory. This includes "/",
+ "." and "..", as well as procfs-style symlinks and mountpoint
+ traversal.

- In this case, we are less concerned with whether the dentry is still
- fully correct, but rather that the inode is still valid. As with
- d_revalidate, most local filesystems will set this to NULL since their
- dcache entries are always valid.
+ In this case, we are less concerned with whether the dentry is
+ still fully correct, but rather that the inode is still valid.
+ As with d_revalidate, most local filesystems will set this to
+ NULL since their dcache entries are always valid.

- This function has the same return code semantics as d_revalidate.
+ This function has the same return code semantics as
+ d_revalidate.

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
to be hashed into.

Same locking and synchronisation rules as d_compare regarding
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
- the child dentry. len and name string are properties of the dentry
- to be compared. qstr is the name to compare it with.
+ the child dentry. len and name string are properties of the
+ dentry to be compared. qstr is the name to compare it with.

Must be constant and idempotent, and should not take locks if
- possible, and should not or store into the dentry.
- Should not dereference pointers outside the dentry without
- lots of care (eg. d_parent, d_inode, d_name should not be used).
-
- However, our vfsmount is pinned, and RCU held, so the dentries and
- inodes won't disappear, neither will our sb or filesystem module.
- ->d_sb may be used.
-
- It is a tricky calling convention because it needs to be called under
- "rcu-walk", ie. without any locks or references on things.
-
-``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
- immediately, or 0 to cache the dentry. Default is NULL which means to
- always cache a reachable dentry. d_delete must be constant and
- idempotent.
-
-``d_init``: called when a dentry is allocated
-
-``d_release``: called when a dentry is really deallocated
-
-``d_iput``: called when a dentry loses its inode (just prior to its
- being deallocated). The default when this is NULL is that the
- VFS calls iput(). If you define this method, you must call
- iput() yourself
-
-``d_dname``: called when the pathname of a dentry should be generated.
- Useful for some pseudo filesystems (sockfs, pipefs, ...) to delay
- pathname generation. (Instead of doing it when dentry is created,
- it's done only when the path is needed.). Real filesystems probably
- dont want to use it, because their dentries are present in global
- dcache hash, so their hash should be an invariant. As no lock is
- held, d_dname() should not try to modify the dentry itself, unless
- appropriate SMP safety is used. CAUTION : d_path() logic is quite
- tricky. The correct way to return for example "Hello" is to put it
- at the end of the buffer, and returns a pointer to the first char.
- dynamic_dname() helper function is provided to take care of this.
+ possible, and should not or store into the dentry. Should not
+ dereference pointers outside the dentry without lots of care
+ (eg. d_parent, d_inode, d_name should not be used).
+
+ However, our vfsmount is pinned, and RCU held, so the dentries
+ and inodes won't disappear, neither will our sb or filesystem
+ module. ->d_sb may be used.
+
+ It is a tricky calling convention because it needs to be called
+ under "rcu-walk", ie. without any locks or references on things.
+
+``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 immediately, or 0 to cache the dentry. Default is NULL
+ which means to always cache a reachable dentry. d_delete must
+ be constant and idempotent.
+
+``d_init``
+ called when a dentry is allocated
+
+``d_release``
+ called when a dentry is really deallocated
+
+``d_iput``
+ called when a dentry loses its inode (just prior to its being
+ deallocated). The default when this is NULL is that the VFS
+ calls iput(). If you define this method, you must call iput()
+ yourself
+
+``d_dname``
+ called when the pathname of a dentry should be generated.
+ Useful for some pseudo filesystems (sockfs, pipefs, ...) to
+ delay pathname generation. (Instead of doing it when dentry is
+ created, it's done only when the path is needed.). Real
+ filesystems probably dont want to use it, because their dentries
+ are present in global dcache hash, so their hash should be an
+ invariant. As no lock is held, d_dname() should not try to
+ modify the dentry itself, unless appropriate SMP safety is used.
+ CAUTION : d_path() logic is quite tricky. The correct way to
+ return for example "Hello" is to put it at the end of the
+ buffer, and returns a pointer to the first char.
+ dynamic_dname() helper function is provided to take care of
+ this.

Example :

@@ -1130,52 +1256,57 @@ defined:
dentry->d_inode->i_ino);
}

-``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
- caller. The caller is supplied with a path parameter giving the
- automount directory to describe the automount target and the parent
- VFS mount record to provide inheritable mount parameters. NULL should
- be returned if someone else managed to make the automount first. If
- the vfsmount creation failed, then an error code should be returned.
- If -EISDIR is returned, then the directory will be treated as an
- ordinary directory and returned to pathwalk to continue walking.
-
- If a vfsmount is returned, the caller will attempt to mount it on the
- mountpoint and will remove the vfsmount from its expiration list in
- the case of failure. The vfsmount should be returned with 2 refs on
- it to prevent automatic expiration - the caller will clean up the
- additional ref.
-
- This function is only used if DCACHE_NEED_AUTOMOUNT is set on the
- dentry. This is set by __d_instantiate() if S_AUTOMOUNT is set on the
- inode being added.
-
-``d_manage``: called to allow the filesystem to manage the transition from a
- dentry (optional). This allows autofs, for example, to hold up clients
- waiting to explore behind a 'mountpoint' while letting the daemon go
- past and construct the subtree there. 0 should be returned to let the
- calling process continue. -EISDIR can be returned to tell pathwalk to
- use this directory as an ordinary directory and to ignore anything
- mounted on it and not to check the automount flag. Any other error
- code will abort pathwalk completely.
+``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 caller. The caller is supplied with a path parameter
+ giving the automount directory to describe the automount target
+ and the parent VFS mount record to provide inheritable mount
+ parameters. NULL should be returned if someone else managed to
+ make the automount first. If the vfsmount creation failed, then
+ an error code should be returned. If -EISDIR is returned, then
+ the directory will be treated as an ordinary directory and
+ returned to pathwalk to continue walking.
+
+ If a vfsmount is returned, the caller will attempt to mount it
+ on the mountpoint and will remove the vfsmount from its
+ expiration list in the case of failure. The vfsmount should be
+ returned with 2 refs on it to prevent automatic expiration - the
+ caller will clean up the additional ref.
+
+ This function is only used if DCACHE_NEED_AUTOMOUNT is set on
+ the dentry. This is set by __d_instantiate() if S_AUTOMOUNT is
+ set on the inode being added.
+
+``d_manage``
+ called to allow the filesystem to manage the transition from a
+ dentry (optional). This allows autofs, for example, to hold up
+ clients waiting to explore behind a 'mountpoint' while letting
+ the daemon go past and construct the subtree there. 0 should be
+ returned to let the calling process continue. -EISDIR can be
+ returned to tell pathwalk to use this directory as an ordinary
+ directory and to ignore anything mounted on it and not to check
+ the automount flag. Any other error code will abort pathwalk
+ completely.

If the 'rcu_walk' parameter is true, then the caller is doing a
- pathwalk in RCU-walk mode. Sleeping is not permitted in this mode,
- and the caller can be asked to leave it and call again by returning
- -ECHILD. -EISDIR may also be returned to tell pathwalk to
- ignore d_automount or any mounts.
+ pathwalk in RCU-walk mode. Sleeping is not permitted in this
+ mode, and the caller can be asked to leave it and call again by
+ returning -ECHILD. -EISDIR may also be returned to tell
+ pathwalk to ignore d_automount or any mounts.

- This function is only used if DCACHE_MANAGE_TRANSIT is set on the
- dentry being transited from.
+ This function is only used if DCACHE_MANAGE_TRANSIT is set on
+ the dentry being transited from.

-``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
- different modes:
+``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 different modes:

- Called from file_dentry() it returns the real dentry matching the inode
- argument. The real dentry may be from a lower layer already copied up,
- but still referenced from the file. This mode is selected with a
- non-NULL inode argument.
+ Called from file_dentry() it returns the real dentry matching
+ the inode argument. The real dentry may be from a lower layer
+ already copied up, but still referenced from the file. This
+ mode is selected with a non-NULL inode argument.

With NULL inode the topmost real underlying dentry is returned.

@@ -1190,40 +1321,47 @@ Directory Entry Cache API
There are a number of functions defined which permit a filesystem to
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)

-``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
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
- is not hashed, it is deleted. Otherwise cached dentries are put
- into an LRU list to be reclaimed on memory shortage.
-
-``d_drop``: this unhashes a dentry from its parents hash list. A
- subsequent call to dput() will deallocate the dentry if its
- usage count drops to 0
-
-``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 d_iput() method is called). If there are other
- references, then d_drop() is called instead
-
-``d_add``: add a dentry to its parents hash list and then calls
+ 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 into an LRU list to be reclaimed on memory shortage.
+
+``d_drop``
+ this unhashes a dentry from its parents hash list. A subsequent
+ call to dput() will deallocate the dentry if its usage count
+ drops to 0
+
+``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
+ d_iput() method is called). If there are other references, then
+ d_drop() is called instead
+
+``d_add``
+ add a dentry to its parents hash list and then calls
d_instantiate()

-``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
- inode structure should be set/incremented. If the inode
- pointer is NULL, the dentry is called a "negative
- dentry". This function is commonly called when an inode is
- created for an existing negative dentry
-
-``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
- hash table. If it is found, the reference count is incremented
- and the dentry is returned. The caller must use dput()
- to free the dentry when it finishes using it.
+``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 inode
+ structure should be set/incremented. If the inode pointer is
+ NULL, the dentry is called a "negative dentry". This function
+ is commonly called when an inode is created for an existing
+ negative dentry
+
+``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 hash
+ table. If it is found, the reference count is incremented and
+ the dentry is returned. The caller must use dput() to free the
+ dentry when it finishes using it.


Mount Options
--
2.21.0
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