[PATCH 08/10] autofs - rename autofs documentation files
From: Ian Kent
Date: Sun May 13 2018 - 23:14:57 EST
There are two files in Documentation/filsystems that should now
use autofs rather than autofs4 in their names.
Signed-off-by: Ian Kent <raven@xxxxxxxxxx>
---
Documentation/filesystems/autofs-mount-control.txt | 407 +++++++++++++++
Documentation/filesystems/autofs.txt | 529 ++++++++++++++++++++
.../filesystems/autofs4-mount-control.txt | 407 ---------------
Documentation/filesystems/autofs4.txt | 529 --------------------
4 files changed, 936 insertions(+), 936 deletions(-)
create mode 100644 Documentation/filesystems/autofs-mount-control.txt
create mode 100644 Documentation/filesystems/autofs.txt
delete mode 100644 Documentation/filesystems/autofs4-mount-control.txt
delete mode 100644 Documentation/filesystems/autofs4.txt
diff --git a/Documentation/filesystems/autofs-mount-control.txt b/Documentation/filesystems/autofs-mount-control.txt
new file mode 100644
index 000000000000..e5177cb31a04
--- /dev/null
+++ b/Documentation/filesystems/autofs-mount-control.txt
@@ -0,0 +1,407 @@
+
+Miscellaneous Device control operations for the autofs4 kernel module
+====================================================================
+
+The problem
+===========
+
+There is a problem with active restarts in autofs (that is to say
+restarting autofs when there are busy mounts).
+
+During normal operation autofs uses a file descriptor opened on the
+directory that is being managed in order to be able to issue control
+operations. Using a file descriptor gives ioctl operations access to
+autofs specific information stored in the super block. The operations
+are things such as setting an autofs mount catatonic, setting the
+expire timeout and requesting expire checks. As is explained below,
+certain types of autofs triggered mounts can end up covering an autofs
+mount itself which prevents us being able to use open(2) to obtain a
+file descriptor for these operations if we don't already have one open.
+
+Currently autofs uses "umount -l" (lazy umount) to clear active mounts
+at restart. While using lazy umount works for most cases, anything that
+needs to walk back up the mount tree to construct a path, such as
+getcwd(2) and the proc file system /proc/<pid>/cwd, no longer works
+because the point from which the path is constructed has been detached
+from the mount tree.
+
+The actual problem with autofs is that it can't reconnect to existing
+mounts. Immediately one thinks of just adding the ability to remount
+autofs file systems would solve it, but alas, that can't work. This is
+because autofs direct mounts and the implementation of "on demand mount
+and expire" of nested mount trees have the file system mounted directly
+on top of the mount trigger directory dentry.
+
+For example, there are two types of automount maps, direct (in the kernel
+module source you will see a third type called an offset, which is just
+a direct mount in disguise) and indirect.
+
+Here is a master map with direct and indirect map entries:
+
+/- /etc/auto.direct
+/test /etc/auto.indirect
+
+and the corresponding map files:
+
+/etc/auto.direct:
+
+/automount/dparse/g6 budgie:/autofs/export1
+/automount/dparse/g1 shark:/autofs/export1
+and so on.
+
+/etc/auto.indirect:
+
+g1 shark:/autofs/export1
+g6 budgie:/autofs/export1
+and so on.
+
+For the above indirect map an autofs file system is mounted on /test and
+mounts are triggered for each sub-directory key by the inode lookup
+operation. So we see a mount of shark:/autofs/export1 on /test/g1, for
+example.
+
+The way that direct mounts are handled is by making an autofs mount on
+each full path, such as /automount/dparse/g1, and using it as a mount
+trigger. So when we walk on the path we mount shark:/autofs/export1 "on
+top of this mount point". Since these are always directories we can
+use the follow_link inode operation to trigger the mount.
+
+But, each entry in direct and indirect maps can have offsets (making
+them multi-mount map entries).
+
+For example, an indirect mount map entry could also be:
+
+g1 \
+ / shark:/autofs/export5/testing/test \
+ /s1 shark:/autofs/export/testing/test/s1 \
+ /s2 shark:/autofs/export5/testing/test/s2 \
+ /s1/ss1 shark:/autofs/export1 \
+ /s2/ss2 shark:/autofs/export2
+
+and a similarly a direct mount map entry could also be:
+
+/automount/dparse/g1 \
+ / shark:/autofs/export5/testing/test \
+ /s1 shark:/autofs/export/testing/test/s1 \
+ /s2 shark:/autofs/export5/testing/test/s2 \
+ /s1/ss1 shark:/autofs/export2 \
+ /s2/ss2 shark:/autofs/export2
+
+One of the issues with version 4 of autofs was that, when mounting an
+entry with a large number of offsets, possibly with nesting, we needed
+to mount and umount all of the offsets as a single unit. Not really a
+problem, except for people with a large number of offsets in map entries.
+This mechanism is used for the well known "hosts" map and we have seen
+cases (in 2.4) where the available number of mounts are exhausted or
+where the number of privileged ports available is exhausted.
+
+In version 5 we mount only as we go down the tree of offsets and
+similarly for expiring them which resolves the above problem. There is
+somewhat more detail to the implementation but it isn't needed for the
+sake of the problem explanation. The one important detail is that these
+offsets are implemented using the same mechanism as the direct mounts
+above and so the mount points can be covered by a mount.
+
+The current autofs implementation uses an ioctl file descriptor opened
+on the mount point for control operations. The references held by the
+descriptor are accounted for in checks made to determine if a mount is
+in use and is also used to access autofs file system information held
+in the mount super block. So the use of a file handle needs to be
+retained.
+
+
+The Solution
+============
+
+To be able to restart autofs leaving existing direct, indirect and
+offset mounts in place we need to be able to obtain a file handle
+for these potentially covered autofs mount points. Rather than just
+implement an isolated operation it was decided to re-implement the
+existing ioctl interface and add new operations to provide this
+functionality.
+
+In addition, to be able to reconstruct a mount tree that has busy mounts,
+the uid and gid of the last user that triggered the mount needs to be
+available because these can be used as macro substitution variables in
+autofs maps. They are recorded at mount request time and an operation
+has been added to retrieve them.
+
+Since we're re-implementing the control interface, a couple of other
+problems with the existing interface have been addressed. First, when
+a mount or expire operation completes a status is returned to the
+kernel by either a "send ready" or a "send fail" operation. The
+"send fail" operation of the ioctl interface could only ever send
+ENOENT so the re-implementation allows user space to send an actual
+status. Another expensive operation in user space, for those using
+very large maps, is discovering if a mount is present. Usually this
+involves scanning /proc/mounts and since it needs to be done quite
+often it can introduce significant overhead when there are many entries
+in the mount table. An operation to lookup the mount status of a mount
+point dentry (covered or not) has also been added.
+
+Current kernel development policy recommends avoiding the use of the
+ioctl mechanism in favor of systems such as Netlink. An implementation
+using this system was attempted to evaluate its suitability and it was
+found to be inadequate, in this case. The Generic Netlink system was
+used for this as raw Netlink would lead to a significant increase in
+complexity. There's no question that the Generic Netlink system is an
+elegant solution for common case ioctl functions but it's not a complete
+replacement probably because its primary purpose in life is to be a
+message bus implementation rather than specifically an ioctl replacement.
+While it would be possible to work around this there is one concern
+that lead to the decision to not use it. This is that the autofs
+expire in the daemon has become far to complex because umount
+candidates are enumerated, almost for no other reason than to "count"
+the number of times to call the expire ioctl. This involves scanning
+the mount table which has proved to be a big overhead for users with
+large maps. The best way to improve this is try and get back to the
+way the expire was done long ago. That is, when an expire request is
+issued for a mount (file handle) we should continually call back to
+the daemon until we can't umount any more mounts, then return the
+appropriate status to the daemon. At the moment we just expire one
+mount at a time. A Generic Netlink implementation would exclude this
+possibility for future development due to the requirements of the
+message bus architecture.
+
+
+autofs4 Miscellaneous Device mount control interface
+====================================================
+
+The control interface is opening a device node, typically /dev/autofs.
+
+All the ioctls use a common structure to pass the needed parameter
+information and return operation results:
+
+struct autofs_dev_ioctl {
+ __u32 ver_major;
+ __u32 ver_minor;
+ __u32 size; /* total size of data passed in
+ * including this struct */
+ __s32 ioctlfd; /* automount command fd */
+
+ /* Command parameters */
+ union {
+ struct args_protover protover;
+ struct args_protosubver protosubver;
+ struct args_openmount openmount;
+ struct args_ready ready;
+ struct args_fail fail;
+ struct args_setpipefd setpipefd;
+ struct args_timeout timeout;
+ struct args_requester requester;
+ struct args_expire expire;
+ struct args_askumount askumount;
+ struct args_ismountpoint ismountpoint;
+ };
+
+ char path[0];
+};
+
+The ioctlfd field is a mount point file descriptor of an autofs mount
+point. It is returned by the open call and is used by all calls except
+the check for whether a given path is a mount point, where it may
+optionally be used to check a specific mount corresponding to a given
+mount point file descriptor, and when requesting the uid and gid of the
+last successful mount on a directory within the autofs file system.
+
+The union is used to communicate parameters and results of calls made
+as described below.
+
+The path field is used to pass a path where it is needed and the size field
+is used account for the increased structure length when translating the
+structure sent from user space.
+
+This structure can be initialized before setting specific fields by using
+the void function call init_autofs_dev_ioctl(struct autofs_dev_ioctl *).
+
+All of the ioctls perform a copy of this structure from user space to
+kernel space and return -EINVAL if the size parameter is smaller than
+the structure size itself, -ENOMEM if the kernel memory allocation fails
+or -EFAULT if the copy itself fails. Other checks include a version check
+of the compiled in user space version against the module version and a
+mismatch results in a -EINVAL return. If the size field is greater than
+the structure size then a path is assumed to be present and is checked to
+ensure it begins with a "/" and is NULL terminated, otherwise -EINVAL is
+returned. Following these checks, for all ioctl commands except
+AUTOFS_DEV_IOCTL_VERSION_CMD, AUTOFS_DEV_IOCTL_OPENMOUNT_CMD and
+AUTOFS_DEV_IOCTL_CLOSEMOUNT_CMD the ioctlfd is validated and if it is
+not a valid descriptor or doesn't correspond to an autofs mount point
+an error of -EBADF, -ENOTTY or -EINVAL (not an autofs descriptor) is
+returned.
+
+
+The ioctls
+==========
+
+An example of an implementation which uses this interface can be seen
+in autofs version 5.0.4 and later in file lib/dev-ioctl-lib.c of the
+distribution tar available for download from kernel.org in directory
+/pub/linux/daemons/autofs/v5.
+
+The device node ioctl operations implemented by this interface are:
+
+
+AUTOFS_DEV_IOCTL_VERSION
+------------------------
+
+Get the major and minor version of the autofs4 device ioctl kernel module
+implementation. It requires an initialized struct autofs_dev_ioctl as an
+input parameter and sets the version information in the passed in structure.
+It returns 0 on success or the error -EINVAL if a version mismatch is
+detected.
+
+
+AUTOFS_DEV_IOCTL_PROTOVER_CMD and AUTOFS_DEV_IOCTL_PROTOSUBVER_CMD
+------------------------------------------------------------------
+
+Get the major and minor version of the autofs4 protocol version understood
+by loaded module. This call requires an initialized struct autofs_dev_ioctl
+with the ioctlfd field set to a valid autofs mount point descriptor
+and sets the requested version number in version field of struct args_protover
+or sub_version field of struct args_protosubver. These commands return
+0 on success or one of the negative error codes if validation fails.
+
+
+AUTOFS_DEV_IOCTL_OPENMOUNT and AUTOFS_DEV_IOCTL_CLOSEMOUNT
+----------------------------------------------------------
+
+Obtain and release a file descriptor for an autofs managed mount point
+path. The open call requires an initialized struct autofs_dev_ioctl with
+the path field set and the size field adjusted appropriately as well
+as the devid field of struct args_openmount set to the device number of
+the autofs mount. The device number can be obtained from the mount options
+shown in /proc/mounts. The close call requires an initialized struct
+autofs_dev_ioct with the ioctlfd field set to the descriptor obtained
+from the open call. The release of the file descriptor can also be done
+with close(2) so any open descriptors will also be closed at process exit.
+The close call is included in the implemented operations largely for
+completeness and to provide for a consistent user space implementation.
+
+
+AUTOFS_DEV_IOCTL_READY_CMD and AUTOFS_DEV_IOCTL_FAIL_CMD
+--------------------------------------------------------
+
+Return mount and expire result status from user space to the kernel.
+Both of these calls require an initialized struct autofs_dev_ioctl
+with the ioctlfd field set to the descriptor obtained from the open
+call and the token field of struct args_ready or struct args_fail set
+to the wait queue token number, received by user space in the foregoing
+mount or expire request. The status field of struct args_fail is set to
+the errno of the operation. It is set to 0 on success.
+
+
+AUTOFS_DEV_IOCTL_SETPIPEFD_CMD
+------------------------------
+
+Set the pipe file descriptor used for kernel communication to the daemon.
+Normally this is set at mount time using an option but when reconnecting
+to a existing mount we need to use this to tell the autofs mount about
+the new kernel pipe descriptor. In order to protect mounts against
+incorrectly setting the pipe descriptor we also require that the autofs
+mount be catatonic (see next call).
+
+The call requires an initialized struct autofs_dev_ioctl with the
+ioctlfd field set to the descriptor obtained from the open call and
+the pipefd field of struct args_setpipefd set to descriptor of the pipe.
+On success the call also sets the process group id used to identify the
+controlling process (eg. the owning automount(8) daemon) to the process
+group of the caller.
+
+
+AUTOFS_DEV_IOCTL_CATATONIC_CMD
+------------------------------
+
+Make the autofs mount point catatonic. The autofs mount will no longer
+issue mount requests, the kernel communication pipe descriptor is released
+and any remaining waits in the queue released.
+
+The call requires an initialized struct autofs_dev_ioctl with the
+ioctlfd field set to the descriptor obtained from the open call.
+
+
+AUTOFS_DEV_IOCTL_TIMEOUT_CMD
+----------------------------
+
+Set the expire timeout for mounts within an autofs mount point.
+
+The call requires an initialized struct autofs_dev_ioctl with the
+ioctlfd field set to the descriptor obtained from the open call.
+
+
+AUTOFS_DEV_IOCTL_REQUESTER_CMD
+------------------------------
+
+Return the uid and gid of the last process to successfully trigger a the
+mount on the given path dentry.
+
+The call requires an initialized struct autofs_dev_ioctl with the path
+field set to the mount point in question and the size field adjusted
+appropriately. Upon return the uid field of struct args_requester contains
+the uid and gid field the gid.
+
+When reconstructing an autofs mount tree with active mounts we need to
+re-connect to mounts that may have used the original process uid and
+gid (or string variations of them) for mount lookups within the map entry.
+This call provides the ability to obtain this uid and gid so they may be
+used by user space for the mount map lookups.
+
+
+AUTOFS_DEV_IOCTL_EXPIRE_CMD
+---------------------------
+
+Issue an expire request to the kernel for an autofs mount. Typically
+this ioctl is called until no further expire candidates are found.
+
+The call requires an initialized struct autofs_dev_ioctl with the
+ioctlfd field set to the descriptor obtained from the open call. In
+addition an immediate expire, independent of the mount timeout, can be
+requested by setting the how field of struct args_expire to 1. If no
+expire candidates can be found the ioctl returns -1 with errno set to
+EAGAIN.
+
+This call causes the kernel module to check the mount corresponding
+to the given ioctlfd for mounts that can be expired, issues an expire
+request back to the daemon and waits for completion.
+
+AUTOFS_DEV_IOCTL_ASKUMOUNT_CMD
+------------------------------
+
+Checks if an autofs mount point is in use.
+
+The call requires an initialized struct autofs_dev_ioctl with the
+ioctlfd field set to the descriptor obtained from the open call and
+it returns the result in the may_umount field of struct args_askumount,
+1 for busy and 0 otherwise.
+
+
+AUTOFS_DEV_IOCTL_ISMOUNTPOINT_CMD
+---------------------------------
+
+Check if the given path is a mountpoint.
+
+The call requires an initialized struct autofs_dev_ioctl. There are two
+possible variations. Both use the path field set to the path of the mount
+point to check and the size field adjusted appropriately. One uses the
+ioctlfd field to identify a specific mount point to check while the other
+variation uses the path and optionally in.type field of struct args_ismountpoint
+set to an autofs mount type. The call returns 1 if this is a mount point
+and sets out.devid field to the device number of the mount and out.magic
+field to the relevant super block magic number (described below) or 0 if
+it isn't a mountpoint. In both cases the the device number (as returned
+by new_encode_dev()) is returned in out.devid field.
+
+If supplied with a file descriptor we're looking for a specific mount,
+not necessarily at the top of the mounted stack. In this case the path
+the descriptor corresponds to is considered a mountpoint if it is itself
+a mountpoint or contains a mount, such as a multi-mount without a root
+mount. In this case we return 1 if the descriptor corresponds to a mount
+point and and also returns the super magic of the covering mount if there
+is one or 0 if it isn't a mountpoint.
+
+If a path is supplied (and the ioctlfd field is set to -1) then the path
+is looked up and is checked to see if it is the root of a mount. If a
+type is also given we are looking for a particular autofs mount and if
+a match isn't found a fail is returned. If the the located path is the
+root of a mount 1 is returned along with the super magic of the mount
+or 0 otherwise.
+
diff --git a/Documentation/filesystems/autofs.txt b/Documentation/filesystems/autofs.txt
new file mode 100644
index 000000000000..f10dd590f69f
--- /dev/null
+++ b/Documentation/filesystems/autofs.txt
@@ -0,0 +1,529 @@
+<head>
+<style> p { max-width:50em} ol, ul {max-width: 40em}</style>
+</head>
+
+autofs - how it works
+=====================
+
+Purpose
+-------
+
+The goal of autofs is to provide on-demand mounting and race free
+automatic unmounting of various other filesystems. This provides two
+key advantages:
+
+1. There is no need to delay boot until all filesystems that
+ might be needed are mounted. Processes that try to access those
+ slow filesystems might be delayed but other processes can
+ continue freely. This is particularly important for
+ network filesystems (e.g. NFS) or filesystems stored on
+ media with a media-changing robot.
+
+2. The names and locations of filesystems can be stored in
+ a remote database and can change at any time. The content
+ in that data base at the time of access will be used to provide
+ a target for the access. The interpretation of names in the
+ filesystem can even be programmatic rather than database-backed,
+ allowing wildcards for example, and can vary based on the user who
+ first accessed a name.
+
+Context
+-------
+
+The "autofs4" filesystem module is only one part of an autofs system.
+There also needs to be a user-space program which looks up names
+and mounts filesystems. This will often be the "automount" program,
+though other tools including "systemd" can make use of "autofs4".
+This document describes only the kernel module and the interactions
+required with any user-space program. Subsequent text refers to this
+as the "automount daemon" or simply "the daemon".
+
+"autofs4" is a Linux kernel module with provides the "autofs"
+filesystem type. Several "autofs" filesystems can be mounted and they
+can each be managed separately, or all managed by the same daemon.
+
+Content
+-------
+
+An autofs filesystem can contain 3 sorts of objects: directories,
+symbolic links and mount traps. Mount traps are directories with
+extra properties as described in the next section.
+
+Objects can only be created by the automount daemon: symlinks are
+created with a regular `symlink` system call, while directories and
+mount traps are created with `mkdir`. The determination of whether a
+directory should be a mount trap or not is quite _ad hoc_, largely for
+historical reasons, and is determined in part by the
+*direct*/*indirect*/*offset* mount options, and the *maxproto* mount option.
+
+If neither the *direct* or *offset* mount options are given (so the
+mount is considered to be *indirect*), then the root directory is
+always a regular directory, otherwise it is a mount trap when it is
+empty and a regular directory when not empty. Note that *direct* and
+*offset* are treated identically so a concise summary is that the root
+directory is a mount trap only if the filesystem is mounted *direct*
+and the root is empty.
+
+Directories created in the root directory are mount traps only if the
+filesystem is mounted *indirect* and they are empty.
+
+Directories further down the tree depend on the *maxproto* mount
+option and particularly whether it is less than five or not.
+When *maxproto* is five, no directories further down the
+tree are ever mount traps, they are always regular directories. When
+the *maxproto* is four (or three), these directories are mount traps
+precisely when they are empty.
+
+So: non-empty (i.e. non-leaf) directories are never mount traps. Empty
+directories are sometimes mount traps, and sometimes not depending on
+where in the tree they are (root, top level, or lower), the *maxproto*,
+and whether the mount was *indirect* or not.
+
+Mount Traps
+---------------
+
+A core element of the implementation of autofs is the Mount Traps
+which are provided by the Linux VFS. Any directory provided by a
+filesystem can be designated as a trap. This involves two separate
+features that work together to allow autofs to do its job.
+
+**DCACHE_NEED_AUTOMOUNT**
+
+If a dentry has the DCACHE_NEED_AUTOMOUNT flag set (which gets set if
+the inode has S_AUTOMOUNT set, or can be set directly) then it is
+(potentially) a mount trap. Any access to this directory beyond a
+"`stat`" will (normally) cause the `d_op->d_automount()` dentry operation
+to be called. The task of this method is to find the filesystem that
+should be mounted on the directory and to return it. The VFS is
+responsible for actually mounting the root of this filesystem on the
+directory.
+
+autofs doesn't find the filesystem itself but sends a message to the
+automount daemon asking it to find and mount the filesystem. The
+autofs `d_automount` method then waits for the daemon to report that
+everything is ready. It will then return "`NULL`" indicating that the
+mount has already happened. The VFS doesn't try to mount anything but
+follows down the mount that is already there.
+
+This functionality is sufficient for some users of mount traps such
+as NFS which creates traps so that mountpoints on the server can be
+reflected on the client. However it is not sufficient for autofs. As
+mounting onto a directory is considered to be "beyond a `stat`", the
+automount daemon would not be able to mount a filesystem on the 'trap'
+directory without some way to avoid getting caught in the trap. For
+that purpose there is another flag.
+
+**DCACHE_MANAGE_TRANSIT**
+
+If a dentry has DCACHE_MANAGE_TRANSIT set then two very different but
+related behaviors are invoked, both using the `d_op->d_manage()`
+dentry operation.
+
+Firstly, before checking to see if any filesystem is mounted on the
+directory, d_manage() will be called with the `rcu_walk` parameter set
+to `false`. It may return one of three things:
+
+- A return value of zero indicates that there is nothing special
+ about this dentry and normal checks for mounts and automounts
+ should proceed.
+
+ autofs normally returns zero, but first waits for any
+ expiry (automatic unmounting of the mounted filesystem) to
+ complete. This avoids races.
+
+- A return value of `-EISDIR` tells the VFS to ignore any mounts
+ on the directory and to not consider calling `->d_automount()`.
+ This effectively disables the **DCACHE_NEED_AUTOMOUNT** flag
+ causing the directory not be a mount trap after all.
+
+ autofs returns this if it detects that the process performing the
+ lookup is the automount daemon and that the mount has been
+ requested but has not yet completed. How it determines this is
+ discussed later. This allows the automount daemon not to get
+ caught in the mount trap.
+
+ There is a subtlety here. It is possible that a second autofs
+ filesystem can be mounted below the first and for both of them to
+ be managed by the same daemon. For the daemon to be able to mount
+ something on the second it must be able to "walk" down past the
+ first. This means that d_manage cannot *always* return -EISDIR for
+ the automount daemon. It must only return it when a mount has
+ been requested, but has not yet completed.
+
+ `d_manage` also returns `-EISDIR` if the dentry shouldn't be a
+ mount trap, either because it is a symbolic link or because it is
+ not empty.
+
+- Any other negative value is treated as an error and returned
+ to the caller.
+
+ autofs can return
+
+ - -ENOENT if the automount daemon failed to mount anything,
+ - -ENOMEM if it ran out of memory,
+ - -EINTR if a signal arrived while waiting for expiry to
+ complete
+ - or any other error sent down by the automount daemon.
+
+
+The second use case only occurs during an "RCU-walk" and so `rcu_walk`
+will be set.
+
+An RCU-walk is a fast and lightweight process for walking down a
+filename path (i.e. it is like running on tip-toes). RCU-walk cannot
+cope with all situations so when it finds a difficulty it falls back
+to "REF-walk", which is slower but more robust.
+
+RCU-walk will never call `->d_automount`; the filesystems must already
+be mounted or RCU-walk cannot handle the path.
+To determine if a mount-trap is safe for RCU-walk mode it calls
+`->d_manage()` with `rcu_walk` set to `true`.
+
+In this case `d_manage()` must avoid blocking and should avoid taking
+spinlocks if at all possible. Its sole purpose is to determine if it
+would be safe to follow down into any mounted directory and the only
+reason that it might not be is if an expiry of the mount is
+underway.
+
+In the `rcu_walk` case, `d_manage()` cannot return -EISDIR to tell the
+VFS that this is a directory that doesn't require d_automount. If
+`rcu_walk` sees a dentry with DCACHE_NEED_AUTOMOUNT set but nothing
+mounted, it *will* fall back to REF-walk. `d_manage()` cannot make the
+VFS remain in RCU-walk mode, but can only tell it to get out of
+RCU-walk mode by returning `-ECHILD`.
+
+So `d_manage()`, when called with `rcu_walk` set, should either return
+-ECHILD if there is any reason to believe it is unsafe to end the
+mounted filesystem, and otherwise should return 0.
+
+autofs will return `-ECHILD` if an expiry of the filesystem has been
+initiated or is being considered, otherwise it returns 0.
+
+
+Mountpoint expiry
+-----------------
+
+The VFS has a mechanism for automatically expiring unused mounts,
+much as it can expire any unused dentry information from the dcache.
+This is guided by the MNT_SHRINKABLE flag. This only applies to
+mounts that were created by `d_automount()` returning a filesystem to be
+mounted. As autofs doesn't return such a filesystem but leaves the
+mounting to the automount daemon, it must involve the automount daemon
+in unmounting as well. This also means that autofs has more control
+of expiry.
+
+The VFS also supports "expiry" of mounts using the MNT_EXPIRE flag to
+the `umount` system call. Unmounting with MNT_EXPIRE will fail unless
+a previous attempt had been made, and the filesystem has been inactive
+and untouched since that previous attempt. autofs4 does not depend on
+this but has its own internal tracking of whether filesystems were
+recently used. This allows individual names in the autofs directory
+to expire separately.
+
+With version 4 of the protocol, the automount daemon can try to
+unmount any filesystems mounted on the autofs filesystem or remove any
+symbolic links or empty directories any time it likes. If the unmount
+or removal is successful the filesystem will be returned to the state
+it was before the mount or creation, so that any access of the name
+will trigger normal auto-mount processing. In particlar, `rmdir` and
+`unlink` do not leave negative entries in the dcache as a normal
+filesystem would, so an attempt to access a recently-removed object is
+passed to autofs for handling.
+
+With version 5, this is not safe except for unmounting from top-level
+directories. As lower-level directories are never mount traps, other
+processes will see an empty directory as soon as the filesystem is
+unmounted. So it is generally safest to use the autofs expiry
+protocol described below.
+
+Normally the daemon only wants to remove entries which haven't been
+used for a while. For this purpose autofs maintains a "`last_used`"
+time stamp on each directory or symlink. For symlinks it genuinely
+does record the last time the symlink was "used" or followed to find
+out where it points to. For directories the field is a slight
+misnomer. It actually records the last time that autofs checked if
+the directory or one of its descendents was busy and found that it
+was. This is just as useful and doesn't require updating the field so
+often.
+
+The daemon is able to ask autofs if anything is due to be expired,
+using an `ioctl` as discussed later. For a *direct* mount, autofs
+considers if the entire mount-tree can be unmounted or not. For an
+*indirect* mount, autofs considers each of the names in the top level
+directory to determine if any of those can be unmounted and cleaned
+up.
+
+There is an option with indirect mounts to consider each of the leaves
+that has been mounted on instead of considering the top-level names.
+This is intended for compatability with version 4 of autofs and should
+be considered as deprecated.
+
+When autofs considers a directory it checks the `last_used` time and
+compares it with the "timeout" value set when the filesystem was
+mounted, though this check is ignored in some cases. It also checks if
+the directory or anything below it is in use. For symbolic links,
+only the `last_used` time is ever considered.
+
+If both appear to support expiring the directory or symlink, an action
+is taken.
+
+There are two ways to ask autofs to consider expiry. The first is to
+use the **AUTOFS_IOC_EXPIRE** ioctl. This only works for indirect
+mounts. If it finds something in the root directory to expire it will
+return the name of that thing. Once a name has been returned the
+automount daemon needs to unmount any filesystems mounted below the
+name normally. As described above, this is unsafe for non-toplevel
+mounts in a version-5 autofs. For this reason the current `automountd`
+does not use this ioctl.
+
+The second mechanism uses either the **AUTOFS_DEV_IOCTL_EXPIRE_CMD** or
+the **AUTOFS_IOC_EXPIRE_MULTI** ioctl. This will work for both direct and
+indirect mounts. If it selects an object to expire, it will notify
+the daemon using the notification mechanism described below. This
+will block until the daemon acknowledges the expiry notification.
+This implies that the "`EXPIRE`" ioctl must be sent from a different
+thread than the one which handles notification.
+
+While the ioctl is blocking, the entry is marked as "expiring" and
+`d_manage` will block until the daemon affirms that the unmount has
+completed (together with removing any directories that might have been
+necessary), or has been aborted.
+
+Communicating with autofs: detecting the daemon
+-----------------------------------------------
+
+There are several forms of communication between the automount daemon
+and the filesystem. As we have already seen, the daemon can create and
+remove directories and symlinks using normal filesystem operations.
+autofs knows whether a process requesting some operation is the daemon
+or not based on its process-group id number (see getpgid(1)).
+
+When an autofs filesystem is mounted the pgid of the mounting
+processes is recorded unless the "pgrp=" option is given, in which
+case that number is recorded instead. Any request arriving from a
+process in that process group is considered to come from the daemon.
+If the daemon ever has to be stopped and restarted a new pgid can be
+provided through an ioctl as will be described below.
+
+Communicating with autofs: the event pipe
+-----------------------------------------
+
+When an autofs filesystem is mounted, the 'write' end of a pipe must
+be passed using the 'fd=' mount option. autofs will write
+notification messages to this pipe for the daemon to respond to.
+For version 5, the format of the message is:
+
+ struct autofs_v5_packet {
+ int proto_version; /* Protocol version */
+ int type; /* Type of packet */
+ autofs_wqt_t wait_queue_token;
+ __u32 dev;
+ __u64 ino;
+ __u32 uid;
+ __u32 gid;
+ __u32 pid;
+ __u32 tgid;
+ __u32 len;
+ char name[NAME_MAX+1];
+ };
+
+where the type is one of
+
+ autofs_ptype_missing_indirect
+ autofs_ptype_expire_indirect
+ autofs_ptype_missing_direct
+ autofs_ptype_expire_direct
+
+so messages can indicate that a name is missing (something tried to
+access it but it isn't there) or that it has been selected for expiry.
+
+The pipe will be set to "packet mode" (equivalent to passing
+`O_DIRECT`) to _pipe2(2)_ so that a read from the pipe will return at
+most one packet, and any unread portion of a packet will be discarded.
+
+The `wait_queue_token` is a unique number which can identify a
+particular request to be acknowledged. When a message is sent over
+the pipe the affected dentry is marked as either "active" or
+"expiring" and other accesses to it block until the message is
+acknowledged using one of the ioctls below and the relevant
+`wait_queue_token`.
+
+Communicating with autofs: root directory ioctls
+------------------------------------------------
+
+The root directory of an autofs filesystem will respond to a number of
+ioctls. The process issuing the ioctl must have the CAP_SYS_ADMIN
+capability, or must be the automount daemon.
+
+The available ioctl commands are:
+
+- **AUTOFS_IOC_READY**: a notification has been handled. The argument
+ to the ioctl command is the "wait_queue_token" number
+ corresponding to the notification being acknowledged.
+- **AUTOFS_IOC_FAIL**: similar to above, but indicates failure with
+ the error code `ENOENT`.
+- **AUTOFS_IOC_CATATONIC**: Causes the autofs to enter "catatonic"
+ mode meaning that it stops sending notifications to the daemon.
+ This mode is also entered if a write to the pipe fails.
+- **AUTOFS_IOC_PROTOVER**: This returns the protocol version in use.
+- **AUTOFS_IOC_PROTOSUBVER**: Returns the protocol sub-version which
+ is really a version number for the implementation. It is
+ currently 2.
+- **AUTOFS_IOC_SETTIMEOUT**: This passes a pointer to an unsigned
+ long. The value is used to set the timeout for expiry, and
+ the current timeout value is stored back through the pointer.
+- **AUTOFS_IOC_ASKUMOUNT**: Returns, in the pointed-to `int`, 1 if
+ the filesystem could be unmounted. This is only a hint as
+ the situation could change at any instant. This call can be
+ use to avoid a more expensive full unmount attempt.
+- **AUTOFS_IOC_EXPIRE**: as described above, this asks if there is
+ anything suitable to expire. A pointer to a packet:
+
+ struct autofs_packet_expire_multi {
+ int proto_version; /* Protocol version */
+ int type; /* Type of packet */
+ autofs_wqt_t wait_queue_token;
+ int len;
+ char name[NAME_MAX+1];
+ };
+
+ is required. This is filled in with the name of something
+ that can be unmounted or removed. If nothing can be expired,
+ `errno` is set to `EAGAIN`. Even though a `wait_queue_token`
+ is present in the structure, no "wait queue" is established
+ and no acknowledgment is needed.
+- **AUTOFS_IOC_EXPIRE_MULTI**: This is similar to
+ **AUTOFS_IOC_EXPIRE** except that it causes notification to be
+ sent to the daemon, and it blocks until the daemon acknowledges.
+ The argument is an integer which can contain two different flags.
+
+ **AUTOFS_EXP_IMMEDIATE** causes `last_used` time to be ignored
+ and objects are expired if the are not in use.
+
+ **AUTOFS_EXP_LEAVES** will select a leaf rather than a top-level
+ name to expire. This is only safe when *maxproto* is 4.
+
+Communicating with autofs: char-device ioctls
+---------------------------------------------
+
+It is not always possible to open the root of an autofs filesystem,
+particularly a *direct* mounted filesystem. If the automount daemon
+is restarted there is no way for it to regain control of existing
+mounts using any of the above communication channels. To address this
+need there is a "miscellaneous" character device (major 10, minor 235)
+which can be used to communicate directly with the autofs filesystem.
+It requires CAP_SYS_ADMIN for access.
+
+The `ioctl`s that can be used on this device are described in a separate
+document `autofs4-mount-control.txt`, and are summarized briefly here.
+Each ioctl is passed a pointer to an `autofs_dev_ioctl` structure:
+
+ struct autofs_dev_ioctl {
+ __u32 ver_major;
+ __u32 ver_minor;
+ __u32 size; /* total size of data passed in
+ * including this struct */
+ __s32 ioctlfd; /* automount command fd */
+
+ /* Command parameters */
+ union {
+ struct args_protover protover;
+ struct args_protosubver protosubver;
+ struct args_openmount openmount;
+ struct args_ready ready;
+ struct args_fail fail;
+ struct args_setpipefd setpipefd;
+ struct args_timeout timeout;
+ struct args_requester requester;
+ struct args_expire expire;
+ struct args_askumount askumount;
+ struct args_ismountpoint ismountpoint;
+ };
+
+ char path[0];
+ };
+
+For the **OPEN_MOUNT** and **IS_MOUNTPOINT** commands, the target
+filesystem is identified by the `path`. All other commands identify
+the filesystem by the `ioctlfd` which is a file descriptor open on the
+root, and which can be returned by **OPEN_MOUNT**.
+
+The `ver_major` and `ver_minor` are in/out parameters which check that
+the requested version is supported, and report the maximum version
+that the kernel module can support.
+
+Commands are:
+
+- **AUTOFS_DEV_IOCTL_VERSION_CMD**: does nothing, except validate and
+ set version numbers.
+- **AUTOFS_DEV_IOCTL_OPENMOUNT_CMD**: return an open file descriptor
+ on the root of an autofs filesystem. The filesystem is identified
+ by name and device number, which is stored in `openmount.devid`.
+ Device numbers for existing filesystems can be found in
+ `/proc/self/mountinfo`.
+- **AUTOFS_DEV_IOCTL_CLOSEMOUNT_CMD**: same as `close(ioctlfd)`.
+- **AUTOFS_DEV_IOCTL_SETPIPEFD_CMD**: if the filesystem is in
+ catatonic mode, this can provide the write end of a new pipe
+ in `setpipefd.pipefd` to re-establish communication with a daemon.
+ The process group of the calling process is used to identify the
+ daemon.
+- **AUTOFS_DEV_IOCTL_REQUESTER_CMD**: `path` should be a
+ name within the filesystem that has been auto-mounted on.
+ On successful return, `requester.uid` and `requester.gid` will be
+ the UID and GID of the process which triggered that mount.
+- **AUTOFS_DEV_IOCTL_ISMOUNTPOINT_CMD**: Check if path is a
+ mountpoint of a particular type - see separate documentation for
+ details.
+- **AUTOFS_DEV_IOCTL_PROTOVER_CMD**:
+- **AUTOFS_DEV_IOCTL_PROTOSUBVER_CMD**:
+- **AUTOFS_DEV_IOCTL_READY_CMD**:
+- **AUTOFS_DEV_IOCTL_FAIL_CMD**:
+- **AUTOFS_DEV_IOCTL_CATATONIC_CMD**:
+- **AUTOFS_DEV_IOCTL_TIMEOUT_CMD**:
+- **AUTOFS_DEV_IOCTL_EXPIRE_CMD**:
+- **AUTOFS_DEV_IOCTL_ASKUMOUNT_CMD**: These all have the same
+ function as the similarly named **AUTOFS_IOC** ioctls, except
+ that **FAIL** can be given an explicit error number in `fail.status`
+ instead of assuming `ENOENT`, and this **EXPIRE** command
+ corresponds to **AUTOFS_IOC_EXPIRE_MULTI**.
+
+Catatonic mode
+--------------
+
+As mentioned, an autofs mount can enter "catatonic" mode. This
+happens if a write to the notification pipe fails, or if it is
+explicitly requested by an `ioctl`.
+
+When entering catatonic mode, the pipe is closed and any pending
+notifications are acknowledged with the error `ENOENT`.
+
+Once in catatonic mode attempts to access non-existing names will
+result in `ENOENT` while attempts to access existing directories will
+be treated in the same way as if they came from the daemon, so mount
+traps will not fire.
+
+When the filesystem is mounted a _uid_ and _gid_ can be given which
+set the ownership of directories and symbolic links. When the
+filesystem is in catatonic mode, any process with a matching UID can
+create directories or symlinks in the root directory, but not in other
+directories.
+
+Catatonic mode can only be left via the
+**AUTOFS_DEV_IOCTL_OPENMOUNT_CMD** ioctl on the `/dev/autofs`.
+
+autofs, name spaces, and shared mounts
+--------------------------------------
+
+With bind mounts and name spaces it is possible for an autofs
+filesystem to appear at multiple places in one or more filesystem
+name spaces. For this to work sensibly, the autofs filesystem should
+always be mounted "shared". e.g.
+
+> `mount --make-shared /autofs/mount/point`
+
+The automount daemon is only able to manage a single mount location for
+an autofs filesystem and if mounts on that are not 'shared', other
+locations will not behave as expected. In particular access to those
+other locations will likely result in the `ELOOP` error
+
+> Too many levels of symbolic links
diff --git a/Documentation/filesystems/autofs4-mount-control.txt b/Documentation/filesystems/autofs4-mount-control.txt
deleted file mode 100644
index e5177cb31a04..000000000000
--- a/Documentation/filesystems/autofs4-mount-control.txt
+++ /dev/null
@@ -1,407 +0,0 @@
-
-Miscellaneous Device control operations for the autofs4 kernel module
-====================================================================
-
-The problem
-===========
-
-There is a problem with active restarts in autofs (that is to say
-restarting autofs when there are busy mounts).
-
-During normal operation autofs uses a file descriptor opened on the
-directory that is being managed in order to be able to issue control
-operations. Using a file descriptor gives ioctl operations access to
-autofs specific information stored in the super block. The operations
-are things such as setting an autofs mount catatonic, setting the
-expire timeout and requesting expire checks. As is explained below,
-certain types of autofs triggered mounts can end up covering an autofs
-mount itself which prevents us being able to use open(2) to obtain a
-file descriptor for these operations if we don't already have one open.
-
-Currently autofs uses "umount -l" (lazy umount) to clear active mounts
-at restart. While using lazy umount works for most cases, anything that
-needs to walk back up the mount tree to construct a path, such as
-getcwd(2) and the proc file system /proc/<pid>/cwd, no longer works
-because the point from which the path is constructed has been detached
-from the mount tree.
-
-The actual problem with autofs is that it can't reconnect to existing
-mounts. Immediately one thinks of just adding the ability to remount
-autofs file systems would solve it, but alas, that can't work. This is
-because autofs direct mounts and the implementation of "on demand mount
-and expire" of nested mount trees have the file system mounted directly
-on top of the mount trigger directory dentry.
-
-For example, there are two types of automount maps, direct (in the kernel
-module source you will see a third type called an offset, which is just
-a direct mount in disguise) and indirect.
-
-Here is a master map with direct and indirect map entries:
-
-/- /etc/auto.direct
-/test /etc/auto.indirect
-
-and the corresponding map files:
-
-/etc/auto.direct:
-
-/automount/dparse/g6 budgie:/autofs/export1
-/automount/dparse/g1 shark:/autofs/export1
-and so on.
-
-/etc/auto.indirect:
-
-g1 shark:/autofs/export1
-g6 budgie:/autofs/export1
-and so on.
-
-For the above indirect map an autofs file system is mounted on /test and
-mounts are triggered for each sub-directory key by the inode lookup
-operation. So we see a mount of shark:/autofs/export1 on /test/g1, for
-example.
-
-The way that direct mounts are handled is by making an autofs mount on
-each full path, such as /automount/dparse/g1, and using it as a mount
-trigger. So when we walk on the path we mount shark:/autofs/export1 "on
-top of this mount point". Since these are always directories we can
-use the follow_link inode operation to trigger the mount.
-
-But, each entry in direct and indirect maps can have offsets (making
-them multi-mount map entries).
-
-For example, an indirect mount map entry could also be:
-
-g1 \
- / shark:/autofs/export5/testing/test \
- /s1 shark:/autofs/export/testing/test/s1 \
- /s2 shark:/autofs/export5/testing/test/s2 \
- /s1/ss1 shark:/autofs/export1 \
- /s2/ss2 shark:/autofs/export2
-
-and a similarly a direct mount map entry could also be:
-
-/automount/dparse/g1 \
- / shark:/autofs/export5/testing/test \
- /s1 shark:/autofs/export/testing/test/s1 \
- /s2 shark:/autofs/export5/testing/test/s2 \
- /s1/ss1 shark:/autofs/export2 \
- /s2/ss2 shark:/autofs/export2
-
-One of the issues with version 4 of autofs was that, when mounting an
-entry with a large number of offsets, possibly with nesting, we needed
-to mount and umount all of the offsets as a single unit. Not really a
-problem, except for people with a large number of offsets in map entries.
-This mechanism is used for the well known "hosts" map and we have seen
-cases (in 2.4) where the available number of mounts are exhausted or
-where the number of privileged ports available is exhausted.
-
-In version 5 we mount only as we go down the tree of offsets and
-similarly for expiring them which resolves the above problem. There is
-somewhat more detail to the implementation but it isn't needed for the
-sake of the problem explanation. The one important detail is that these
-offsets are implemented using the same mechanism as the direct mounts
-above and so the mount points can be covered by a mount.
-
-The current autofs implementation uses an ioctl file descriptor opened
-on the mount point for control operations. The references held by the
-descriptor are accounted for in checks made to determine if a mount is
-in use and is also used to access autofs file system information held
-in the mount super block. So the use of a file handle needs to be
-retained.
-
-
-The Solution
-============
-
-To be able to restart autofs leaving existing direct, indirect and
-offset mounts in place we need to be able to obtain a file handle
-for these potentially covered autofs mount points. Rather than just
-implement an isolated operation it was decided to re-implement the
-existing ioctl interface and add new operations to provide this
-functionality.
-
-In addition, to be able to reconstruct a mount tree that has busy mounts,
-the uid and gid of the last user that triggered the mount needs to be
-available because these can be used as macro substitution variables in
-autofs maps. They are recorded at mount request time and an operation
-has been added to retrieve them.
-
-Since we're re-implementing the control interface, a couple of other
-problems with the existing interface have been addressed. First, when
-a mount or expire operation completes a status is returned to the
-kernel by either a "send ready" or a "send fail" operation. The
-"send fail" operation of the ioctl interface could only ever send
-ENOENT so the re-implementation allows user space to send an actual
-status. Another expensive operation in user space, for those using
-very large maps, is discovering if a mount is present. Usually this
-involves scanning /proc/mounts and since it needs to be done quite
-often it can introduce significant overhead when there are many entries
-in the mount table. An operation to lookup the mount status of a mount
-point dentry (covered or not) has also been added.
-
-Current kernel development policy recommends avoiding the use of the
-ioctl mechanism in favor of systems such as Netlink. An implementation
-using this system was attempted to evaluate its suitability and it was
-found to be inadequate, in this case. The Generic Netlink system was
-used for this as raw Netlink would lead to a significant increase in
-complexity. There's no question that the Generic Netlink system is an
-elegant solution for common case ioctl functions but it's not a complete
-replacement probably because its primary purpose in life is to be a
-message bus implementation rather than specifically an ioctl replacement.
-While it would be possible to work around this there is one concern
-that lead to the decision to not use it. This is that the autofs
-expire in the daemon has become far to complex because umount
-candidates are enumerated, almost for no other reason than to "count"
-the number of times to call the expire ioctl. This involves scanning
-the mount table which has proved to be a big overhead for users with
-large maps. The best way to improve this is try and get back to the
-way the expire was done long ago. That is, when an expire request is
-issued for a mount (file handle) we should continually call back to
-the daemon until we can't umount any more mounts, then return the
-appropriate status to the daemon. At the moment we just expire one
-mount at a time. A Generic Netlink implementation would exclude this
-possibility for future development due to the requirements of the
-message bus architecture.
-
-
-autofs4 Miscellaneous Device mount control interface
-====================================================
-
-The control interface is opening a device node, typically /dev/autofs.
-
-All the ioctls use a common structure to pass the needed parameter
-information and return operation results:
-
-struct autofs_dev_ioctl {
- __u32 ver_major;
- __u32 ver_minor;
- __u32 size; /* total size of data passed in
- * including this struct */
- __s32 ioctlfd; /* automount command fd */
-
- /* Command parameters */
- union {
- struct args_protover protover;
- struct args_protosubver protosubver;
- struct args_openmount openmount;
- struct args_ready ready;
- struct args_fail fail;
- struct args_setpipefd setpipefd;
- struct args_timeout timeout;
- struct args_requester requester;
- struct args_expire expire;
- struct args_askumount askumount;
- struct args_ismountpoint ismountpoint;
- };
-
- char path[0];
-};
-
-The ioctlfd field is a mount point file descriptor of an autofs mount
-point. It is returned by the open call and is used by all calls except
-the check for whether a given path is a mount point, where it may
-optionally be used to check a specific mount corresponding to a given
-mount point file descriptor, and when requesting the uid and gid of the
-last successful mount on a directory within the autofs file system.
-
-The union is used to communicate parameters and results of calls made
-as described below.
-
-The path field is used to pass a path where it is needed and the size field
-is used account for the increased structure length when translating the
-structure sent from user space.
-
-This structure can be initialized before setting specific fields by using
-the void function call init_autofs_dev_ioctl(struct autofs_dev_ioctl *).
-
-All of the ioctls perform a copy of this structure from user space to
-kernel space and return -EINVAL if the size parameter is smaller than
-the structure size itself, -ENOMEM if the kernel memory allocation fails
-or -EFAULT if the copy itself fails. Other checks include a version check
-of the compiled in user space version against the module version and a
-mismatch results in a -EINVAL return. If the size field is greater than
-the structure size then a path is assumed to be present and is checked to
-ensure it begins with a "/" and is NULL terminated, otherwise -EINVAL is
-returned. Following these checks, for all ioctl commands except
-AUTOFS_DEV_IOCTL_VERSION_CMD, AUTOFS_DEV_IOCTL_OPENMOUNT_CMD and
-AUTOFS_DEV_IOCTL_CLOSEMOUNT_CMD the ioctlfd is validated and if it is
-not a valid descriptor or doesn't correspond to an autofs mount point
-an error of -EBADF, -ENOTTY or -EINVAL (not an autofs descriptor) is
-returned.
-
-
-The ioctls
-==========
-
-An example of an implementation which uses this interface can be seen
-in autofs version 5.0.4 and later in file lib/dev-ioctl-lib.c of the
-distribution tar available for download from kernel.org in directory
-/pub/linux/daemons/autofs/v5.
-
-The device node ioctl operations implemented by this interface are:
-
-
-AUTOFS_DEV_IOCTL_VERSION
-------------------------
-
-Get the major and minor version of the autofs4 device ioctl kernel module
-implementation. It requires an initialized struct autofs_dev_ioctl as an
-input parameter and sets the version information in the passed in structure.
-It returns 0 on success or the error -EINVAL if a version mismatch is
-detected.
-
-
-AUTOFS_DEV_IOCTL_PROTOVER_CMD and AUTOFS_DEV_IOCTL_PROTOSUBVER_CMD
-------------------------------------------------------------------
-
-Get the major and minor version of the autofs4 protocol version understood
-by loaded module. This call requires an initialized struct autofs_dev_ioctl
-with the ioctlfd field set to a valid autofs mount point descriptor
-and sets the requested version number in version field of struct args_protover
-or sub_version field of struct args_protosubver. These commands return
-0 on success or one of the negative error codes if validation fails.
-
-
-AUTOFS_DEV_IOCTL_OPENMOUNT and AUTOFS_DEV_IOCTL_CLOSEMOUNT
-----------------------------------------------------------
-
-Obtain and release a file descriptor for an autofs managed mount point
-path. The open call requires an initialized struct autofs_dev_ioctl with
-the path field set and the size field adjusted appropriately as well
-as the devid field of struct args_openmount set to the device number of
-the autofs mount. The device number can be obtained from the mount options
-shown in /proc/mounts. The close call requires an initialized struct
-autofs_dev_ioct with the ioctlfd field set to the descriptor obtained
-from the open call. The release of the file descriptor can also be done
-with close(2) so any open descriptors will also be closed at process exit.
-The close call is included in the implemented operations largely for
-completeness and to provide for a consistent user space implementation.
-
-
-AUTOFS_DEV_IOCTL_READY_CMD and AUTOFS_DEV_IOCTL_FAIL_CMD
---------------------------------------------------------
-
-Return mount and expire result status from user space to the kernel.
-Both of these calls require an initialized struct autofs_dev_ioctl
-with the ioctlfd field set to the descriptor obtained from the open
-call and the token field of struct args_ready or struct args_fail set
-to the wait queue token number, received by user space in the foregoing
-mount or expire request. The status field of struct args_fail is set to
-the errno of the operation. It is set to 0 on success.
-
-
-AUTOFS_DEV_IOCTL_SETPIPEFD_CMD
-------------------------------
-
-Set the pipe file descriptor used for kernel communication to the daemon.
-Normally this is set at mount time using an option but when reconnecting
-to a existing mount we need to use this to tell the autofs mount about
-the new kernel pipe descriptor. In order to protect mounts against
-incorrectly setting the pipe descriptor we also require that the autofs
-mount be catatonic (see next call).
-
-The call requires an initialized struct autofs_dev_ioctl with the
-ioctlfd field set to the descriptor obtained from the open call and
-the pipefd field of struct args_setpipefd set to descriptor of the pipe.
-On success the call also sets the process group id used to identify the
-controlling process (eg. the owning automount(8) daemon) to the process
-group of the caller.
-
-
-AUTOFS_DEV_IOCTL_CATATONIC_CMD
-------------------------------
-
-Make the autofs mount point catatonic. The autofs mount will no longer
-issue mount requests, the kernel communication pipe descriptor is released
-and any remaining waits in the queue released.
-
-The call requires an initialized struct autofs_dev_ioctl with the
-ioctlfd field set to the descriptor obtained from the open call.
-
-
-AUTOFS_DEV_IOCTL_TIMEOUT_CMD
-----------------------------
-
-Set the expire timeout for mounts within an autofs mount point.
-
-The call requires an initialized struct autofs_dev_ioctl with the
-ioctlfd field set to the descriptor obtained from the open call.
-
-
-AUTOFS_DEV_IOCTL_REQUESTER_CMD
-------------------------------
-
-Return the uid and gid of the last process to successfully trigger a the
-mount on the given path dentry.
-
-The call requires an initialized struct autofs_dev_ioctl with the path
-field set to the mount point in question and the size field adjusted
-appropriately. Upon return the uid field of struct args_requester contains
-the uid and gid field the gid.
-
-When reconstructing an autofs mount tree with active mounts we need to
-re-connect to mounts that may have used the original process uid and
-gid (or string variations of them) for mount lookups within the map entry.
-This call provides the ability to obtain this uid and gid so they may be
-used by user space for the mount map lookups.
-
-
-AUTOFS_DEV_IOCTL_EXPIRE_CMD
----------------------------
-
-Issue an expire request to the kernel for an autofs mount. Typically
-this ioctl is called until no further expire candidates are found.
-
-The call requires an initialized struct autofs_dev_ioctl with the
-ioctlfd field set to the descriptor obtained from the open call. In
-addition an immediate expire, independent of the mount timeout, can be
-requested by setting the how field of struct args_expire to 1. If no
-expire candidates can be found the ioctl returns -1 with errno set to
-EAGAIN.
-
-This call causes the kernel module to check the mount corresponding
-to the given ioctlfd for mounts that can be expired, issues an expire
-request back to the daemon and waits for completion.
-
-AUTOFS_DEV_IOCTL_ASKUMOUNT_CMD
-------------------------------
-
-Checks if an autofs mount point is in use.
-
-The call requires an initialized struct autofs_dev_ioctl with the
-ioctlfd field set to the descriptor obtained from the open call and
-it returns the result in the may_umount field of struct args_askumount,
-1 for busy and 0 otherwise.
-
-
-AUTOFS_DEV_IOCTL_ISMOUNTPOINT_CMD
----------------------------------
-
-Check if the given path is a mountpoint.
-
-The call requires an initialized struct autofs_dev_ioctl. There are two
-possible variations. Both use the path field set to the path of the mount
-point to check and the size field adjusted appropriately. One uses the
-ioctlfd field to identify a specific mount point to check while the other
-variation uses the path and optionally in.type field of struct args_ismountpoint
-set to an autofs mount type. The call returns 1 if this is a mount point
-and sets out.devid field to the device number of the mount and out.magic
-field to the relevant super block magic number (described below) or 0 if
-it isn't a mountpoint. In both cases the the device number (as returned
-by new_encode_dev()) is returned in out.devid field.
-
-If supplied with a file descriptor we're looking for a specific mount,
-not necessarily at the top of the mounted stack. In this case the path
-the descriptor corresponds to is considered a mountpoint if it is itself
-a mountpoint or contains a mount, such as a multi-mount without a root
-mount. In this case we return 1 if the descriptor corresponds to a mount
-point and and also returns the super magic of the covering mount if there
-is one or 0 if it isn't a mountpoint.
-
-If a path is supplied (and the ioctlfd field is set to -1) then the path
-is looked up and is checked to see if it is the root of a mount. If a
-type is also given we are looking for a particular autofs mount and if
-a match isn't found a fail is returned. If the the located path is the
-root of a mount 1 is returned along with the super magic of the mount
-or 0 otherwise.
-
diff --git a/Documentation/filesystems/autofs4.txt b/Documentation/filesystems/autofs4.txt
deleted file mode 100644
index f10dd590f69f..000000000000
--- a/Documentation/filesystems/autofs4.txt
+++ /dev/null
@@ -1,529 +0,0 @@
-<head>
-<style> p { max-width:50em} ol, ul {max-width: 40em}</style>
-</head>
-
-autofs - how it works
-=====================
-
-Purpose
--------
-
-The goal of autofs is to provide on-demand mounting and race free
-automatic unmounting of various other filesystems. This provides two
-key advantages:
-
-1. There is no need to delay boot until all filesystems that
- might be needed are mounted. Processes that try to access those
- slow filesystems might be delayed but other processes can
- continue freely. This is particularly important for
- network filesystems (e.g. NFS) or filesystems stored on
- media with a media-changing robot.
-
-2. The names and locations of filesystems can be stored in
- a remote database and can change at any time. The content
- in that data base at the time of access will be used to provide
- a target for the access. The interpretation of names in the
- filesystem can even be programmatic rather than database-backed,
- allowing wildcards for example, and can vary based on the user who
- first accessed a name.
-
-Context
--------
-
-The "autofs4" filesystem module is only one part of an autofs system.
-There also needs to be a user-space program which looks up names
-and mounts filesystems. This will often be the "automount" program,
-though other tools including "systemd" can make use of "autofs4".
-This document describes only the kernel module and the interactions
-required with any user-space program. Subsequent text refers to this
-as the "automount daemon" or simply "the daemon".
-
-"autofs4" is a Linux kernel module with provides the "autofs"
-filesystem type. Several "autofs" filesystems can be mounted and they
-can each be managed separately, or all managed by the same daemon.
-
-Content
--------
-
-An autofs filesystem can contain 3 sorts of objects: directories,
-symbolic links and mount traps. Mount traps are directories with
-extra properties as described in the next section.
-
-Objects can only be created by the automount daemon: symlinks are
-created with a regular `symlink` system call, while directories and
-mount traps are created with `mkdir`. The determination of whether a
-directory should be a mount trap or not is quite _ad hoc_, largely for
-historical reasons, and is determined in part by the
-*direct*/*indirect*/*offset* mount options, and the *maxproto* mount option.
-
-If neither the *direct* or *offset* mount options are given (so the
-mount is considered to be *indirect*), then the root directory is
-always a regular directory, otherwise it is a mount trap when it is
-empty and a regular directory when not empty. Note that *direct* and
-*offset* are treated identically so a concise summary is that the root
-directory is a mount trap only if the filesystem is mounted *direct*
-and the root is empty.
-
-Directories created in the root directory are mount traps only if the
-filesystem is mounted *indirect* and they are empty.
-
-Directories further down the tree depend on the *maxproto* mount
-option and particularly whether it is less than five or not.
-When *maxproto* is five, no directories further down the
-tree are ever mount traps, they are always regular directories. When
-the *maxproto* is four (or three), these directories are mount traps
-precisely when they are empty.
-
-So: non-empty (i.e. non-leaf) directories are never mount traps. Empty
-directories are sometimes mount traps, and sometimes not depending on
-where in the tree they are (root, top level, or lower), the *maxproto*,
-and whether the mount was *indirect* or not.
-
-Mount Traps
----------------
-
-A core element of the implementation of autofs is the Mount Traps
-which are provided by the Linux VFS. Any directory provided by a
-filesystem can be designated as a trap. This involves two separate
-features that work together to allow autofs to do its job.
-
-**DCACHE_NEED_AUTOMOUNT**
-
-If a dentry has the DCACHE_NEED_AUTOMOUNT flag set (which gets set if
-the inode has S_AUTOMOUNT set, or can be set directly) then it is
-(potentially) a mount trap. Any access to this directory beyond a
-"`stat`" will (normally) cause the `d_op->d_automount()` dentry operation
-to be called. The task of this method is to find the filesystem that
-should be mounted on the directory and to return it. The VFS is
-responsible for actually mounting the root of this filesystem on the
-directory.
-
-autofs doesn't find the filesystem itself but sends a message to the
-automount daemon asking it to find and mount the filesystem. The
-autofs `d_automount` method then waits for the daemon to report that
-everything is ready. It will then return "`NULL`" indicating that the
-mount has already happened. The VFS doesn't try to mount anything but
-follows down the mount that is already there.
-
-This functionality is sufficient for some users of mount traps such
-as NFS which creates traps so that mountpoints on the server can be
-reflected on the client. However it is not sufficient for autofs. As
-mounting onto a directory is considered to be "beyond a `stat`", the
-automount daemon would not be able to mount a filesystem on the 'trap'
-directory without some way to avoid getting caught in the trap. For
-that purpose there is another flag.
-
-**DCACHE_MANAGE_TRANSIT**
-
-If a dentry has DCACHE_MANAGE_TRANSIT set then two very different but
-related behaviors are invoked, both using the `d_op->d_manage()`
-dentry operation.
-
-Firstly, before checking to see if any filesystem is mounted on the
-directory, d_manage() will be called with the `rcu_walk` parameter set
-to `false`. It may return one of three things:
-
-- A return value of zero indicates that there is nothing special
- about this dentry and normal checks for mounts and automounts
- should proceed.
-
- autofs normally returns zero, but first waits for any
- expiry (automatic unmounting of the mounted filesystem) to
- complete. This avoids races.
-
-- A return value of `-EISDIR` tells the VFS to ignore any mounts
- on the directory and to not consider calling `->d_automount()`.
- This effectively disables the **DCACHE_NEED_AUTOMOUNT** flag
- causing the directory not be a mount trap after all.
-
- autofs returns this if it detects that the process performing the
- lookup is the automount daemon and that the mount has been
- requested but has not yet completed. How it determines this is
- discussed later. This allows the automount daemon not to get
- caught in the mount trap.
-
- There is a subtlety here. It is possible that a second autofs
- filesystem can be mounted below the first and for both of them to
- be managed by the same daemon. For the daemon to be able to mount
- something on the second it must be able to "walk" down past the
- first. This means that d_manage cannot *always* return -EISDIR for
- the automount daemon. It must only return it when a mount has
- been requested, but has not yet completed.
-
- `d_manage` also returns `-EISDIR` if the dentry shouldn't be a
- mount trap, either because it is a symbolic link or because it is
- not empty.
-
-- Any other negative value is treated as an error and returned
- to the caller.
-
- autofs can return
-
- - -ENOENT if the automount daemon failed to mount anything,
- - -ENOMEM if it ran out of memory,
- - -EINTR if a signal arrived while waiting for expiry to
- complete
- - or any other error sent down by the automount daemon.
-
-
-The second use case only occurs during an "RCU-walk" and so `rcu_walk`
-will be set.
-
-An RCU-walk is a fast and lightweight process for walking down a
-filename path (i.e. it is like running on tip-toes). RCU-walk cannot
-cope with all situations so when it finds a difficulty it falls back
-to "REF-walk", which is slower but more robust.
-
-RCU-walk will never call `->d_automount`; the filesystems must already
-be mounted or RCU-walk cannot handle the path.
-To determine if a mount-trap is safe for RCU-walk mode it calls
-`->d_manage()` with `rcu_walk` set to `true`.
-
-In this case `d_manage()` must avoid blocking and should avoid taking
-spinlocks if at all possible. Its sole purpose is to determine if it
-would be safe to follow down into any mounted directory and the only
-reason that it might not be is if an expiry of the mount is
-underway.
-
-In the `rcu_walk` case, `d_manage()` cannot return -EISDIR to tell the
-VFS that this is a directory that doesn't require d_automount. If
-`rcu_walk` sees a dentry with DCACHE_NEED_AUTOMOUNT set but nothing
-mounted, it *will* fall back to REF-walk. `d_manage()` cannot make the
-VFS remain in RCU-walk mode, but can only tell it to get out of
-RCU-walk mode by returning `-ECHILD`.
-
-So `d_manage()`, when called with `rcu_walk` set, should either return
--ECHILD if there is any reason to believe it is unsafe to end the
-mounted filesystem, and otherwise should return 0.
-
-autofs will return `-ECHILD` if an expiry of the filesystem has been
-initiated or is being considered, otherwise it returns 0.
-
-
-Mountpoint expiry
------------------
-
-The VFS has a mechanism for automatically expiring unused mounts,
-much as it can expire any unused dentry information from the dcache.
-This is guided by the MNT_SHRINKABLE flag. This only applies to
-mounts that were created by `d_automount()` returning a filesystem to be
-mounted. As autofs doesn't return such a filesystem but leaves the
-mounting to the automount daemon, it must involve the automount daemon
-in unmounting as well. This also means that autofs has more control
-of expiry.
-
-The VFS also supports "expiry" of mounts using the MNT_EXPIRE flag to
-the `umount` system call. Unmounting with MNT_EXPIRE will fail unless
-a previous attempt had been made, and the filesystem has been inactive
-and untouched since that previous attempt. autofs4 does not depend on
-this but has its own internal tracking of whether filesystems were
-recently used. This allows individual names in the autofs directory
-to expire separately.
-
-With version 4 of the protocol, the automount daemon can try to
-unmount any filesystems mounted on the autofs filesystem or remove any
-symbolic links or empty directories any time it likes. If the unmount
-or removal is successful the filesystem will be returned to the state
-it was before the mount or creation, so that any access of the name
-will trigger normal auto-mount processing. In particlar, `rmdir` and
-`unlink` do not leave negative entries in the dcache as a normal
-filesystem would, so an attempt to access a recently-removed object is
-passed to autofs for handling.
-
-With version 5, this is not safe except for unmounting from top-level
-directories. As lower-level directories are never mount traps, other
-processes will see an empty directory as soon as the filesystem is
-unmounted. So it is generally safest to use the autofs expiry
-protocol described below.
-
-Normally the daemon only wants to remove entries which haven't been
-used for a while. For this purpose autofs maintains a "`last_used`"
-time stamp on each directory or symlink. For symlinks it genuinely
-does record the last time the symlink was "used" or followed to find
-out where it points to. For directories the field is a slight
-misnomer. It actually records the last time that autofs checked if
-the directory or one of its descendents was busy and found that it
-was. This is just as useful and doesn't require updating the field so
-often.
-
-The daemon is able to ask autofs if anything is due to be expired,
-using an `ioctl` as discussed later. For a *direct* mount, autofs
-considers if the entire mount-tree can be unmounted or not. For an
-*indirect* mount, autofs considers each of the names in the top level
-directory to determine if any of those can be unmounted and cleaned
-up.
-
-There is an option with indirect mounts to consider each of the leaves
-that has been mounted on instead of considering the top-level names.
-This is intended for compatability with version 4 of autofs and should
-be considered as deprecated.
-
-When autofs considers a directory it checks the `last_used` time and
-compares it with the "timeout" value set when the filesystem was
-mounted, though this check is ignored in some cases. It also checks if
-the directory or anything below it is in use. For symbolic links,
-only the `last_used` time is ever considered.
-
-If both appear to support expiring the directory or symlink, an action
-is taken.
-
-There are two ways to ask autofs to consider expiry. The first is to
-use the **AUTOFS_IOC_EXPIRE** ioctl. This only works for indirect
-mounts. If it finds something in the root directory to expire it will
-return the name of that thing. Once a name has been returned the
-automount daemon needs to unmount any filesystems mounted below the
-name normally. As described above, this is unsafe for non-toplevel
-mounts in a version-5 autofs. For this reason the current `automountd`
-does not use this ioctl.
-
-The second mechanism uses either the **AUTOFS_DEV_IOCTL_EXPIRE_CMD** or
-the **AUTOFS_IOC_EXPIRE_MULTI** ioctl. This will work for both direct and
-indirect mounts. If it selects an object to expire, it will notify
-the daemon using the notification mechanism described below. This
-will block until the daemon acknowledges the expiry notification.
-This implies that the "`EXPIRE`" ioctl must be sent from a different
-thread than the one which handles notification.
-
-While the ioctl is blocking, the entry is marked as "expiring" and
-`d_manage` will block until the daemon affirms that the unmount has
-completed (together with removing any directories that might have been
-necessary), or has been aborted.
-
-Communicating with autofs: detecting the daemon
------------------------------------------------
-
-There are several forms of communication between the automount daemon
-and the filesystem. As we have already seen, the daemon can create and
-remove directories and symlinks using normal filesystem operations.
-autofs knows whether a process requesting some operation is the daemon
-or not based on its process-group id number (see getpgid(1)).
-
-When an autofs filesystem is mounted the pgid of the mounting
-processes is recorded unless the "pgrp=" option is given, in which
-case that number is recorded instead. Any request arriving from a
-process in that process group is considered to come from the daemon.
-If the daemon ever has to be stopped and restarted a new pgid can be
-provided through an ioctl as will be described below.
-
-Communicating with autofs: the event pipe
------------------------------------------
-
-When an autofs filesystem is mounted, the 'write' end of a pipe must
-be passed using the 'fd=' mount option. autofs will write
-notification messages to this pipe for the daemon to respond to.
-For version 5, the format of the message is:
-
- struct autofs_v5_packet {
- int proto_version; /* Protocol version */
- int type; /* Type of packet */
- autofs_wqt_t wait_queue_token;
- __u32 dev;
- __u64 ino;
- __u32 uid;
- __u32 gid;
- __u32 pid;
- __u32 tgid;
- __u32 len;
- char name[NAME_MAX+1];
- };
-
-where the type is one of
-
- autofs_ptype_missing_indirect
- autofs_ptype_expire_indirect
- autofs_ptype_missing_direct
- autofs_ptype_expire_direct
-
-so messages can indicate that a name is missing (something tried to
-access it but it isn't there) or that it has been selected for expiry.
-
-The pipe will be set to "packet mode" (equivalent to passing
-`O_DIRECT`) to _pipe2(2)_ so that a read from the pipe will return at
-most one packet, and any unread portion of a packet will be discarded.
-
-The `wait_queue_token` is a unique number which can identify a
-particular request to be acknowledged. When a message is sent over
-the pipe the affected dentry is marked as either "active" or
-"expiring" and other accesses to it block until the message is
-acknowledged using one of the ioctls below and the relevant
-`wait_queue_token`.
-
-Communicating with autofs: root directory ioctls
-------------------------------------------------
-
-The root directory of an autofs filesystem will respond to a number of
-ioctls. The process issuing the ioctl must have the CAP_SYS_ADMIN
-capability, or must be the automount daemon.
-
-The available ioctl commands are:
-
-- **AUTOFS_IOC_READY**: a notification has been handled. The argument
- to the ioctl command is the "wait_queue_token" number
- corresponding to the notification being acknowledged.
-- **AUTOFS_IOC_FAIL**: similar to above, but indicates failure with
- the error code `ENOENT`.
-- **AUTOFS_IOC_CATATONIC**: Causes the autofs to enter "catatonic"
- mode meaning that it stops sending notifications to the daemon.
- This mode is also entered if a write to the pipe fails.
-- **AUTOFS_IOC_PROTOVER**: This returns the protocol version in use.
-- **AUTOFS_IOC_PROTOSUBVER**: Returns the protocol sub-version which
- is really a version number for the implementation. It is
- currently 2.
-- **AUTOFS_IOC_SETTIMEOUT**: This passes a pointer to an unsigned
- long. The value is used to set the timeout for expiry, and
- the current timeout value is stored back through the pointer.
-- **AUTOFS_IOC_ASKUMOUNT**: Returns, in the pointed-to `int`, 1 if
- the filesystem could be unmounted. This is only a hint as
- the situation could change at any instant. This call can be
- use to avoid a more expensive full unmount attempt.
-- **AUTOFS_IOC_EXPIRE**: as described above, this asks if there is
- anything suitable to expire. A pointer to a packet:
-
- struct autofs_packet_expire_multi {
- int proto_version; /* Protocol version */
- int type; /* Type of packet */
- autofs_wqt_t wait_queue_token;
- int len;
- char name[NAME_MAX+1];
- };
-
- is required. This is filled in with the name of something
- that can be unmounted or removed. If nothing can be expired,
- `errno` is set to `EAGAIN`. Even though a `wait_queue_token`
- is present in the structure, no "wait queue" is established
- and no acknowledgment is needed.
-- **AUTOFS_IOC_EXPIRE_MULTI**: This is similar to
- **AUTOFS_IOC_EXPIRE** except that it causes notification to be
- sent to the daemon, and it blocks until the daemon acknowledges.
- The argument is an integer which can contain two different flags.
-
- **AUTOFS_EXP_IMMEDIATE** causes `last_used` time to be ignored
- and objects are expired if the are not in use.
-
- **AUTOFS_EXP_LEAVES** will select a leaf rather than a top-level
- name to expire. This is only safe when *maxproto* is 4.
-
-Communicating with autofs: char-device ioctls
----------------------------------------------
-
-It is not always possible to open the root of an autofs filesystem,
-particularly a *direct* mounted filesystem. If the automount daemon
-is restarted there is no way for it to regain control of existing
-mounts using any of the above communication channels. To address this
-need there is a "miscellaneous" character device (major 10, minor 235)
-which can be used to communicate directly with the autofs filesystem.
-It requires CAP_SYS_ADMIN for access.
-
-The `ioctl`s that can be used on this device are described in a separate
-document `autofs4-mount-control.txt`, and are summarized briefly here.
-Each ioctl is passed a pointer to an `autofs_dev_ioctl` structure:
-
- struct autofs_dev_ioctl {
- __u32 ver_major;
- __u32 ver_minor;
- __u32 size; /* total size of data passed in
- * including this struct */
- __s32 ioctlfd; /* automount command fd */
-
- /* Command parameters */
- union {
- struct args_protover protover;
- struct args_protosubver protosubver;
- struct args_openmount openmount;
- struct args_ready ready;
- struct args_fail fail;
- struct args_setpipefd setpipefd;
- struct args_timeout timeout;
- struct args_requester requester;
- struct args_expire expire;
- struct args_askumount askumount;
- struct args_ismountpoint ismountpoint;
- };
-
- char path[0];
- };
-
-For the **OPEN_MOUNT** and **IS_MOUNTPOINT** commands, the target
-filesystem is identified by the `path`. All other commands identify
-the filesystem by the `ioctlfd` which is a file descriptor open on the
-root, and which can be returned by **OPEN_MOUNT**.
-
-The `ver_major` and `ver_minor` are in/out parameters which check that
-the requested version is supported, and report the maximum version
-that the kernel module can support.
-
-Commands are:
-
-- **AUTOFS_DEV_IOCTL_VERSION_CMD**: does nothing, except validate and
- set version numbers.
-- **AUTOFS_DEV_IOCTL_OPENMOUNT_CMD**: return an open file descriptor
- on the root of an autofs filesystem. The filesystem is identified
- by name and device number, which is stored in `openmount.devid`.
- Device numbers for existing filesystems can be found in
- `/proc/self/mountinfo`.
-- **AUTOFS_DEV_IOCTL_CLOSEMOUNT_CMD**: same as `close(ioctlfd)`.
-- **AUTOFS_DEV_IOCTL_SETPIPEFD_CMD**: if the filesystem is in
- catatonic mode, this can provide the write end of a new pipe
- in `setpipefd.pipefd` to re-establish communication with a daemon.
- The process group of the calling process is used to identify the
- daemon.
-- **AUTOFS_DEV_IOCTL_REQUESTER_CMD**: `path` should be a
- name within the filesystem that has been auto-mounted on.
- On successful return, `requester.uid` and `requester.gid` will be
- the UID and GID of the process which triggered that mount.
-- **AUTOFS_DEV_IOCTL_ISMOUNTPOINT_CMD**: Check if path is a
- mountpoint of a particular type - see separate documentation for
- details.
-- **AUTOFS_DEV_IOCTL_PROTOVER_CMD**:
-- **AUTOFS_DEV_IOCTL_PROTOSUBVER_CMD**:
-- **AUTOFS_DEV_IOCTL_READY_CMD**:
-- **AUTOFS_DEV_IOCTL_FAIL_CMD**:
-- **AUTOFS_DEV_IOCTL_CATATONIC_CMD**:
-- **AUTOFS_DEV_IOCTL_TIMEOUT_CMD**:
-- **AUTOFS_DEV_IOCTL_EXPIRE_CMD**:
-- **AUTOFS_DEV_IOCTL_ASKUMOUNT_CMD**: These all have the same
- function as the similarly named **AUTOFS_IOC** ioctls, except
- that **FAIL** can be given an explicit error number in `fail.status`
- instead of assuming `ENOENT`, and this **EXPIRE** command
- corresponds to **AUTOFS_IOC_EXPIRE_MULTI**.
-
-Catatonic mode
---------------
-
-As mentioned, an autofs mount can enter "catatonic" mode. This
-happens if a write to the notification pipe fails, or if it is
-explicitly requested by an `ioctl`.
-
-When entering catatonic mode, the pipe is closed and any pending
-notifications are acknowledged with the error `ENOENT`.
-
-Once in catatonic mode attempts to access non-existing names will
-result in `ENOENT` while attempts to access existing directories will
-be treated in the same way as if they came from the daemon, so mount
-traps will not fire.
-
-When the filesystem is mounted a _uid_ and _gid_ can be given which
-set the ownership of directories and symbolic links. When the
-filesystem is in catatonic mode, any process with a matching UID can
-create directories or symlinks in the root directory, but not in other
-directories.
-
-Catatonic mode can only be left via the
-**AUTOFS_DEV_IOCTL_OPENMOUNT_CMD** ioctl on the `/dev/autofs`.
-
-autofs, name spaces, and shared mounts
---------------------------------------
-
-With bind mounts and name spaces it is possible for an autofs
-filesystem to appear at multiple places in one or more filesystem
-name spaces. For this to work sensibly, the autofs filesystem should
-always be mounted "shared". e.g.
-
-> `mount --make-shared /autofs/mount/point`
-
-The automount daemon is only able to manage a single mount location for
-an autofs filesystem and if mounts on that are not 'shared', other
-locations will not behave as expected. In particular access to those
-other locations will likely result in the `ELOOP` error
-
-> Too many levels of symbolic links