[PATCH/RFC] autofs: the documentation I wanted to read

From: NeilBrown
Date: Mon Jul 28 2014 - 22:00:33 EST

This documents autofs from the perspective of what the module actually
supports rather than how automount is expected to use it.
It is based mostly on code review and very little on testing so it
may be inaccurate in some places.

The document assumes the functionality added by the RCU-walk patches
that I posted recently.

It is formatted using "markdown" and works best with Markdown.pl
(markdown_py doesn't like some constructs).

Signed-off-by: NeilBrown <neilb@xxxxxxx>

diff --git a/Documentation/filesystems/autofs4.txt b/Documentation/filesystems/autofs4.txt
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+<style> p { max-width:50em} ol, ul {max-width: 40em}</style>
+autofs - how it works
+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 programatic rather than database-backed,
+ allowing wildcards for example, and can vary based on the user who
+ first accessed a name.
+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.
+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` systemcall, 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 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 *max_proto* mount
+option and particularly whether it is less than five or not.
+When *max_proto* is five, no directories further down the
+tree are ever mount traps, they are always regular directories. When
+the *max_proto* 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.
+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
+responsibly for actually mounting the root of this filesystem on the
+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.
+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 possibly 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 light weight 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
+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 mechansim 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 be 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` systemcall. Unmounting with MNT_EXPIRE will fail unless
+a previous attempt had been made, and the filesystem was 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
+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
+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 it's process-group id number (see getpgid(1)).
+When an autofs filesystem it mounted the pgid of the mounting
+processes is recorded unless that "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 messages is
+acknowledged using one of the ioctls below and the relevant
+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, not "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 a 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 */
+ __u32 arg1; /* Command parameters */
+ __u32 arg2;
+ 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 a device number, which is stored in `arg1`. 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 `arg1` 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 as been auto-mounted on.
+ arg1 is the dev number of the underlying autofs. On successful
+ return, `arg1` and `arg2` 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_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 `arg1`
+ 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 is 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 symlink in the root directory, but not in other
+Catatonic mode can only be left via the
+**AUTOFS_DEV_IOCTL_OPENMOUNT_CMD** ioctl on the `/dev/autofs`.

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