docs: filesystems: convert autofs.txt to reST
Convert autofs.txt to reST. The following changes abound: - Introduce reST formatting for headings, lists et al. - Add an indentation of an 8 space tab wherever suitable, so as to maintain consistency. - Remove indentation of the description of the ioctls which are similar to the AUTOFS_IOC ioctls, as it does not come out quite right in HTML. - Add an entry for autofs in the index. Signed-off-by: Jaskaran Singh <jaskaransingh7654321@gmail.com> Signed-off-by: Jonathan Corbet <corbet@lwn.net>alistair/sunxi64-5.5-dsi
Jaskaran Singh
Jonathan Corbet
parent
14d3fe428b
commit
f11f2a3c54
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@ -1,12 +1,9 @@
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<head>
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<style> p { max-width:50em} ol, ul {max-width: 40em}</style>
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</head>
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=====================
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autofs - how it works
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=====================
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Purpose
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-------
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=======
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The goal of autofs is to provide on-demand mounting and race free
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automatic unmounting of various other filesystems. This provides two
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@ -28,7 +25,7 @@ key advantages:
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first accessed a name.
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Context
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-------
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=======
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The "autofs" filesystem module is only one part of an autofs system.
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There also needs to be a user-space program which looks up names
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@ -43,7 +40,7 @@ filesystem type. Several "autofs" filesystems can be mounted and they
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can each be managed separately, or all managed by the same daemon.
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Content
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-------
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=======
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An autofs filesystem can contain 3 sorts of objects: directories,
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symbolic links and mount traps. Mount traps are directories with
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@ -52,7 +49,7 @@ extra properties as described in the next section.
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Objects can only be created by the automount daemon: symlinks are
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created with a regular `symlink` system call, while directories and
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mount traps are created with `mkdir`. The determination of whether a
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directory should be a mount trap or not is quite _ad hoc_, largely for
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directory should be a mount trap or not is quite ad hoc, largely for
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historical reasons, and is determined in part by the
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*direct*/*indirect*/*offset* mount options, and the *maxproto* mount option.
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@ -80,7 +77,7 @@ where in the tree they are (root, top level, or lower), the *maxproto*,
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and whether the mount was *indirect* or not.
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Mount Traps
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---------------
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===========
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A core element of the implementation of autofs is the Mount Traps
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which are provided by the Linux VFS. Any directory provided by a
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@ -201,7 +198,7 @@ initiated or is being considered, otherwise it returns 0.
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Mountpoint expiry
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-----------------
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=================
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The VFS has a mechanism for automatically expiring unused mounts,
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much as it can expire any unused dentry information from the dcache.
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@ -301,7 +298,7 @@ completed (together with removing any directories that might have been
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necessary), or has been aborted.
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Communicating with autofs: detecting the daemon
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-----------------------------------------------
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===============================================
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There are several forms of communication between the automount daemon
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and the filesystem. As we have already seen, the daemon can create and
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@ -317,33 +314,33 @@ If the daemon ever has to be stopped and restarted a new pgid can be
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provided through an ioctl as will be described below.
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Communicating with autofs: the event pipe
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-----------------------------------------
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=========================================
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When an autofs filesystem is mounted, the 'write' end of a pipe must
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be passed using the 'fd=' mount option. autofs will write
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notification messages to this pipe for the daemon to respond to.
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For version 5, the format of the message is:
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For version 5, the format of the message is::
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struct autofs_v5_packet {
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int proto_version; /* Protocol version */
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int type; /* Type of packet */
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autofs_wqt_t wait_queue_token;
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__u32 dev;
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__u64 ino;
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__u32 uid;
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__u32 gid;
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__u32 pid;
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__u32 tgid;
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__u32 len;
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char name[NAME_MAX+1];
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struct autofs_v5_packet {
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int proto_version; /* Protocol version */
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int type; /* Type of packet */
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autofs_wqt_t wait_queue_token;
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__u32 dev;
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__u64 ino;
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__u32 uid;
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__u32 gid;
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__u32 pid;
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__u32 tgid;
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__u32 len;
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char name[NAME_MAX+1];
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};
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where the type is one of
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where the type is one of ::
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autofs_ptype_missing_indirect
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autofs_ptype_expire_indirect
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autofs_ptype_missing_direct
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autofs_ptype_expire_direct
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autofs_ptype_missing_indirect
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autofs_ptype_expire_indirect
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autofs_ptype_missing_direct
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autofs_ptype_expire_direct
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so messages can indicate that a name is missing (something tried to
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access it but it isn't there) or that it has been selected for expiry.
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@ -360,7 +357,7 @@ acknowledged using one of the ioctls below with the relevant
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`wait_queue_token`.
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Communicating with autofs: root directory ioctls
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------------------------------------------------
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================================================
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The root directory of an autofs filesystem will respond to a number of
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ioctls. The process issuing the ioctl must have the CAP_SYS_ADMIN
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@ -368,58 +365,67 @@ capability, or must be the automount daemon.
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The available ioctl commands are:
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- **AUTOFS_IOC_READY**: a notification has been handled. The argument
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to the ioctl command is the "wait_queue_token" number
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corresponding to the notification being acknowledged.
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- **AUTOFS_IOC_FAIL**: similar to above, but indicates failure with
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the error code `ENOENT`.
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- **AUTOFS_IOC_CATATONIC**: Causes the autofs to enter "catatonic"
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mode meaning that it stops sending notifications to the daemon.
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This mode is also entered if a write to the pipe fails.
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- **AUTOFS_IOC_PROTOVER**: This returns the protocol version in use.
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- **AUTOFS_IOC_PROTOSUBVER**: Returns the protocol sub-version which
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is really a version number for the implementation.
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- **AUTOFS_IOC_SETTIMEOUT**: This passes a pointer to an unsigned
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long. The value is used to set the timeout for expiry, and
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the current timeout value is stored back through the pointer.
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- **AUTOFS_IOC_ASKUMOUNT**: Returns, in the pointed-to `int`, 1 if
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the filesystem could be unmounted. This is only a hint as
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the situation could change at any instant. This call can be
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used to avoid a more expensive full unmount attempt.
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- **AUTOFS_IOC_EXPIRE**: as described above, this asks if there is
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anything suitable to expire. A pointer to a packet:
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- **AUTOFS_IOC_READY**:
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a notification has been handled. The argument
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to the ioctl command is the "wait_queue_token" number
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corresponding to the notification being acknowledged.
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- **AUTOFS_IOC_FAIL**:
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similar to above, but indicates failure with
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the error code `ENOENT`.
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- **AUTOFS_IOC_CATATONIC**:
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Causes the autofs to enter "catatonic"
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mode meaning that it stops sending notifications to the daemon.
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This mode is also entered if a write to the pipe fails.
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- **AUTOFS_IOC_PROTOVER**:
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This returns the protocol version in use.
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- **AUTOFS_IOC_PROTOSUBVER**:
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Returns the protocol sub-version which
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is really a version number for the implementation.
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- **AUTOFS_IOC_SETTIMEOUT**:
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This passes a pointer to an unsigned
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long. The value is used to set the timeout for expiry, and
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the current timeout value is stored back through the pointer.
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- **AUTOFS_IOC_ASKUMOUNT**:
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Returns, in the pointed-to `int`, 1 if
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the filesystem could be unmounted. This is only a hint as
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the situation could change at any instant. This call can be
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used to avoid a more expensive full unmount attempt.
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- **AUTOFS_IOC_EXPIRE**:
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as described above, this asks if there is
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anything suitable to expire. A pointer to a packet::
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struct autofs_packet_expire_multi {
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int proto_version; /* Protocol version */
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int type; /* Type of packet */
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autofs_wqt_t wait_queue_token;
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int len;
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char name[NAME_MAX+1];
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};
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struct autofs_packet_expire_multi {
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int proto_version; /* Protocol version */
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int type; /* Type of packet */
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autofs_wqt_t wait_queue_token;
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int len;
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char name[NAME_MAX+1];
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};
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is required. This is filled in with the name of something
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that can be unmounted or removed. If nothing can be expired,
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`errno` is set to `EAGAIN`. Even though a `wait_queue_token`
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is present in the structure, no "wait queue" is established
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and no acknowledgment is needed.
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- **AUTOFS_IOC_EXPIRE_MULTI**: This is similar to
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**AUTOFS_IOC_EXPIRE** except that it causes notification to be
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sent to the daemon, and it blocks until the daemon acknowledges.
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The argument is an integer which can contain two different flags.
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is required. This is filled in with the name of something
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that can be unmounted or removed. If nothing can be expired,
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`errno` is set to `EAGAIN`. Even though a `wait_queue_token`
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is present in the structure, no "wait queue" is established
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and no acknowledgment is needed.
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- **AUTOFS_IOC_EXPIRE_MULTI**:
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This is similar to
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**AUTOFS_IOC_EXPIRE** except that it causes notification to be
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sent to the daemon, and it blocks until the daemon acknowledges.
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The argument is an integer which can contain two different flags.
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**AUTOFS_EXP_IMMEDIATE** causes `last_used` time to be ignored
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and objects are expired if the are not in use.
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**AUTOFS_EXP_IMMEDIATE** causes `last_used` time to be ignored
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and objects are expired if the are not in use.
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**AUTOFS_EXP_FORCED** causes the in use status to be ignored
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and objects are expired ieven if they are in use. This assumes
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that the daemon has requested this because it is capable of
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performing the umount.
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**AUTOFS_EXP_FORCED** causes the in use status to be ignored
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and objects are expired ieven if they are in use. This assumes
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that the daemon has requested this because it is capable of
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performing the umount.
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**AUTOFS_EXP_LEAVES** will select a leaf rather than a top-level
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name to expire. This is only safe when *maxproto* is 4.
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**AUTOFS_EXP_LEAVES** will select a leaf rather than a top-level
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name to expire. This is only safe when *maxproto* is 4.
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Communicating with autofs: char-device ioctls
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---------------------------------------------
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=============================================
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It is not always possible to open the root of an autofs filesystem,
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particularly a *direct* mounted filesystem. If the automount daemon
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@ -429,9 +435,9 @@ need there is a "miscellaneous" character device (major 10, minor 235)
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which can be used to communicate directly with the autofs filesystem.
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It requires CAP_SYS_ADMIN for access.
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The `ioctl`s that can be used on this device are described in a separate
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The 'ioctl's that can be used on this device are described in a separate
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document `autofs-mount-control.txt`, and are summarised briefly here.
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Each ioctl is passed a pointer to an `autofs_dev_ioctl` structure:
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Each ioctl is passed a pointer to an `autofs_dev_ioctl` structure::
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struct autofs_dev_ioctl {
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__u32 ver_major;
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@ -469,41 +475,50 @@ that the kernel module can support.
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Commands are:
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- **AUTOFS_DEV_IOCTL_VERSION_CMD**: does nothing, except validate and
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set version numbers.
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- **AUTOFS_DEV_IOCTL_OPENMOUNT_CMD**: return an open file descriptor
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on the root of an autofs filesystem. The filesystem is identified
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by name and device number, which is stored in `openmount.devid`.
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Device numbers for existing filesystems can be found in
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`/proc/self/mountinfo`.
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- **AUTOFS_DEV_IOCTL_CLOSEMOUNT_CMD**: same as `close(ioctlfd)`.
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- **AUTOFS_DEV_IOCTL_SETPIPEFD_CMD**: if the filesystem is in
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catatonic mode, this can provide the write end of a new pipe
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in `setpipefd.pipefd` to re-establish communication with a daemon.
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The process group of the calling process is used to identify the
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daemon.
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- **AUTOFS_DEV_IOCTL_REQUESTER_CMD**: `path` should be a
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name within the filesystem that has been auto-mounted on.
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On successful return, `requester.uid` and `requester.gid` will be
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the UID and GID of the process which triggered that mount.
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- **AUTOFS_DEV_IOCTL_ISMOUNTPOINT_CMD**: Check if path is a
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mountpoint of a particular type - see separate documentation for
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details.
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- **AUTOFS_DEV_IOCTL_PROTOVER_CMD**:
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- **AUTOFS_DEV_IOCTL_PROTOSUBVER_CMD**:
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- **AUTOFS_DEV_IOCTL_READY_CMD**:
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- **AUTOFS_DEV_IOCTL_FAIL_CMD**:
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- **AUTOFS_DEV_IOCTL_CATATONIC_CMD**:
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- **AUTOFS_DEV_IOCTL_TIMEOUT_CMD**:
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- **AUTOFS_DEV_IOCTL_EXPIRE_CMD**:
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- **AUTOFS_DEV_IOCTL_ASKUMOUNT_CMD**: These all have the same
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function as the similarly named **AUTOFS_IOC** ioctls, except
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that **FAIL** can be given an explicit error number in `fail.status`
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instead of assuming `ENOENT`, and this **EXPIRE** command
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corresponds to **AUTOFS_IOC_EXPIRE_MULTI**.
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- **AUTOFS_DEV_IOCTL_VERSION_CMD**:
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does nothing, except validate and
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set version numbers.
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- **AUTOFS_DEV_IOCTL_OPENMOUNT_CMD**:
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return an open file descriptor
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on the root of an autofs filesystem. The filesystem is identified
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by name and device number, which is stored in `openmount.devid`.
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Device numbers for existing filesystems can be found in
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`/proc/self/mountinfo`.
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- **AUTOFS_DEV_IOCTL_CLOSEMOUNT_CMD**:
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same as `close(ioctlfd)`.
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- **AUTOFS_DEV_IOCTL_SETPIPEFD_CMD**:
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if the filesystem is in
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catatonic mode, this can provide the write end of a new pipe
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in `setpipefd.pipefd` to re-establish communication with a daemon.
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The process group of the calling process is used to identify the
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daemon.
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- **AUTOFS_DEV_IOCTL_REQUESTER_CMD**:
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`path` should be a
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name within the filesystem that has been auto-mounted on.
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On successful return, `requester.uid` and `requester.gid` will be
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the UID and GID of the process which triggered that mount.
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- **AUTOFS_DEV_IOCTL_ISMOUNTPOINT_CMD**:
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Check if path is a
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mountpoint of a particular type - see separate documentation for
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details.
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- **AUTOFS_DEV_IOCTL_PROTOVER_CMD**
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- **AUTOFS_DEV_IOCTL_PROTOSUBVER_CMD**
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- **AUTOFS_DEV_IOCTL_READY_CMD**
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- **AUTOFS_DEV_IOCTL_FAIL_CMD**
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- **AUTOFS_DEV_IOCTL_CATATONIC_CMD**
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- **AUTOFS_DEV_IOCTL_TIMEOUT_CMD**
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- **AUTOFS_DEV_IOCTL_EXPIRE_CMD**
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- **AUTOFS_DEV_IOCTL_ASKUMOUNT_CMD**
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These all have the same
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function as the similarly named **AUTOFS_IOC** ioctls, except
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that **FAIL** can be given an explicit error number in `fail.status`
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instead of assuming `ENOENT`, and this **EXPIRE** command
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corresponds to **AUTOFS_IOC_EXPIRE_MULTI**.
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Catatonic mode
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--------------
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==============
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As mentioned, an autofs mount can enter "catatonic" mode. This
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happens if a write to the notification pipe fails, or if it is
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@ -527,7 +542,7 @@ Catatonic mode can only be left via the
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**AUTOFS_DEV_IOCTL_OPENMOUNT_CMD** ioctl on the `/dev/autofs`.
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The "ignore" mount option
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-------------------------
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=========================
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The "ignore" mount option can be used to provide a generic indicator
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to applications that the mount entry should be ignored when displaying
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@ -542,18 +557,18 @@ This is intended to be used by user space programs to exclude autofs
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mounts from consideration when reading the mounts list.
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autofs, name spaces, and shared mounts
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--------------------------------------
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======================================
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With bind mounts and name spaces it is possible for an autofs
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filesystem to appear at multiple places in one or more filesystem
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name spaces. For this to work sensibly, the autofs filesystem should
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always be mounted "shared". e.g.
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always be mounted "shared". e.g. ::
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> `mount --make-shared /autofs/mount/point`
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mount --make-shared /autofs/mount/point
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The automount daemon is only able to manage a single mount location for
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an autofs filesystem and if mounts on that are not 'shared', other
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locations will not behave as expected. In particular access to those
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other locations will likely result in the `ELOOP` error
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other locations will likely result in the `ELOOP` error ::
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> Too many levels of symbolic links
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Too many levels of symbolic links
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@ -46,4 +46,5 @@ Documentation for filesystem implementations.
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.. toctree::
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:maxdepth: 2
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autofs
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virtiofs
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Reference in New Issue