[RFC PATCH] docs: filesystems: convert autofs.txt to reST
From: Jaskaran Singh
Date: Wed Nov 13 2019 - 02:38:31 EST
This patch converts autofs.txt to reST.
- Most of the changes pertain to reST formatting.
- Some of the code snippets are updated to reflect current source.
- A definition of the autofs packet header has been added in the chapter
"Communicating with autofs: the event pipe".
- An indentation of an 8 space tab has been added wherever suitable, so
as to maintain consistency.
- Removed 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.
Signed-off-by: Jaskaran Singh <jaskaransingh7654321@xxxxxxxxx>
---
.../filesystems/{autofs.txt => autofs.rst} | 258 ++++++++++--------
Documentation/filesystems/index.rst | 1 +
2 files changed, 140 insertions(+), 119 deletions(-)
rename Documentation/filesystems/{autofs.txt => autofs.rst} (77%)
diff --git a/Documentation/filesystems/autofs.txt b/Documentation/filesystems/autofs.rst
similarity index 77%
rename from Documentation/filesystems/autofs.txt
rename to Documentation/filesystems/autofs.rst
index 3af38c7fd26d..a130cba76f07 100644
--- a/Documentation/filesystems/autofs.txt
+++ b/Documentation/filesystems/autofs.rst
@@ -1,12 +1,9 @@
-<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
@@ -28,7 +25,7 @@ key advantages:
first accessed a name.
Context
--------
+=======
The "autofs" filesystem module is only one part of an autofs system.
There also needs to be a user-space program which looks up names
@@ -43,7 +40,7 @@ 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
@@ -52,7 +49,7 @@ 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
+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.
@@ -80,7 +77,7 @@ 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
@@ -201,7 +198,7 @@ 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.
@@ -301,7 +298,7 @@ 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
@@ -317,33 +314,39 @@ 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];
+For version 5, the format of the message is: ::
+
+ struct autofs_v5_packet {
+ struct autofs_packet_hdr hdr;
+ 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
+And the format of the header is: ::
+
+ struct autofs_packet_hdr {
+ int proto_version; /* Protocol version */
+ int type; /* Type of packet */
+ };
- autofs_ptype_missing_indirect
- autofs_ptype_expire_indirect
- autofs_ptype_missing_direct
- autofs_ptype_expire_direct
+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.
@@ -360,7 +363,7 @@ acknowledged using one of the ioctls below with 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
@@ -368,58 +371,66 @@ 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.
-- **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
- used 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];
- };
+- **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.
+- **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
+ used 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 {
+ struct autofs_packet_hdr hdr;
+ 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.
+ 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_IMMEDIATE** causes `last_used` time to be ignored
+ and objects are expired if the are not in use.
- **AUTOFS_EXP_FORCED** causes the in use status to be ignored
- and objects are expired ieven if they are in use. This assumes
- that the daemon has requested this because it is capable of
- performing the umount.
+ **AUTOFS_EXP_FORCED** causes the in use status to be ignored
+ and objects are expired ieven if they are in use. This assumes
+ that the daemon has requested this because it is capable of
+ performing the umount.
- **AUTOFS_EXP_LEAVES** will select a leaf rather than a top-level
- name to expire. This is only safe when *maxproto* is 4.
+ **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
@@ -429,9 +440,9 @@ 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
+The `ioctl`\s that can be used on this device are described in a separate
document `autofs-mount-control.txt`, and are summarised briefly here.
-Each ioctl is passed a pointer to an `autofs_dev_ioctl` structure:
+Each ioctl is passed a pointer to an `autofs_dev_ioctl` structure: ::
struct autofs_dev_ioctl {
__u32 ver_major;
@@ -469,41 +480,50 @@ 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**.
+- **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
@@ -527,7 +547,7 @@ Catatonic mode can only be left via the
**AUTOFS_DEV_IOCTL_OPENMOUNT_CMD** ioctl on the `/dev/autofs`.
The "ignore" mount option
--------------------------
+=========================
The "ignore" mount option can be used to provide a generic indicator
to applications that the mount entry should be ignored when displaying
@@ -542,18 +562,18 @@ This is intended to be used by user space programs to exclude autofs
mounts from consideration when reading the mounts list.
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.
+always be mounted "shared". e.g. ::
-> `mount --make-shared /autofs/mount/point`
+ 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
+other locations will likely result in the `ELOOP` error ::
-> Too many levels of symbolic links
+ Too many levels of symbolic links
diff --git a/Documentation/filesystems/index.rst b/Documentation/filesystems/index.rst
index 2c3a9f761205..ad6315a48d14 100644
--- a/Documentation/filesystems/index.rst
+++ b/Documentation/filesystems/index.rst
@@ -46,4 +46,5 @@ Documentation for filesystem implementations.
.. toctree::
:maxdepth: 2
+ autofs
virtiofs
--
2.21.0