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diff --git a/Documentation/filesystems/autofs-mount-control.txt b/Documentation/filesystems/autofs-mount-control.txt deleted file mode 100644 index acc02fc57993..000000000000 --- a/Documentation/filesystems/autofs-mount-control.txt +++ /dev/null @@ -1,408 +0,0 @@ - -Miscellaneous Device control operations for the autofs 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. - - -autofs 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 autofs 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 autofs 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 that's independent of the mount timeout, -and a forced expire that's independent of whether the mount is busy, -can be requested by setting the how field of struct args_expire to -AUTOFS_EXP_IMMEDIATE or AUTOFS_EXP_FORCED, respectively . 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. |