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   mount    ( 2 )

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Описание (Description)

mount() attaches the filesystem specified by source (which is often a pathname referring to a device, but can also be the pathname of a directory or file, or a dummy string) to the location (a directory or file) specified by the pathname in target.

Appropriate privilege (Linux: the CAP_SYS_ADMIN capability) is required to mount filesystems.

Values for the filesystemtype argument supported by the kernel are listed in /proc/filesystems (e.g., "btrfs", "ext4", "jfs", "xfs", "vfat", "fuse", "tmpfs", "cgroup", "proc", "mqueue", "nfs", "cifs", "iso9660"). Further types may become available when the appropriate modules are loaded.

The data argument is interpreted by the different filesystems. Typically it is a string of comma-separated options understood by this filesystem. See mount(8) for details of the options available for each filesystem type. This argument may be specified as NULL, if there are no options.

A call to mount() performs one of a number of general types of operation, depending on the bits specified in mountflags. The choice of which operation to perform is determined by testing the bits set in mountflags, with the tests being conducted in the order listed here:

* Remount an existing mount: mountflags includes MS_REMOUNT.

* Create a bind mount: mountflags includes MS_BIND.

* Change the propagation type of an existing mount: mountflags includes one of MS_SHARED, MS_PRIVATE, MS_SLAVE, or MS_UNBINDABLE.

* Move an existing mount to a new location: mountflags includes MS_MOVE.

* Create a new mount: mountflags includes none of the above flags.

Each of these operations is detailed later in this page. Further flags may be specified in mountflags to modify the behavior of mount(), as described below.

Additional mount flags The list below describes the additional flags that can be specified in mountflags. Note that some operation types ignore some or all of these flags, as described later in this page.

MS_DIRSYNC (since Linux 2.5.19) Make directory changes on this filesystem synchronous. (This property can be obtained for individual directories or subtrees using chattr(1).)

MS_LAZYTIME (since Linux 4.0) Reduce on-disk updates of inode timestamps (atime, mtime, ctime) by maintaining these changes only in memory. The on-disk timestamps are updated only when:

(a) the inode needs to be updated for some change unrelated to file timestamps;

(b) the application employs fsync(2), syncfs(2), or sync(2);

(c) an undeleted inode is evicted from memory; or

(d) more than 24 hours have passed since the inode was written to disk.

This mount option significantly reduces writes needed to update the inode's timestamps, especially mtime and atime. However, in the event of a system crash, the atime and mtime fields on disk might be out of date by up to 24 hours.

Examples of workloads where this option could be of significant benefit include frequent random writes to preallocated files, as well as cases where the MS_STRICTATIME mount option is also enabled. (The advantage of combining MS_STRICTATIME and MS_LAZYTIME is that stat(2) will return the correctly updated atime, but the atime updates will be flushed to disk only in the cases listed above.)

MS_MANDLOCK Permit mandatory locking on files in this filesystem. (Mandatory locking must still be enabled on a per-file basis, as described in fcntl(2).) Since Linux 4.5, this mount option requires the CAP_SYS_ADMIN capability and a kernel configured with the CONFIG_MANDATORY_FILE_LOCKING option.

MS_NOATIME Do not update access times for (all types of) files on this filesystem.

MS_NODEV Do not allow access to devices (special files) on this filesystem.

MS_NODIRATIME Do not update access times for directories on this filesystem. This flag provides a subset of the functionality provided by MS_NOATIME; that is, MS_NOATIME implies MS_NODIRATIME.

MS_NOEXEC Do not allow programs to be executed from this filesystem.

MS_NOSUID Do not honor set-user-ID and set-group-ID bits or file capabilities when executing programs from this filesystem. In addition, SELinux domain transitions require the permission nosuid_transition, which in turn needs also the policy capability nnp_nosuid_transition.

MS_RDONLY Mount filesystem read-only.

MS_REC (since Linux 2.4.11) Used in conjunction with MS_BIND to create a recursive bind mount, and in conjunction with the propagation type flags to recursively change the propagation type of all of the mounts in a subtree. See below for further details.

MS_RELATIME (since Linux 2.6.20) When a file on this filesystem is accessed, update the file's last access time (atime) only if the current value of atime is less than or equal to the file's last modification time (mtime) or last status change time (ctime). This option is useful for programs, such as mutt(1), that need to know when a file has been read since it was last modified. Since Linux 2.6.30, the kernel defaults to the behavior provided by this flag (unless MS_NOATIME was specified), and the MS_STRICTATIME flag is required to obtain traditional semantics. In addition, since Linux 2.6.30, the file's last access time is always updated if it is more than 1 day old.

MS_SILENT (since Linux 2.6.17) Suppress the display of certain (printk()) warning messages in the kernel log. This flag supersedes the misnamed and obsolete MS_VERBOSE flag (available since Linux 2.4.12), which has the same meaning.

MS_STRICTATIME (since Linux 2.6.30) Always update the last access time (atime) when files on this filesystem are accessed. (This was the default behavior before Linux 2.6.30.) Specifying this flag overrides the effect of setting the MS_NOATIME and MS_RELATIME flags.

MS_SYNCHRONOUS Make writes on this filesystem synchronous (as though the O_SYNC flag to open(2) was specified for all file opens to this filesystem).

MS_NOSYMFOLLOW (since Linux 5.10) Do not follow symbolic links when resolving paths. Symbolic links can still be created, and readlink(1), readlink(2), realpath(1), and realpath(3) all still work properly.

From Linux 2.4 onward, some of the above flags are settable on a per-mount basis, while others apply to the superblock of the mounted filesystem, meaning that all mounts of the same filesystem share those flags. (Previously, all of the flags were per-superblock.)

The per-mount-point flags are as follows:

* Since Linux 2.4: MS_NODEV, MS_NOEXEC, and MS_NOSUID flags are settable on a per-mount-point basis.

* Additionally, since Linux 2.6.16: MS_NOATIME and MS_NODIRATIME.

* Additionally, since Linux 2.6.20: MS_RELATIME.

The following flags are per-superblock: MS_DIRSYNC, MS_LAZYTIME, MS_MANDLOCK, MS_SILENT, and MS_SYNCHRONOUS. The initial settings of these flags are determined on the first mount of the filesystem, and will be shared by all subsequent mounts of the same filesystem. Subsequently, the settings of the flags can be changed via a remount operation (see below). Such changes will be visible via all mounts associated with the filesystem.

Since Linux 2.6.16, MS_RDONLY can be set or cleared on a per- mount-point basis as well as on the underlying filesystem superblock. The mounted filesystem will be writable only if neither the filesystem nor the mountpoint are flagged as read- only.

Remounting an existing mount An existing mount may be remounted by specifying MS_REMOUNT in mountflags. This allows you to change the mountflags and data of an existing mount without having to unmount and remount the filesystem. target should be the same value specified in the initial mount() call.

The source and filesystemtype arguments are ignored.

The mountflags and data arguments should match the values used in the original mount() call, except for those parameters that are being deliberately changed.

The following mountflags can be changed: MS_LAZYTIME, MS_MANDLOCK, MS_NOATIME, MS_NODEV, MS_NODIRATIME, MS_NOEXEC, MS_NOSUID, MS_RELATIME, MS_RDONLY, MS_STRICTATIME (whose effect is to clear the MS_NOATIME and MS_RELATIME flags), and MS_SYNCHRONOUS. Attempts to change the setting of the MS_DIRSYNC and MS_SILENT flags during a remount are silently ignored. Note that changes to per-superblock flags are visible via all mounts of the associated filesystem (because the per-superblock flags are shared by all mounts).

Since Linux 3.17, if none of MS_NOATIME, MS_NODIRATIME, MS_RELATIME, or MS_STRICTATIME is specified in mountflags, then the remount operation preserves the existing values of these flags (rather than defaulting to MS_RELATIME).

Since Linux 2.6.26, the MS_REMOUNT flag can be used with MS_BIND to modify only the per-mount-point flags. This is particularly useful for setting or clearing the "read-only" flag on a mount without changing the underlying filesystem. Specifying mountflags as:

MS_REMOUNT | MS_BIND | MS_RDONLY

will make access through this mountpoint read-only, without affecting other mounts.

Creating a bind mount If mountflags includes MS_BIND (available since Linux 2.4), then perform a bind mount. A bind mount makes a file or a directory subtree visible at another point within the single directory hierarchy. Bind mounts may cross filesystem boundaries and span chroot(2) jails.

The filesystemtype and data arguments are ignored.

The remaining bits (other than MS_REC, described below) in the mountflags argument are also ignored. (The bind mount has the same mount options as the underlying mount.) However, see the discussion of remounting above, for a method of making an existing bind mount read-only.

By default, when a directory is bind mounted, only that directory is mounted; if there are any submounts under the directory tree, they are not bind mounted. If the MS_REC flag is also specified, then a recursive bind mount operation is performed: all submounts under the source subtree (other than unbindable mounts) are also bind mounted at the corresponding location in the target subtree.

Changing the propagation type of an existing mount If mountflags includes one of MS_SHARED, MS_PRIVATE, MS_SLAVE, or MS_UNBINDABLE (all available since Linux 2.6.15), then the propagation type of an existing mount is changed. If more than one of these flags is specified, an error results.

The only other flags that can be specified while changing the propagation type are MS_REC (described below) and MS_SILENT (which is ignored).

The source, filesystemtype, and data arguments are ignored.

The meanings of the propagation type flags are as follows:

MS_SHARED Make this mount shared. Mount and unmount events immediately under this mount will propagate to the other mounts that are members of this mount's peer group. Propagation here means that the same mount or unmount will automatically occur under all of the other mounts in the peer group. Conversely, mount and unmount events that take place under peer mounts will propagate to this mount.

MS_PRIVATE Make this mount private. Mount and unmount events do not propagate into or out of this mount.

MS_SLAVE If this is a shared mount that is a member of a peer group that contains other members, convert it to a slave mount. If this is a shared mount that is a member of a peer group that contains no other members, convert it to a private mount. Otherwise, the propagation type of the mount is left unchanged.

When a mount is a slave, mount and unmount events propagate into this mount from the (master) shared peer group of which it was formerly a member. Mount and unmount events under this mount do not propagate to any peer.

A mount can be the slave of another peer group while at the same time sharing mount and unmount events with a peer group of which it is a member.

MS_UNBINDABLE Make this mount unbindable. This is like a private mount, and in addition this mount can't be bind mounted. When a recursive bind mount (mount() with the MS_BIND and MS_REC flags) is performed on a directory subtree, any unbindable mounts within the subtree are automatically pruned (i.e., not replicated) when replicating that subtree to produce the target subtree.

By default, changing the propagation type affects only the target mount. If the MS_REC flag is also specified in mountflags, then the propagation type of all mounts under target is also changed.

For further details regarding mount propagation types (including the default propagation type assigned to new mounts), see mount_namespaces(7).

Moving a mount If mountflags contains the flag MS_MOVE (available since Linux 2.4.18), then move a subtree: source specifies an existing mount and target specifies the new location to which that mount is to be relocated. The move is atomic: at no point is the subtree unmounted.

The remaining bits in the mountflags argument are ignored, as are the filesystemtype and data arguments.

Creating a new mount If none of MS_REMOUNT, MS_BIND, MS_MOVE, MS_SHARED, MS_PRIVATE, MS_SLAVE, or MS_UNBINDABLE is specified in mountflags, then mount() performs its default action: creating a new mount. source specifies the source for the new mount, and target specifies the directory at which to create the mount point.

The filesystemtype and data arguments are employed, and further bits may be specified in mountflags to modify the behavior of the call.