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:

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 However, in the event of a system crash, the atime and mtime fields on disk might be out of date by up to 24 hours.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.

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.

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).

From Linux 2.4 onward, the MS_NODEV, MS_NOEXEC, and MS_NOSUID flags are settable on a per-mount-point basis. From kernel 2.6.16 onward, MS_NOATIME and MS_NODIRATIME are also settable on a per-mount-point basis. The MS_RELATIME flag is also settable on a per-mount-point basis. 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. 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. Another exception is that MS_BIND has a different meaning for remount, and it should be included only if explicitly desired.

The following mountflags can be changed: MS_LAZYTIME, MS_MANDLOCK, MS_NOATIME, MS_NODEV, MS_NODIRATIME, MS_NOEXEC, MS_NOSUID, MS_RELATIME, MS_RDONLY, and MS_SYNCHRONOUS. Attempts to change the setting of the MS_DIRSYNC flag during a remount are silently ignored.

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, this 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 point 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 mount points.

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 in the mountflags argument are also ignored, with the exception of MS_REC. (The bind mount has the same mount options as the underlying mount point.) 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 flags that can be used with changing the propagation type are MS_REC and MS_SILENT.

The source, filesystemtype, and data arguments are ignored.

The meanings of the propagation type flags are as follows:

MS_SHARED

Make this mount point shared. Mount and unmount events immediately under this mount point will propagate to the other mount points 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 mount points in the peer group. Conversely, mount and unmount events that take place under peer mount points will propagate to this mount point.

MS_PRIVATE

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

MS_SLAVE

If this is a shared mount point that is a member of a peer group that contains other members, convert it to a slave mount. If this is a shared mount point 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 point is left unchanged.

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

A mount point 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 bind 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 point. If the MS_REC flag is also specified in mountflags, then the propagation type of all mount points 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 point and target specifies the new location to which that mount point 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 point¶

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 point. source specifies the source for the new mount point, 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.

Per-process namespaces¶

Starting with kernel 2.4.19, Linux provides per-process mount namespaces. A mount namespace is the set of filesystem mounts that are visible to a process. Mount-point namespaces can be (and usually are) shared between multiple processes, and changes to the namespace (i.e., mounts and unmounts) by one process are visible to all other processes sharing the same namespace. (The pre-2.4.19 Linux situation can be considered as one in which a single namespace was shared by every process on the system.)

A child process created by fork(2) shares its parent's mount namespace; the mount namespace is preserved across an execve(2).

A process can obtain a private mount namespace if: it was created using the clone(2) CLONE_NEWNS flag, in which case its new namespace is initialized to be a copy of the namespace of the process that called clone(2); or it calls unshare(2) with the CLONE_NEWNS flag, which causes the caller's mount namespace to obtain a private copy of the namespace that it was previously sharing with other processes, so that future mounts and unmounts by the caller are invisible to other processes (except child processes that the caller subsequently creates) and vice versa.

The Linux-specific /proc/[pid]/mounts file exposes the list of mount points in the mount namespace of the process with the specified ID; see proc(5) for details.