1 .\" Copyright (C) 1993 Rickard E. Faith <faith@cs.unc.edu>
2 .\" and Copyright (C) 1994 Andries E. Brouwer <aeb@cwi.nl>
3 .\" and Copyright (C) 2002, 2005, 2016 Michael Kerrisk <mtk.manpages@gmail.com>
5 .\" %%%LICENSE_START(VERBATIM)
6 .\" Permission is granted to make and distribute verbatim copies of this
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10 .\" Permission is granted to copy and distribute modified versions of this
11 .\" manual under the conditions for verbatim copying, provided that the
12 .\" entire resulting derived work is distributed under the terms of a
13 .\" permission notice identical to this one.
15 .\" Since the Linux kernel and libraries are constantly changing, this
16 .\" manual page may be incorrect or out-of-date. The author(s) assume no
17 .\" responsibility for errors or omissions, or for damages resulting from
18 .\" the use of the information contained herein. The author(s) may not
19 .\" have taken the same level of care in the production of this manual,
20 .\" which is licensed free of charge, as they might when working
23 .\" Formatted or processed versions of this manual, if unaccompanied by
24 .\" the source, must acknowledge the copyright and authors of this work.
27 .\" Modified 1996-11-04 by Eric S. Raymond <esr@thyrsus.com>
28 .\" Modified 2001-10-13 by Michael Kerrisk <mtk.manpages@gmail.com>
29 .\" Added note on historical behavior of MS_NOSUID
30 .\" Modified 2002-05-16 by Michael Kerrisk <mtk.manpages@gmail.com>
31 .\" Extensive changes and additions
32 .\" Modified 2002-05-27 by aeb
33 .\" Modified 2002-06-11 by Michael Kerrisk <mtk.manpages@gmail.com>
34 .\" Enhanced descriptions of MS_MOVE, MS_BIND, and MS_REMOUNT
35 .\" Modified 2004-06-17 by Michael Kerrisk <mtk.manpages@gmail.com>
36 .\" 2005-05-18, mtk, Added MNT_EXPIRE, plus a few other tidy-ups.
37 .\" 2008-10-06, mtk: move umount*() material into separate umount.2 page.
38 .\" 2008-10-06, mtk: Add discussion of namespaces.
40 .TH MOUNT 2 2016-12-12 "Linux" "Linux Programmer's Manual"
42 mount \- mount filesystem
45 .B "#include <sys/mount.h>"
47 .BI "int mount(const char *" source ", const char *" target ,
48 .BI " const char *" filesystemtype ", unsigned long " mountflags ,
49 .BI " const void *" data );
53 attaches the filesystem specified by
55 (which is often a pathname referring to a device,
56 but can also be the pathname of a directory or file,
57 or a dummy string) to the location (a directory or file)
58 specified by the pathname in
61 Appropriate privilege (Linux: the
63 capability) is required to mount filesystems.
67 argument supported by the kernel are listed in
69 (e.g., "btrfs", "ext4", "jfs", "xfs", "vfat", "fuse",
70 "tmpfs", "cgroup", "proc", "mqueue", "nfs", "cifs", "iso9660").
71 Further types may become available when the appropriate modules
76 argument is interpreted by the different filesystems.
77 Typically it is a string of comma-separated options
78 understood by this filesystem.
81 for details of the options available for each filesystem type.
85 performs one of a number of general types of operation,
86 depending on the bits specified in
88 The choice of which operation to perform is determined by
89 testing the bits set in
91 with the tests being conducted in the order listed here:
93 Remount an existing mount:
103 Change the propagation type of an existing mount:
112 Move an existing mount to a new location:
119 includes none of the above flags.
121 Each of these operations is detailed later in this page.
122 Further flags may be specified in
124 to modify the behavior of
128 .SS Additional mount flags
129 The list below describes the additional flags that can be specified in
131 Note that some operation types ignore some or all of these flags,
132 as described later in this page.
134 .\" FIXME 2.6.25 Added MS_I_VERSION, which needs to be documented.
137 .BR MS_DIRSYNC " (since Linux 2.5.19)"
138 Make directory changes on this filesystem synchronous.
139 (This property can be obtained for individual directories
143 .BR MS_LAZYTIME " (since Linux 4.0)"
144 .\" commit 0ae45f63d4ef8d8eeec49c7d8b44a1775fff13e8
145 .\" commit fe032c422c5ba562ba9c2d316f55e258e03259c6
146 .\" commit a26f49926da938f47561f386be56a83dd37a496d
147 Reduce on-disk updates of inode timestamps (atime, mtime, ctime)
148 by maintaining these changes only in memory.
149 The on-disk timestamps are updated only when:
152 the inode needs to be updated for some change unrelated to file timestamps;
154 the application employs
160 an undeleted inode is evicted from memory; or
162 more than 24 hours have passed since the inode was written to disk.
165 This mount option significantly reduces writes
166 needed to update the inode's timestamps, especially mtime and atime.
167 However, in the event of a system crash, the atime and mtime fields
168 on disk might be out of date by up to 24 hours.
170 Examples of workloads where this option could be of significant benefit
171 include frequent random writes to preallocated files,
172 as well as cases where the
174 mount option is also enabled.
175 (The advantage of combining
181 will return the correctly updated atime, but the atime updates
182 will be flushed to disk only in the cases listed above.)
185 Permit mandatory locking on files in this filesystem.
186 (Mandatory locking must still be enabled on a per-file basis,
190 .\" commit 95ace75414f312f9a7b93d873f386987b92a5301
191 this mount option requires the
196 Do not update access times for (all types of) files on this filesystem.
199 Do not allow access to devices (special files) on this filesystem.
202 Do not update access times for directories on this filesystem.
203 This flag provides a subset of the functionality provided by
211 Do not allow programs to be executed from this filesystem.
212 .\" (Possibly useful for a filesystem that contains non-Linux executables.
213 .\" Often used as a security feature, e.g., to make sure that restricted
214 .\" users cannot execute files uploaded using ftp or so.)
217 Do not honor set-user-ID and set-group-ID bits or file capabilities
218 when executing programs from this filesystem.
219 .\" (This is a security feature to prevent users executing set-user-ID and
220 .\" set-group-ID programs from removable disk devices.)
223 Mount filesystem read-only.
225 .BR MS_REC " (since Linux 2.4.11)"
226 Used in conjunction with
228 to create a recursive bind mount,
229 and in conjunction with the propagation type flags to recursively change
230 the propagation type of all of the mounts in a subtree.
231 See below for further details.
233 .BR MS_RELATIME " (since Linux 2.6.20)"
234 When a file on this filesystem is accessed,
235 update the file's last access time (atime) only if the current value
236 of atime is less than or equal to the file's last modification time (mtime)
237 or last status change time (ctime).
238 This option is useful for programs, such as
240 that need to know when a file has been read since it was last modified.
241 Since Linux 2.6.30, the kernel defaults to the behavior provided
244 was specified), and the
246 flag is required to obtain traditional semantics.
247 In addition, since Linux 2.6.30,
248 the file's last access time is always updated if it
249 is more than 1 day old.
250 .\" Matthew Garrett notes in the patch that added this behavior
251 .\" that this lets utilities such as tmpreaper (which deletes
252 .\" files based on last access time) work correctly.
254 .BR MS_SILENT " (since Linux 2.6.17)"
255 Suppress the display of certain
257 warning messages in the kernel log.
258 This flag supersedes the misnamed and obsolete
260 flag (available since Linux 2.4.12), which has the same meaning.
262 .BR MS_STRICTATIME " (since Linux 2.6.30)"
263 Always update the last access time (atime) when files on this
264 filesystem are accessed.
265 (This was the default behavior before Linux 2.6.30.)
266 Specifying this flag overrides the effect of setting the
273 Make writes on this filesystem synchronous (as though
278 was specified for all file opens to this filesystem).
280 From Linux 2.4 onward, the
281 .BR MS_NODEV ", " MS_NOEXEC ", and " MS_NOSUID
282 flags are settable on a per-mount-point basis.
283 From kernel 2.6.16 onward,
287 are also settable on a per-mount-point basis.
290 flag is also settable on a per-mount-point basis.
293 can be set or cleared on a per-mount-point basis as well as on
294 the underlying filesystem. The mounted filesystem will only be
295 writable if neither the filesystem nor the mountpoint are flagged as
298 .SS Remounting an existing mount
299 An existing mount may be remounted by specifying
303 This allows you to change the
307 of an existing mount without having to unmount and remount the filesystem.
309 should be the same value specified in the initial
317 arguments are ignored.
323 arguments should match the values used in the original
325 call, except for those parameters that are being deliberately changed.
326 Another exception is that
328 has a different meaning for remount, and it should only be included if
345 Attempts to change the setting of the
346 .\" See the definition of MS_RMT_MASK in include/uapi/linux/fs.h
348 flag during a remount are silently ignored.
351 .\" commit ffbc6f0ead47fa5a1dc9642b0331cb75c20a640e
360 then the remount operation preserves the existing values of these flags
361 (rather than defaulting to
364 Since Linux 2.6.26, this flag can be used with
366 to only modify the per-mount-point flags.
367 .\" See https://lwn.net/Articles/281157/
368 This is particularly useful for setting or clearing the "read-only"
369 flag on a mount point without changing the underlying filesystem.
374 MS_REMOUNT | MS_BIND | MS_RDONLY
376 will make access through this mountpoint read-only, without affecting
379 .SS Creating a bind mount
384 (available since Linux 2.4),
385 .\" since 2.4.0-test9
386 then perform a bind mount.
387 A bind mount makes a file or a directory subtree visible at
388 another point within the single directory hierarchy.
389 Bind mounts may cross filesystem boundaries and span
397 arguments are ignored.
399 The remaining bits in the
401 argument are also ignored, with the exception of
403 (The bind mount has the same mount options as
404 the underlying mount point.)
405 However, see the discussion of remounting above,
406 for a method of making an existing bind mount read-only.
408 By default, when a directory is bind mounted,
409 only that directory is mounted;
410 if there are any submounts under the directory tree,
411 they are not bind mounted.
414 flag is also specified, then a recursive bind mount operation is performed:
415 all submounts under the
417 subtree (other than unbindable mounts)
418 are also bind mounted at the corresponding location in the
422 .SS Changing the propagation type of an existing mount
431 (all available since Linux 2.6.15),
432 then the propagation type of an existing mount is changed.
433 If more than one of these flags is specified, an error results.
435 The only flags that can be used with changing the propagation type are
445 arguments are ignored.
447 The meanings of the propagation type flags are as follows:
450 Make this mount point shared.
451 Mount and unmount events immediately under this mount point will propagate
452 to the other mount points that are members of this mount's peer group.
453 Propagation here means that the same mount or unmount will automatically
454 occur under all of the other mount points in the peer group.
455 Conversely, mount and unmount events that take place under
456 peer mount points will propagate to this mount point.
459 Make this mount point private.
460 Mount and unmount events do not propagate into or out of this mount point.
463 If this is a shared mount point that is a member of a peer group
464 that contains other members, convert it to a slave mount.
465 If this is a shared mount point that is a member of a peer group
466 that contains no other members, convert it to a private mount.
467 Otherwise, the propagation type of the mount point is left unchanged.
469 When a mount point is a slave,
470 mount and unmount events propagate into this mount point from
471 the (master) shared peer group of which it was formerly a member.
472 Mount and unmount events under this mount point do not propagate to any peer.
474 A mount point can be the slave of another peer group
475 while at the same time sharing mount and unmount events
476 with a peer group of which it is a member.
479 Make this mount unbindable.
480 This is like a private mount,
481 and in addition this mount can't be bind mounted.
482 When a recursive bind mount
488 flags) is performed on a directory subtree,
489 any bind mounts within the subtree are automatically pruned
490 (i.e., not replicated)
491 when replicating that subtree to produce the target subtree.
493 By default, changing the propagation type affects only the
498 flag is also specified in
500 then the propagation type of all mount points under
504 For further details regarding mount propagation types
505 (including the default propagation type assigned to new mounts), see
506 .BR mount_namespaces (7).
513 (available since Linux 2.4.18),
516 specifies an existing mount point and
518 specifies the new location to which that mount point is to be relocated.
519 The move is atomic: at no point is the subtree unmounted.
521 The remaining bits in the
523 argument are ignored, as are the
529 .SS Creating a new mount point
543 performs its default action: creating a new mount point.
545 specifies the source for the new mount point, and
547 specifies the directory at which to create the mount point.
553 arguments are employed, and further bits may be specified in
555 to modify the behavior of the call.
558 On success, zero is returned.
559 On error, \-1 is returned, and
561 is set appropriately.
563 The error values given below result from filesystem type independent
565 Each filesystem type may have its own special errors and its
566 own special behavior.
567 See the Linux kernel source code for details.
570 A component of a path was not searchable.
572 .BR path_resolution (7).)
575 Mounting a read-only filesystem was attempted without giving the
582 is located on a filesystem mounted with the
585 .\" mtk: Probably: write permission is required for MS_BIND, with
586 .\" the error EPERM if not present; CAP_DAC_OVERRIDE is required.
594 cannot be remounted read-only,
595 because it still holds files open for writing.
603 is still busy (it is the working directory of some thread,
604 the mount point of another device, has open files, etc.).
607 One of the pointer arguments points outside the user address space.
611 had an invalid superblock.
618 was not already mounted on
626 was not a mount point, or was \(aq/\(aq.
630 includes more than one of
645 and also includes a flag other than
651 An attempt was made to bind mount an unbindable mount.
654 Too many links encountered during pathname resolution.
657 A move operation was attempted, and
663 (In case no block device is required:)
664 Table of dummy devices is full.
667 A pathname was longer than
672 not configured in the kernel.
675 A pathname was empty or had a nonexistent component.
678 The kernel could not allocate a free page to copy filenames or data into.
682 is not a block device (and a device was required).
691 The major number of the block device
696 The caller does not have the required privileges.
709 were added to glibc headers in version 2.12.
712 This function is Linux-specific and should not be used in
713 programs intended to be portable.
715 Since Linux 2.4 a single filesystem can be mounted at
716 multiple mount points, and multiple mounts can be stacked
717 on the same mount point.
718 .\" Multiple mounts on same mount point: since 2.3.99pre7.
722 argument may have the magic number 0xC0ED (\fBMS_MGC_VAL\fP)
724 (All of the other flags discussed in DESCRIPTION
725 occupy the low order 16 bits of
729 was required in kernel versions prior to 2.4,
730 but since Linux 2.4 is no longer required and is ignored if specified.
739 was added to \fI<mman.h>\fP.
741 Before Linux 2.4 an attempt to execute a set-user-ID or set-group-ID program
742 on a filesystem mounted with
746 Since Linux 2.4 the set-user-ID and set-group-ID bits are
747 just silently ignored in this case.
748 .\" The change is in patch-2.4.0-prerelease.
751 .SS Per-process namespaces
752 Starting with kernel 2.4.19, Linux provides
753 per-process mount namespaces.
754 A mount namespace is the set of filesystem mounts that
755 are visible to a process.
756 Mount-point namespaces can be (and usually are)
757 shared between multiple processes,
758 and changes to the namespace (i.e., mounts and unmounts) by one process
759 are visible to all other processes sharing the same namespace.
760 (The pre-2.4.19 Linux situation can be considered as one in which
761 a single namespace was shared by every process on the system.)
763 A child process created by
765 shares its parent's mount namespace;
766 the mount namespace is preserved across an
769 A process can obtain a private mount namespace if:
770 it was created using the
774 in which case its new namespace is initialized to be a
776 of the namespace of the process that called
783 which causes the caller's mount namespace to obtain a private copy
784 of the namespace that it was previously sharing with other processes,
785 so that future mounts and unmounts by the caller are invisible
786 to other processes (except child processes that the caller
787 subsequently creates) and vice versa.
790 .I /proc/[pid]/mounts
791 file exposes the list of mount points in the mount
792 namespace of the process with the specified ID; see
798 .BR mount_namespaces (7),
799 .BR path_resolution (7),