man2/mount.2

   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>
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  26 .\"
  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.
  39 .\"
  40 .TH MOUNT 2 2016-07-17 "Linux" "Linux Programmer's Manual"
  41 .SH NAME
  42 mount \- mount filesystem
  43 .SH SYNOPSIS
  44 .nf
  45 .B "#include <sys/mount.h>"
  46 .sp
  47 .BI "int mount(const char *" source ", const char *" target ,
  48 .BI "          const char *" filesystemtype ", unsigned long " mountflags ,
  49 .BI "          const void *" data );
  50 .fi
  51 .SH DESCRIPTION
  52 .BR mount ()
  53 attaches the filesystem specified by
  54 .I source
  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
  59 .IR target .
  60
  61 Appropriate privilege (Linux: the
  62 .B CAP_SYS_ADMIN
  63 capability) is required to mount filesystems.
  64
  65 Values for the
  66 .I filesystemtype
  67 argument supported by the kernel are listed in
  68 .I /proc/filesystems
  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
  72 are loaded.
  73
  74 The
  75 .I data
  76 argument is interpreted by the different filesystems.
  77 Typically it is a string of comma-separated options
  78 understood by this filesystem.
  79 See
  80 .BR mount (8)
  81 for details of the options available for each filesystem type.
  82
  83 A call to
  84 .BR mount ()
  85 performs one of a number of general types of operation,
  86 depending on the bits specified in
  87 .IR mountflags .
  88 The choice of which operation to perform is determined by
  89 testing the bits set in
  90 .IR mountflags ,
  91 with the tests being conducted in the order listed here:
  92 .IP * 3
  93 Remount an existing mount:
  94 .IR mountflags
  95 includes
  96 .BR MS_REMOUNT .
  97 .IP *
  98 Create a bind mount:
  99 .IR mountflags
 100 includes
 101 .BR MS_BIND .
 102 .IP *
 103 Change the propagation type of an existing mount:
 104 .IR mountflags
 105 includes one of
 106 .BR MS_SHARED ,
 107 .BR MS_PRIVATE ,
 108 .BR MS_SLAVE ,
 109 or
 110 .BR MS_UNBINDABLE .
 111 .IP *
 112 Move an existing mount to a new location:
 113 .IR mountflags
 114 includes
 115 .BR MS_MOVE .
 116 .IP *
 117 Create a new mount:
 118 .IR mountflags
 119 includes none of the above flags.
 120 .PP
 121 Each of these operations is detailed later in this page.
 122 Further flags may be specified in
 123 .IR mountflags
 124 to modify the behavior of
 125 .BR mount (),
 126 as described below.
 127 .\"
 128 .SS Additional mount flags
 129 The list below describes the additional flags that can be specified in
 130 .IR mountflags .
 131 Note that some operation types ignore some or all of these flags,
 132 as described later in this page.
 133 .\"
 134 .\" FIXME 2.6.25 Added MS_I_VERSION, which needs to be documented.
 135 .\"
 136 .TP
 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
 140 or subtrees using
 141 .BR chattr (1).)
 142 .TP
 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:
 150 .RS
 151 .IP (a) 5
 152 the inode needs to be updated for some change unrelated to file timestamps;
 153 .IP (b)
 154 the application employs
 155 .BR fsync (2),
 156 .BR syncfs (2),
 157 or
 158 .BR sync (2);
 159 .IP (c)
 160 an undeleted inode is evicted from memory; or
 161 .IP (d)
 162 more than 24 hours have passed since the inode was written to disk.
 163 .RE
 164 .IP
 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.
 169
 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
 173 .B MS_STRICTATIME
 174 mount option is also enabled.
 175 (The advantage of combining
 176 .BR MS_STRICTATIME
 177 and
 178 .BR MS_LAZYTIME
 179 is that
 180 .BR stat (2)
 181 will return the correctly updated atime, but the atime updates
 182 will be flushed to disk only in the cases listed above.)
 183 .TP
 184 .B MS_MANDLOCK
 185 Permit mandatory locking on files in this filesystem.
 186 (Mandatory locking must still be enabled on a per-file basis,
 187 as described in
 188 .BR fcntl (2).)
 189 Since Linux 4.5,
 190 .\" commit 95ace75414f312f9a7b93d873f386987b92a5301
 191 this mount option requires the
 192 .B CAP_SYS_ADMIN
 193 capability.
 194 .TP
 195 .B MS_NOATIME
 196 Do not update access times for (all types of) files on this filesystem.
 197 .TP
 198 .B MS_NODEV
 199 Do not allow access to devices (special files) on this filesystem.
 200 .TP
 201 .B MS_NODIRATIME
 202 Do not update access times for directories on this filesystem.
 203 This flag provides a subset of the functionality provided by
 204 .BR MS_NOATIME ;
 205 that is,
 206 .BR MS_NOATIME
 207 implies
 208 .BR MS_NODIRATIME .
 209 .TP
 210 .B MS_NOEXEC
 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.)
 215 .TP
 216 .B MS_NOSUID
 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.)
 221 .TP
 222 .B MS_RDONLY
 223 Mount filesystem read-only.
 224 .TP
 225 .BR MS_REC " (since Linux 2.4.11)"
 226 Used in conjunction with
 227 .BR MS_BIND
 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.
 232 .TP
 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
 239 .BR mutt (1),
 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
 242 by this flag (unless
 243 .BR MS_NOATIME
 244 was specified), and the
 245 .B MS_STRICTATIME
 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.
 253 .TP
 254 .BR MS_SILENT " (since Linux 2.6.17)"
 255 Suppress the display of certain
 256 .RI ( printk ())
 257 warning messages in the kernel log.
 258 This flag supersedes the misnamed and obsolete
 259 .BR MS_VERBOSE
 260 flag (available since Linux 2.4.12), which has the same meaning.
 261 .TP
 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
 267 .BR MS_NOATIME
 268 and
 269 .BR MS_RELATIME
 270 flags.
 271 .TP
 272 .B MS_SYNCHRONOUS
 273 Make writes on this filesystem synchronous (as though
 274 the
 275 .B O_SYNC
 276 flag to
 277 .BR open (2)
 278 was specified for all file opens to this filesystem).
 279 .PP
 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,
 284 .B MS_NOATIME
 285 and
 286 .B MS_NODIRATIME
 287 are also settable on a per-mount-point basis.
 288 The
 289 .B MS_RELATIME
 290 flag is also settable on a per-mount-point basis.
 291 .\"
 292 .SS Remounting an existing mount
 293 An existing mount may be remounted by specifying
 294 .B MS_REMOUNT
 295 in
 296 .IR mountflags .
 297 This allows you to change the
 298 .I mountflags
 299 and
 300 .I data
 301 of an existing mount without having to unmount and remount the filesystem.
 302 .I target
 303 should be the same value specified in the initial
 304 .BR mount ()
 305 call.
 306
 307 The
 308 .I source
 309 and
 310 .I filesystemtype
 311 arguments are ignored.
 312
 313 The
 314 .I mountflags
 315 and
 316 .I data
 317 arguments should match the values used in the original
 318 .BR mount ()
 319 call, except for those parameters that are being deliberately changed.
 320
 321 The following
 322 .I mountflags
 323 can be changed:
 324 .BR MS_LAZYTIME ,
 325 .BR MS_MANDLOCK ,
 326 .BR MS_NOATIME ,
 327 .BR MS_NODEV ,
 328 .BR MS_NODIRATIME ,
 329 .BR MS_NOEXEC ,
 330 .BR MS_NOSUID ,
 331 .BR MS_RELATIME ,
 332 .BR MS_RDONLY ,
 333 and
 334 .BR MS_SYNCHRONOUS .
 335 Attempts to change the setting of the
 336 .\" See the definition of MS_RMT_MASK in include/uapi/linux/fs.h
 337 .BR MS_DIRSYNC
 338 flag during a remount are silently ignored.
 339
 340 Since Linux 3.17,
 341 .\" commit ffbc6f0ead47fa5a1dc9642b0331cb75c20a640e
 342 if none of
 343 .BR MS_NOATIME ,
 344 .BR MS_NODIRATIME ,
 345 .BR MS_RELATIME ,
 346 or
 347 .BR MS_STRICTATIME
 348 is specified in
 349 .IR mountflags ,
 350 then the remount operation preserves the existing values of these flags
 351 (rather than defaulting to
 352 .BR MS_RELATIME ).
 353
 354 Since Linux 2.6.26, this flag can also be used to make
 355 .\" See https://lwn.net/Articles/281157/
 356 an existing bind mount read-only by specifying
 357 .IR mountflags
 358 as:
 359
 360     MS_REMOUNT | MS_BIND | MS_RDONLY
 361
 362 Note that only the
 363 .BR MS_RDONLY
 364 setting of the bind mount can be changed in this manner.
 365 .\"
 366 .SS Creating a bind mount
 367 If
 368 .I mountflags
 369 includes
 370 .BR MS_BIND
 371 (available since Linux 2.4),
 372 .\" since 2.4.0-test9
 373 then perform a bind mount.
 374 A bind mount makes a file or a directory subtree visible at
 375 another point within the single directory hierarchy.
 376 Bind mounts may cross filesystem boundaries and span
 377 .BR chroot (2)
 378 jails.
 379
 380 The
 381 .IR filesystemtype
 382 and
 383 .IR data
 384 arguments are ignored.
 385
 386 The remaining bits in the
 387 .I mountflags
 388 argument are also ignored, with the exception of
 389 .BR MS_REC .
 390 (The bind mount has the same mount options as
 391 the underlying mount point.)
 392 However, see the discussion of remounting above,
 393 for a method of making an existing bind mount read-only.
 394
 395 By default, when a directory is bind mounted,
 396 only that directory is mounted;
 397 if there are any submounts under the directory tree,
 398 they are not bind mounted.
 399 If the
 400 .BR MS_REC
 401 flag is also specified, then a recursive bind mount operation is performed:
 402 all submounts under the
 403 .I source
 404 subtree (other than unbindable mounts)
 405 are also bind mounted at the corresponding location in the
 406 .I target
 407 subtree.
 408 .\"
 409 .SS Changing the propagation type of an existing mount
 410 If
 411 .IR mountflags
 412 includes one of
 413 .BR MS_SHARED ,
 414 .BR MS_PRIVATE ,
 415 .BR MS_SLAVE ,
 416 or
 417 .BR MS_UNBINDABLE
 418 (all available since Linux 2.6.15),
 419 then the propagation type of an existing mount is changed.
 420 If more than one of these flags is specified, an error results.
 421
 422 The only flags that can be used with changing the propagation type are
 423 .BR MS_REC
 424 and
 425 .BR MS_SILENT .
 426
 427 The
 428 .IR source ,
 429 .IR filesystemtype ,
 430 and
 431 .IR data
 432 arguments are ignored.
 433
 434 The meanings of the propagation type flags are as follows:
 435 .TP
 436 .BR MS_SHARED
 437 Make this mount point shared.
 438 Mount and unmount events immediately under this mount point will propagate
 439 to the other mount points that are members of this mount's peer group.
 440 Propagation here means that the same mount or unmount will automatically
 441 occur under all of the other mount points in the peer group.
 442 Conversely, mount and unmount events that take place under
 443 peer mount points will propagate to this mount point.
 444 .TP
 445 .BR MS_PRIVATE
 446 Make this mount point private.
 447 Mount and unmount events do not propagate into or out of this mount point.
 448 This is the default propagation type for newly created mount points.
 449 .TP
 450 .BR MS_SLAVE
 451 If this is a shared mount point that is a member of a peer group
 452 that contains other members, convert it to a slave mount.
 453 If this is a shared mount point that is a member of a peer group
 454 that contains no other members, convert it to a private mount.
 455 Otherwise, the propagation type of the mount point is left unchanged.
 456
 457 When a mount point is a slave,
 458 mount and unmount events propagate into this mount point from
 459 the (master) shared peer group of which it was formerly a member.
 460 Mount and unmount events under this mount point do not propagate to any peer.
 461
 462 A mount point can be the slave of another peer group
 463 while at the same time sharing mount and unmount events
 464 with a peer group of which it is a member.
 465 .TP
 466 .BR MS_UNBINDABLE
 467 Make this mount unbindable.
 468 This is like a private mount,
 469 and in addition this mount can't be bind mounted.
 470 When a recursive bind mount
 471 .RB ( mount ()
 472 with the
 473 .BR MS_BIND
 474 and
 475 .BR MS_REC
 476 flags) is performed on a directory subtree,
 477 any bind mounts within the subtree are automatically pruned
 478 (i.e., not replicated)
 479 when replicating that subtree to produce the target subtree.
 480 .PP
 481 By default, changing the propagation type affects only the
 482 .I target
 483 mount point.
 484 If the
 485 .B MS_REC
 486 flag is also specified in
 487 .IR mountflags ,
 488 then the propagation type of all mount points under
 489 .IR target
 490 is also changed.
 491
 492 For further details regarding mount propagation types, see
 493 .BR mount_namespaces (7).
 494 .\"
 495 .SS Moving a mount
 496 If
 497 .I mountflags
 498 contains the flag
 499 .BR MS_MOVE
 500 (available since Linux 2.4.18),
 501 then move a subtree:
 502 .I source
 503 specifies an existing mount point and
 504 .I target
 505 specifies the new location to which that mount point is to be relocated.
 506 The move is atomic: at no point is the subtree unmounted.
 507
 508 The remaining bits in the
 509 .IR mountflags
 510 argument are ignored, as are the
 511 .IR filesystemtype
 512 and
 513 .IR data
 514 arguments.
 515 .\"
 516 .SS Creating a new mount point
 517 If none of
 518 .BR MS_REMOUNT ,
 519 .BR MS_BIND ,
 520 .BR MS_MOVE ,
 521 .BR MS_SHARED ,
 522 .BR MS_PRIVATE ,
 523 .BR MS_SLAVE ,
 524 or
 525 .BR MS_UNBINDABLE
 526 is specified in
 527 .IR mountflags ,
 528 then
 529 .BR mount ()
 530 performs its default action: creating a new mount point.
 531 .IR source
 532 specifies the source for the new mount point, and
 533 .IR target
 534 specifies the directory at which to create the mount point.
 535
 536 The
 537 .I filesystemtype
 538 and
 539 .I data
 540 arguments are employed, and further bits may be specified in
 541 .IR mountflags
 542 to modify the behavior of the call.
 543 .\"
 544 .SH RETURN VALUE
 545 On success, zero is returned.
 546 On error, \-1 is returned, and
 547 .I errno
 548 is set appropriately.
 549 .SH ERRORS
 550 The error values given below result from filesystem type independent
 551 errors.
 552 Each filesystem type may have its own special errors and its
 553 own special behavior.
 554 See the Linux kernel source code for details.
 555 .TP
 556 .B EACCES
 557 A component of a path was not searchable.
 558 (See also
 559 .BR path_resolution (7).)
 560 .TP
 561 .B EACCES
 562 Mounting a read-only filesystem was attempted without giving the
 563 .B MS_RDONLY
 564 flag.
 565 .TP
 566 .B EACCES
 567 The block device
 568 .I source
 569 is located on a filesystem mounted with the
 570 .B MS_NODEV
 571 option.
 572 .\" mtk: Probably: write permission is required for MS_BIND, with
 573 .\" the error EPERM if not present; CAP_DAC_OVERRIDE is required.
 574 .TP
 575 .B EBUSY
 576 .I source
 577 is already mounted.
 578 .TP
 579 .B EBUSY
 580 .I source
 581 cannot be remounted read-only,
 582 because it still holds files open for writing.
 583 .TP
 584 .B EBUSY
 585 .I source
 586 cannot be mounted on
 587 .I target
 588 because
 589 .I target
 590 is still busy (it is the working directory of some thread,
 591 the mount point of another device, has open files, etc.).
 592 .TP
 593 .B EFAULT
 594 One of the pointer arguments points outside the user address space.
 595 .TP
 596 .B EINVAL
 597 .I source
 598 had an invalid superblock.
 599 .TP
 600 .B EINVAL
 601 A remount operation
 602 .RB ( MS_REMOUNT )
 603 was attempted, but
 604 .I source
 605 was not already mounted on
 606 .IR target .
 607 .TP
 608 .B EINVAL
 609 A move operation
 610 .RB ( MS_MOVE )
 611 was attempted, but
 612 .I source
 613 was not a mount point, or was \(aq/\(aq.
 614 .TP
 615 .B EINVAL
 616 .I mountflags
 617 includes more than one of
 618 .BR MS_SHARED ,
 619 .BR MS_PRIVATE ,
 620 .BR MS_SLAVE ,
 621 or
 622 .BR MS_UNBINDABLE .
 623 .TP
 624 .B EINVAL
 625 .I mountflags
 626 includes
 627 .BR MS_SHARED ,
 628 .BR MS_PRIVATE ,
 629 .BR MS_SLAVE ,
 630 or
 631 .BR MS_UNBINDABLE
 632 and also includes a flag other than
 633 .BR MS_REC
 634 or
 635 .BR MS_SILENT .
 636 .TP
 637 .BR EINVAL
 638 An attempt was made to bind mount an unbindable mount.
 639 .TP
 640 .B ELOOP
 641 Too many links encountered during pathname resolution.
 642 .TP
 643 .B ELOOP
 644 A move operation was attempted, and
 645 .I target
 646 is a descendant of
 647 .IR source .
 648 .TP
 649 .B EMFILE
 650 (In case no block device is required:)
 651 Table of dummy devices is full.
 652 .TP
 653 .B ENAMETOOLONG
 654 A pathname was longer than
 655 .BR MAXPATHLEN .
 656 .TP
 657 .B ENODEV
 658 .I filesystemtype
 659 not configured in the kernel.
 660 .TP
 661 .B ENOENT
 662 A pathname was empty or had a nonexistent component.
 663 .TP
 664 .B ENOMEM
 665 The kernel could not allocate a free page to copy filenames or data into.
 666 .TP
 667 .B ENOTBLK
 668 .I source
 669 is not a block device (and a device was required).
 670 .TP
 671 .B ENOTDIR
 672 .IR target ,
 673 or a prefix of
 674 .IR source ,
 675 is not a directory.
 676 .TP
 677 .B ENXIO
 678 The major number of the block device
 679 .I source
 680 is out of range.
 681 .TP
 682 .B EPERM
 683 The caller does not have the required privileges.
 684 .SH VERSIONS
 685 The definitions of
 686 .BR MS_DIRSYNC ,
 687 .BR MS_MOVE ,
 688 .BR MS_PRIVATE ,
 689 .BR MS_REC ,
 690 .BR MS_RELATIME ,
 691 .BR MS_SHARED ,
 692 .BR MS_SLAVED ,
 693 .BR MS_STRICTATIME
 694 and
 695 .BR MS_UNBINDABLE
 696 were added to glibc headers in version 2.12.
 697 .\"
 698 .SH CONFORMING TO
 699 This function is Linux-specific and should not be used in
 700 programs intended to be portable.
 701 .SH NOTES
 702 Since Linux 2.4 a single filesystem can be mounted at
 703 multiple mount points, and multiple mounts can be stacked
 704 on the same mount point.
 705 .\" Multiple mounts on same mount point: since 2.3.99pre7.
 706
 707 The
 708 .I mountflags
 709 argument may have the magic number 0xC0ED (\fBMS_MGC_VAL\fP)
 710 in the top 16 bits.
 711 (All of the other flags discussed in DESCRIPTION
 712 occupy the low order 16 bits of
 713 .IR mountflags .)
 714 Specifying
 715 .BR MS_MGC_VAL
 716 was required in kernel versions prior to 2.4,
 717 but since Linux 2.4 is no longer required and is ignored if specified.
 718
 719 The original
 720 .B MS_SYNC
 721 flag was renamed
 722 .B MS_SYNCHRONOUS
 723 in 1.1.69
 724 when a different
 725 .B MS_SYNC
 726 was added to \fI<mman.h>\fP.
 727 .LP
 728 Before Linux 2.4 an attempt to execute a set-user-ID or set-group-ID program
 729 on a filesystem mounted with
 730 .B MS_NOSUID
 731 would fail with
 732 .BR EPERM .
 733 Since Linux 2.4 the set-user-ID and set-group-ID bits are
 734 just silently ignored in this case.
 735 .\" The change is in patch-2.4.0-prerelease.
 736 .\"
 737
 738 .SS Per-process namespaces
 739 Starting with kernel 2.4.19, Linux provides
 740 per-process mount namespaces.
 741 A mount namespace is the set of filesystem mounts that
 742 are visible to a process.
 743 Mount-point namespaces can be (and usually are)
 744 shared between multiple processes,
 745 and changes to the namespace (i.e., mounts and unmounts) by one process
 746 are visible to all other processes sharing the same namespace.
 747 (The pre-2.4.19 Linux situation can be considered as one in which
 748 a single namespace was shared by every process on the system.)
 749
 750 A child process created by
 751 .BR fork (2)
 752 shares its parent's mount namespace;
 753 the mount namespace is preserved across an
 754 .BR execve (2).
 755
 756 A process can obtain a private mount namespace if:
 757 it was created using the
 758 .BR clone (2)
 759 .BR CLONE_NEWNS
 760 flag,
 761 in which case its new namespace is initialized to be a
 762 .I copy
 763 of the namespace of the process that called
 764 .BR clone (2);
 765 or it calls
 766 .BR unshare (2)
 767 with the
 768 .BR CLONE_NEWNS
 769 flag,
 770 which causes the caller's mount namespace to obtain a private copy
 771 of the namespace that it was previously sharing with other processes,
 772 so that future mounts and unmounts by the caller are invisible
 773 to other processes (except child processes that the caller
 774 subsequently creates) and vice versa.
 775
 776 The Linux-specific
 777 .I /proc/[pid]/mounts
 778 file exposes the list of mount points in the mount
 779 namespace of the process with the specified ID; see
 780 .BR proc (5)
 781 for details.
 782 .SH SEE ALSO
 783 .BR umount (2),
 784 .BR mount_namespaces (7),
 785 .BR path_resolution (7),
 786 .BR findmnt (8),
 787 .BR lsblk (8),
 788 .BR mount (8),
 789 .BR umount (8)