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mount_namespaces.7: Rework the discussion of defaults for mount propagation types 

Add rather more detail. In particular, note the cases where the
default propagation type is MS_PRIVATE vs MS_SHARED.

Reported-by: Rob Landley <rob@landley.net>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Michael Kerrisk 2017-01-19 09:31:56 +13:00
parent 50dedc44d0
commit 3dcc463a03
1 changed files with 27 additions and 5 deletions
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32
man7/mount_namespaces.7
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@ -165,8 +165,6 @@ peer mount points will propagate to this mount point.
.BR MS_PRIVATE
This mount point is private; it does not have a peer group.
Mount and unmount events do not propagate into or out of this mount point.
This is the default propagation type for newly created mount points
(but see NOTES).
.TP
.BR MS_SLAVE
Mount and unmount events propagate into this mount point from
@ -198,6 +196,9 @@ any bind mounts within the subtree are automatically pruned
(i.e., not replicated)
when replicating that subtree to produce the target subtree.
.PP
For a discussion of the propagation type assigned to a new mount,
see NOTES.
The propagation type is a per-mount-point setting;
some mount points may be marked as shared
(with each shared mount point being a member of a distinct peer group),
@ -995,15 +996,29 @@ Mount namespaces first appeared in Linux 2.4.19.
Namespaces are a Linux-specific feature.
.\"
.SH NOTES
The kernel default propagation type for mount points is
The propagation type assigned to a new mount point depends
on the propagation type of the parent directory.
If the mount point has a parent (i.e., it is a non-root mount
point) and the propagation type of the parent is
.BR MS_SHARED ,
then the propagation type of the new mount is also
.BR MS_SHARED .
Otherwise, the propagation type of the new mount is
.BR MS_PRIVATE .
However,
But see also NOTES.
Notwithstanding the fact that the default propagation type
for new mount points is in many cases
.BR MS_PRIVATE ,
.BR MS_SHARED
is typically more commonly required, and for this reason,
is typically more useful.
For this reason,
.BR systemd (1)
automatically remounts all mount points as
.BR MS_SHARED
on system startup.
Thus, on most modern systems, the default propagation type is in practice
.BR MS_SHARED .
Since, when one uses
.BR unshare (1)
@ -1026,6 +1041,13 @@ To prevent this, one can use the
.IR "\-\-propagation\ unchanged"
option to
.BR unshare (1).
For a discussion of propagation types when moving mounts
.RB ( MS_MOVE )
and creating bind mounts
.RB ( MS_BIND ),
see
.IR Documentation/filesystems/sharedsubtree.txt .
.SH SEE ALSO
.BR unshare (1),
.BR clone (2),