[PATCH manpages] umount.2: revise MNT_FORCE description.

[NeilBrown <neilb@suse.com>]

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MNT_FORCE does not allow a busy filesystem to be unmounted.  Only
MNT_DETACH allows that.
MNT_FORCE only tries to abort pending transactions, in the hope that
might help umount not to block,

Also, other filesystems than NFS support MNT_FORCE.

Signed-off-by: NeilBrown <neilb@suse.com>
---

Explanation derived from examination of source code.
Cc to systemd devel in relation to comment I just posted:

https://github.com/systemd/systemd/commit/0c08f5cde749bd2818475e487109cd0d413452df

You may want to move the "linux version when support was added" to a
comment, or discard it.

 man2/umount.2 | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/man2/umount.2 b/man2/umount.2
index 68f76b7c6352..ccd1d07afe88 100644
--- a/man2/umount.2
+++ b/man2/umount.2
@@ -61,9 +61,17 @@ unmounts a target, but allows additional
 controlling the behavior of the operation:
 .TP
 .BR MNT_FORCE " (since Linux 2.1.116)"
-Force unmount even if busy.
-This can cause data loss.
-(Only for NFS mounts.)
+Ask the filesystem to abort pending requests before attempting the
+unmount.  This may allow the unmount to complete without waiting
+for an inaccessible server.  If, after aborting requests, some
+processes still have active references to the filesystem, the
+unmount will still fail. In Linux 4.12, only the filesystems
+9p (2.6.16),
+ceph (2.6.34),
+cifs (2.6.12),
+fuse (2.6.16),
+lustre (3.11),
+and NFS (2.1.116) respond to MNT_FORCE.
 .TP
 .BR MNT_DETACH " (since Linux 2.4.11)"
 Perform a lazy unmount: make the mount point unavailable for new
-- 
2.14.0.rc0.dirty


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