linux-kernel - Re: [PATCH] umount.2: Correct the description of MNT

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Date:	Mon, 29 Sep 2014 18:28:00 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Michael Kerrisk <mtk.manpages@...il.com>,
	Andrey Wagin <avagin@...il.com>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] umount.2: Correct the description of MNT_DETACH

On Mon, Sep 29, 2014 at 6:04 PM, Eric W. Biederman
<ebiederm@...ssion.com> wrote:
>
> I recently realized that I had been reasoning improperly about what
> umount(MNT_DETACH) did based on an insufficient description in
> the umount.2 man page, that matched my intuition but not the
> implementation.
>
> When there are no submounts MNT_DETACH is essentially harmless to
> applications.  Where there are submounts MNT_DETACH changes what
> is visible to applications using the detach directories.
>
> Signed-off-by: Eric W. Biederman <ebiederm@...ssion.com>
> ---
>  man2/umount.2 | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/man2/umount.2 b/man2/umount.2
> index 5ff88152c738..aea39d8306fe 100644
> --- a/man2/umount.2
> +++ b/man2/umount.2
> @@ -66,9 +66,10 @@ This can cause data loss.
>  (Only for NFS mounts.)
>  .TP
>  .BR MNT_DETACH " (since Linux 2.4.11)"
> -Perform a lazy unmount: make the mount point unavailable for
> -new accesses, and actually perform the unmount when the mount point
> -ceases to be busy.
> +Perform a lazy unmount: make the mount point unavailable for new
> +accesses, immediately disconnect the filesystem and all filesystems
> +mounted below it from each other and from the mount table, and
> +actually perform the unmount when the mount point ceases to be busy.

Want to add something like:

MNT_DETACH on a shared mount will propagate unmount events to its peer
group.  That means that recursively bind mounting a shared mount and
then unmounting that recursive bind mount will unmount all submounts
of the original mount.  This behavior can be avoided by remounting a
directory with MS_REC | MS_PRIVATE before unmounting it.

--Andy

>  .TP
>  .BR MNT_EXPIRE " (since Linux 2.6.8)"
>  Mark the mount point as expired.
> --
> 1.9.1
>



-- 
Andy Lutomirski
AMA Capital Management, LLC
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