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

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Date:	Mon, 29 Sep 2014 19:15:02 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Andy Lutomirski <luto@...capital.net>
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\@vger.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] umount.2: Correct the description of MNT_DETACH

Andy Lutomirski <luto@...capital.net> writes:

> 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.

Make that any unmount on a shared mount that will propogate unmount
events to it's peer group.

Eric

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