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

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Date:	Mon, 29 Sep 2014 19:16:24 -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 7:15 PM, Eric W. Biederman
<ebiederm@...ssion.com> wrote:
> 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.

I don't understand your proposed edit.  Can you type it out explicitly?

--Andy

>
> Eric
>



-- 
Andy Lutomirski
AMA Capital Management, LLC
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