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

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Date:	Tue, 28 Oct 2014 14:43:25 +0100
From:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
CC:	Andy Lutomirski <luto@...capital.net>, 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

Hi Eric,

I'm still hoping for a revised patch from you in the light
of Andy's comments and question.

Thanks,

Michael


On 09/30/2014 04:16 AM, Andy Lutomirski wrote:
> 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
>>
> 
> 
> 


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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