linux-kernel - Re: Duplicate inode number when mount --bind some directories to same mountpoint. (from Fedora18 to 4.10-rc3)

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Message-ID: <90248ac2-0253-36d4-ddd0-b283373634f8@nttcom.co.jp>
Date:   Mon, 16 Jan 2017 10:33:40 +0900
From:   Nakajima Akira <nakajima.akira@...com.co.jp>
To:     Al Viro <viro@...IV.linux.org.uk>
CC:     <linux-kernel@...r.kernel.org>
Subject: Re: Duplicate inode number when mount --bind some directories to same
 mountpoint. (from Fedora18 to 4.10-rc3)

 > 120 116 252:1 /home/b /mnt rw,relatime shared:1 - ext4 /dev/vda1 
rw,data=ordered
 > 121 61 252:1 /home/b /home/a rw,relatime shared:1 - ext4 /dev/vda1 
rw,data=ordered

Thanks for your polite explanation.
I can understand this is correct kernel specification
and above Fedora18 and RHEL7.0 is using shared mount as default.


I should have done it as follows.

# mount --make-private --bind a /mnt
# mount --make-private --bind b /mnt
# ls -i
100 a  999 b

# tali /proc/self/mountinfo
200 59 8:3 /home/a /mnt rw,relatime - ext4 /dev/sda3 
rw,stripe=64,data=ordered
205 200 8:3 /home/b /mnt rw,relatime - ext4 /dev/sda3 
rw,stripe=64,data=ordered


On 2017/01/13 12:26, Al Viro wrote:
> On Fri, Jan 13, 2017 at 10:40:08AM +0900, Nakajima Akira wrote:
>> On 2017/01/12 19:24, Al Viro wrote:
>>> On Thu, Jan 12, 2017 at 06:16:35PM +0900, Nakajima Akira wrote:
>>>> Bug:
>>>> Duplicate inode number when mount --bind some directories to same
>>>> mountpoint. (from Fedora18 to 4.10-rc3)
>>>> Fedora17 and earlier works correctly.
>>>
>>> Explain, please.  "Duplicate inode number" between what and what?
>>
>> Duplicate inode number between mounted directories.
>>
>> Example)
>> # cd /home
>> # mkdir a b
>> # ls -i
>> 100 a  999 b
>> # mount --bind a /mnt
>> # mount --bind b /mnt
>> # ls -i
>> 999 a  999 b
>> 	
>> Inode number of directory "a" is changed to "b".
>> Then we see directory "b" when ls "a".
>
> 61 0 252:1 / / rw,relatime shared:1 - ext4 /dev/vda1 rw,data=ordered
>
> Root, marked shared (peer group 1).  /home is not a mountpoint, /mnt
> wasn't one until your mounts (i.e. both are within the same mount as /).
>
> Since /home/a is a subtree of a shared mount, any clone of it will, by
> default, join the same peer group.  Which means that binding it on /mnt
> results in
>
> 116 61 252:1 /home/a /mnt rw,relatime shared:1 - ext4 /dev/vda1 rw,data=ordered
>
> i.e. ext4[vda1]home/a being mounted on /mnt and marked peer of root mount.
> Accordingly, any mount/umount event in either will be duplicated to all
> peers (provided that they contain a counterpart of affected mountpoint).
> In particular, binding /home/b on /mnt (i.e. on top of ext4[vda1]home/mnt)
> propagates to the corresponding points in all peers - including the root
> mount, where it corresponds to /home/a.  Result:
>
> 120 116 252:1 /home/b /mnt rw,relatime shared:1 - ext4 /dev/vda1 rw,data=ordered
> 121 61 252:1 /home/b /home/a rw,relatime shared:1 - ext4 /dev/vda1 rw,data=ordered
>
> The same tree (ext4[vda1]home/b) is mounted on root in mount 116
> (i.e. the thing found on /mnt) and on /home/a in mount 61 (i.e. /home/a).
>
> Since /home/b is on a shared mount, both clones are put in the same peer
> group (i.e. the same group 1).
>
> You asked for it, you've got it...  Well, fedora folks did, actually.
> I'm none too fond of their default setup (root made shared), but that has
> nothing to do with the kernel.  Userland (systemd, as far as I can tell)
> is setting the things up that way, and it's even documented in fedora
> release notes...  Kernel mechanisms involved in that had been there for
> a long time and they are also documented (man 2 mount, look for MS_SHARED
> and related flags in there).
>
> Take it up with fedora folks...