Re: PROBLEM: #89121 BTRFS mixes up mounted devices with their snapshots

[Goffredo Baroncelli <kreijack@inwind.it>]

On 12/02/2014 01:15 AM, MegaBrutal wrote:
> 2014-12-02 0:24 GMT+01:00 Robert White <rwhite@pobox.com>:
>> On 12/01/2014 02:10 PM, MegaBrutal wrote:
>>>
>>> Since having duplicate UUIDs on devices is not a problem for me since
>>> I can tell them apart by LVM names, the discussion is of little
>>> relevance to my use case. Of course it's interesting and I like to
>>> read it along, it is not about the actual problem at hand.
>>>
>>
>> Which is why you use the device= mount option, which would take LVM names
>> and which was repeatedly discussed as solving this very problem.
>>
>> Once you decide to duplicate the UUIDs with LVM snapshots you take up the
>> burden of disambiguating your storage.
>>
>> Which is part of why re-reading was suggested as this was covered in some
>> depth and _is_ _exactly_ about the problem at hand.
> 
> Nope.
> 
> root@reproduce-1391429:~# cat /proc/cmdline
> BOOT_IMAGE=/vmlinuz-3.18.0-031800rc5-generic
> root=/dev/mapper/vg-rootlv ro
> rootflags=device=/dev/mapper/vg-rootlv,subvol=@
> 
> Observe, device= mount option is added.

device= options is needed only in a btrfs multi-volume scenario.
If you have only one disk, this is not needed

> 
> 
> root@reproduce-1391429:~# ./reproduce-1391429.sh
> #!/bin/sh -v
> lvs
>   LV     VG   Attr      LSize   Pool Origin Data%  Move Log Copy%  Convert
>   rootlv vg   -wi-ao---   1.00g
>   swap0  vg   -wi-ao--- 256.00m
> 
> grub-probe --target=device /
> /dev/mapper/vg-rootlv
> 
> grep " / " /proc/mounts
> rootfs / rootfs rw 0 0
> /dev/dm-1 / btrfs rw,relatime,space_cache 0 0
> 
> lvcreate --snapshot --size=128M --name z vg/rootlv
>   Logical volume "z" created
> 
> lvs
>   LV     VG   Attr      LSize   Pool Origin Data%  Move Log Copy%  Convert
>   rootlv vg   owi-aos--   1.00g
>   swap0  vg   -wi-ao--- 256.00m
>   z      vg   swi-a-s-- 128.00m      rootlv   0.11
> 
> ls -l /dev/vg/
> total 0
> lrwxrwxrwx 1 root root 7 Dec  2 00:12 rootlv -> ../dm-1
> lrwxrwxrwx 1 root root 7 Dec  2 00:12 swap0 -> ../dm-0
> lrwxrwxrwx 1 root root 7 Dec  2 00:12 z -> ../dm-2
> 
> grub-probe --target=device /
> /dev/mapper/vg-z
>
> grep " / " /proc/mounts
> rootfs / rootfs rw 0 0
> /dev/dm-2 / btrfs rw,relatime,space_cache 0 0

What /proc/self/mountinfo contains ?

And more important question: it is only the value
returned by /proc/mount wrongly or also the filesystem
content is affected ?

> 
> lvremove --force vg/z
>   Logical volume "z" successfully removed
> 
> grub-probe --target=device /
> /dev/mapper/vg-rootlv
> 
> grep " / " /proc/mounts
> rootfs / rootfs rw 0 0
> /dev/dm-1 / btrfs rw,relatime,space_cache 0 0
> 
> 
> Problem still reproduces.
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