Re: [PATCH v2] common: get fs type again using device canonical name in _fs_type

[Dave Chinner <david@fromorbit.com>]

On Fri, Aug 01, 2014 at 12:02:41PM +0800, Eryu Guan wrote:
> On Fri, Aug 01, 2014 at 10:21:59AM +1000, Dave Chinner wrote:
> > On Thu, Jul 31, 2014 at 06:52:37PM +0800, Eryu Guan wrote:
> > > When testing with lvm, a previous btrfsck run could change df output
> > > from something like
> > > 
> > > /dev/mapper/rhel_hp--dl388eg8--01-testlv1 btrfs 15728640 900 13602172 1% /mnt/btrfs
> > > 
> > > to
> > > 
> > > /dev/dm-3 btrfs 15728640 900 13602172 1% /mnt/btrfs
> > 
> > I don't follow you. Why would running btrfsck change the name of the
> > device? If the filesystem is umounted and mounted again, then the
> > device could change, but btrfsck should not be not doing the
> > unmount/mount, and so unless the TEST_DEV/SCRATCH_DEV is changing
> > the output of df should be identical...
> > 
> > So before we change the _fs_type() code, can you explain exactly
> > how, when and why the device name is changing to me?
> 
> Assume that we have two btrfs filesystems, kernel is 3.16.0-rc4+
> 
> [root@hp-dl388eg8-01 btrfs-progs]# btrfs fi show
> Label: none  uuid: 1aba7da5-ce2b-4af0-a716-db732abc60b2
>         Total devices 1 FS bytes used 384.00KiB
>         devid    1 size 15.00GiB used 2.04GiB path /dev/mapper/rhel_hp--dl388eg8--01-testlv1
> 
> Label: none  uuid: 26ff4f12-f6d9-4cbc-aae2-57febeefde37
>         Total devices 2 FS bytes used 112.00KiB
>         devid    1 size 15.00GiB used 2.03GiB path /dev/mapper/rhel_hp--dl388eg8--01-testlv2
>         devid    2 size 15.00GiB used 2.01GiB path /dev/mapper/rhel_hp--dl388eg8--01-testlv3
> 
> Btrfs v3.14.2
> 
> And testlv1 was mounted at /mnt/btrfs
> 
> [root@hp-dl388eg8-01 btrfs-progs]# df -TP /mnt/btrfs
> Filesystem                                Type  1024-blocks  Used Available Capacity Mounted on
> /dev/mapper/rhel_hp--dl388eg8--01-testlv1 btrfs    15728640   512  13602560       1% /mnt/btrfs
> 
> Now run btrfsck on testlv2, btrfsck will scan all btrfs devices and
> somehow change the device name.
> 
> [root@hp-dl388eg8-01 btrfs-progs]# btrfsck /dev/mapper/rhel_hp--dl388eg8--01-testlv2 >/dev/null 2>&1
> 
> # device name changed in df output and btrfs fi show output
> [root@hp-dl388eg8-01 btrfs-progs]# df -TP /mnt/btrfs
> Filesystem     Type  1024-blocks  Used Available Capacity Mounted on
> /dev/dm-3      btrfs    15728640   512  13602560       1% /mnt/btrfs
> [root@hp-dl388eg8-01 btrfs-progs]# btrfs fi show
> Label: none  uuid: 1aba7da5-ce2b-4af0-a716-db732abc60b2
>         Total devices 1 FS bytes used 384.00KiB
>         devid    1 size 15.00GiB used 2.04GiB path /dev/dm-3
> 
> Label: none  uuid: 26ff4f12-f6d9-4cbc-aae2-57febeefde37
>         Total devices 2 FS bytes used 112.00KiB
>         devid    1 size 15.00GiB used 2.03GiB path /dev/mapper/rhel_hp--dl388eg8--01-testlv2
>         devid    2 size 15.00GiB used 2.01GiB path /dev/mapper/rhel_hp--dl388eg8--01-testlv3
> 
> Btrfs v3.14.2
> 
> This only happens when btrfsck a btrfs with multiple devices, so this
> only affects xfstests run on btrfs with SCRATCH_DEV_POOL set to lvm
> lvs.
> 
> Maybe this is a bug of btrfs-progs and we should fix it there?

Yes, that smells of a btrfs-progs bug. If your /etc/mtab a link to
/proc/mounts? If not, does the contents change when you run btrfsck,
and does the problem go away when you replace /etc/mtab with a link
to /proc/mounts?

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com