Re: [PATCH 2/3] btrfs: include non-missing as a qualifier for the latest_bdev

[David Sterba <dsterba@suse.cz>]

On Tue, Apr 28, 2020 at 11:22:26PM +0800, Anand Jain wrote:
> btrfs_free_extra_devids() reorgs fs_devices::latest_bdev
> to point to the bdev with greatest device::generation number.
> For a typical-missing device the generation number is zero so
> fs_devices::latest_bdev will never point to it.
> 
> But if the missing device is due to alienation [1], then
> device::generation is not-zero and if it is >= to rest of
> device::generation in the list, then fs_devices::latest_bdev
> ends up pointing to the missing device and reports the error
> like this [2]
> 
> [1] We maintain devices of a fsid (as in fs_device::fsid) in the
> fs_devices::devices list, a device is considered as an alien device
> if its fsid does not match with the fs_device::fsid
> 
> $ mkfs.btrfs -fq /dev/sdd && mount /dev/sdd /btrfs

Please put each command on one line for clarity

> $ mkfs.btrfs -fq -draid1 -mraid1 /dev/sdb /dev/sdc
> $ sleep 3 # avoid racing with udev's useless scans if needed
> $ btrfs dev add -f /dev/sdb /btrfs
> $ mount -o degraded /dev/sdc /btrfs1

So the cause is a second mkfs on some devices, but is the degraded mount
supposed to work? The example goes:

- create first filesystem with device A
- create second filesystem with device B and C
- add device B to the first filesystem, effectively making it missing
- mount first filesystem, degraded because of the missing device

For a reproducer that's ok, but is this something that we can expect to
happen in practice? The flag -f should prevent accidental overwrite, but
yes the kernel code needs to deal with that in any case.