Re: "bad tree block start" when trying to mount on ARM

[Hugo Mills <hugo@carfax.org.uk>]

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On Tue, May 21, 2019 at 01:34:42AM -0700, Erik Jensen wrote:
> I have a 5-drive btrfs filesystem. (raid-5 data, dup metadata). I can
> mount it fine on my x86_64 system, and running `btrfs check` there
> reveals no errors. However, I am not able to mount the filesystem on
> my 32-bit ARM board, which I am hoping to use for lower-power file
> serving. dmesg shows the following:
> 
> [   83.066301] BTRFS info (device dm-3): disk space caching is enabled
> [   83.072817] BTRFS info (device dm-3): has skinny extents
> [   83.553973] BTRFS error (device dm-3): bad tree block start, want
> 17628726968320 have 396461950000496896
> [   83.554089] BTRFS error (device dm-3): bad tree block start, want
> 17628727001088 have 5606876608493751477
> [   83.601176] BTRFS error (device dm-3): bad tree block start, want
> 17628727001088 have 5606876608493751477
> [   83.610811] BTRFS error (device dm-3): failed to verify dev extents
> against chunks: -5
> [   83.639058] BTRFS error (device dm-3): open_ctree failed
> 
> Is this expected to work? I did notice that there are gotchas on the
> wiki related to filesystems over 8TiB on 32-bit systems, but it
> sounded like they were mostly related to running the tools, as opposed
> to the filesystem driver itself. (Each of the five drives is
> 8TB/7.28TiB)

   Yes, it should work. We had problems with ARM several years ago,
because of its unusual behaviour with unaligned word accesses, but
those were in userspace, and, as far as I know, fixed now. Looking at
the want/have numbers, it doesn't look like an endianness problem or
an ARM-unaligned-access problem.

> If this isn't expected, what should I do to help track down the issue?

   Can you show us the output of "btrfs check --readonly", on both the
x86_64 machine and the ARM machine? It might give some more insight
into the nature of the breakage.

   Possibly also "btrfs inspect dump-super" on both machines.

> Also potentially relevant: The x86_64 system is currently running
> 4.19.27, while the ARM system is running 5.1.3.

   Shouldn't make a difference.

> Finally, just in case it's relevant, I just finished reencrypting the
> array, which involved doing a `btrfs replace` on each device in the
> array.

   If you can still mount on x86_64, then the FS is at least
reasonably complete and undamaged. I don't think this will make a
difference.  However, it's worth checking whether there are any
funnies about your encryption layer on ARM (I wouldn't expect any,
since it's recognising the decrypted device as btrfs, rather than
random crud).

   Hugo.

-- 
Hugo Mills             | Prisoner unknown: Return to Zenda.
hugo@... carfax.org.uk |
http://carfax.org.uk/  |
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