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

[Erik Jensen <erikjensen@rkjnsn.net>]

The offending system is indeed ARMv7 (specifically a Marvell ARMADA®
388), but I believe the Broadcom BCM2835 in my Raspberry Pi is
actually ARMv6 (with hardware float support).

On Mon, Jan 18, 2021 at 4:01 AM Qu Wenruo <quwenruo.btrfs@gmx.com> wrote:
>
>
>
> On 2021/1/18 下午7:55, Erik Jensen wrote:
> > On Mon, Jan 18, 2021 at 3:07 AM Qu Wenruo <quwenruo.btrfs@gmx.com> wrote:
> >> On 2021/1/18 下午6:33, Erik Jensen wrote:
> >>> I ended up having other priorities occupying my time since 2019, and the
> >>> "solution" of exporting the individual drives on my NAS using NBD and
> >>> mounting them on my desktop worked, even if it wasn't pretty.
> >>>
> >>> However, I am currently looking into Syncthing, which I would like to
> >>> run on the NAS directly. That would, of course, require accessing the
> >>> filesystem directly on the NAS rather than just exporting the raw
> >>> devices, which means circling back to this issue.
> >>>
> >>> After updating my NAS, I have determined that the issue still occurs
> >>> with Linux 5.8.
> >>>
> >>> What's the next best step for debugging the issue? Ideally, I'd like to
> >>> help track down the issue to find a proper fix, rather than just trying
> >>> to bypass the issue. I wasn't sure if the suggestion to comment out
> >>> btrfs_verify_dev_extents() was more geared toward the former or the latter.
> >>
> >> After rewinding my memory on this case, the problem is really that the
> >> ARM btrfs kernel is reading garbage, while X86 or ARM user space tool
> >> works as expected.
> >>
> >> Can you recompile your kernel on the ARM board to add extra debugging
> >> messages?
> >> If possible, we can try to add some extra debug points to bombarding
> >> your dmesg.
> >>
> >> Or do you have other ARM boards to test the same fs?
> >>
> >>
> >> Thanks,
> >> Qu
> >
> > It's pretty easy to build a kernel with custom patches applied, though
> > the actual building takes a while, so I'd be happy to add whatever
> > debug messages would be useful. I also have an old Raspberry Pi
> > (original model B) I can dig out and try to get going, tomorrow. I
> > can't hook it up to the drives directly, but I should be able to
> > access them via NBD like I was doing from my desktop.
>
> RPI 1B would be a little slow but should be enough to expose the
> problem, if the problem is for all arm builds (as long as you're also
> using armv7 for the offending system).
>
> Thanks,
> Qu
>
> > If I can't get
> > that going for whatever reason, I could also try running an emulated
> > ARM system with QEMU.
> >
> >>>
> >>> On Fri, Jun 28, 2019 at 1:15 AM Qu Wenruo <quwenruo.btrfs@gmx.com
> >>> <mailto:quwenruo.btrfs@gmx.com>> wrote:
> >>>
> >>>
> >>>
> >>>      On 2019/6/28 下午4:00, Erik Jensen wrote:
> >>>       >> So it's either the block layer reading some wrong from the disk
> >>>      or btrfs
> >>>       >> layer doesn't do correct endian convert.
> >>>       >
> >>>       > My ARM board is running in little endian mode, so it doesn't seem
> >>>      like
> >>>       > endianness should be an issue. (It is 32-bits versus my desktop's 64,
> >>>       > though.) I've also tried exporting the drives via NBD to my x86_64
> >>>       > system, and that worked fine, so if the problem is under btrfs, it
> >>>       > would have to be in the encryption layer, but fsck succeeding on the
> >>>       > ARM board would seem to rule that out, as well.
> >>>       >
> >>>       >> Would you dump the following data (X86 and ARM should output the
> >>>      same
> >>>       >> content, thus one output is enough).
> >>>       >> # btrfs ins dump-tree -b 17628726968320 /dev/dm-3
> >>>       >> # btrfs ins dump-tree -b 17628727001088 /dev/dm-3
> >>>       >
> >>>       > Attached, and also 17628705964032, since that's the block
> >>>      mentioned in
> >>>       > my most recent mount attempt (see below).
> >>>
> >>>      The trees are completely fine.
> >>>
> >>>      So it should be something else causing the problem.
> >>>
> >>>       >
> >>>       >> And then, for the ARM system, please apply the following diff,
> >>>      and try
> >>>       >> mount again.
> >>>       >> The diff adds extra debug info, to exam the vital members of a
> >>>      tree block.
> >>>       >>
> >>>       >> Correct fs should output something like:
> >>>       >>   BTRFS error (device dm-4): bad tree block start, want 30408704
> >>>      have 0
> >>>       >>   tree block gen=4 owner=5 nritems=2 level=0
> >>>       >>   csum:
> >>>       >>
> >>>      a304e483-0000-0000-0000-00000000000000000000-0000-0000-0000-000000000000
> >>>       >>
> >>>       >> The csum one is the most important one, if there aren't so many
> >>>      zeros,
> >>>       >> it means at that timing, btrfs just got a bunch of garbage, thus we
> >>>       >> could do further debug.
> >>>       >
> >>>       > [  131.725573] BTRFS info (device dm-1): disk space caching is
> >>>      enabled
> >>>       > [  131.731884] BTRFS info (device dm-1): has skinny extents
> >>>       > [  133.046145] BTRFS error (device dm-1): bad tree block start, want
> >>>       > 17628705964032 have 2807793151171243621
> >>>       > [  133.055775] tree block gen=7888986126946982446
> >>>       > owner=11331573954727661546 nritems=4191910623 level=112
> >>>       > [  133.065661] csum:
> >>>       >
> >>>      416a456c-1e68-dbc3-185d-aaad410beaef5493ab3f-3cb9-4ba1-2214-b41cba9656fc
> >>>
> >>>      Completely garbage here, so I'd say the data we got isn't what we want.
> >>>
> >>>       > [  133.108383] BTRFS error (device dm-1): bad tree block start, want
> >>>       > 17628705964032 have 2807793151171243621
> >>>       > [  133.117999] tree block gen=7888986126946982446
> >>>       > owner=11331573954727661546 nritems=4191910623 level=112
> >>>       > [  133.127756] csum:
> >>>       >
> >>>      416a456c-1e68-dbc3-185d-aaad410beaef5493ab3f-3cb9-4ba1-2214-b41cba9656fc
> >>>
> >>>      But strangely, the 2nd try still gives us the same result, if it's
> >>>      really some garbage, we should get some different result.
> >>>
> >>>       > [  133.136241] BTRFS error (device dm-1): failed to verify dev
> >>>      extents
> >>>       > against chunks: -5
> >>>
> >>>      You can try to skip the dev extents verification by commenting out the
> >>>      btrfs_verify_dev_extents() call in disk-io.c::open_ctree().
> >>>
> >>>      It may fail at another location though.
> >>>
> >>>      The more strange part is, we have the device tree root node read out
> >>>      without problem.
> >>>
> >>>      Thanks,
> >>>      Qu
> >>>
> >>>       > [  133.166165] BTRFS error (device dm-1): open_ctree failed
> >>>       >
> >>>       > I copied some files over last time I had it mounted on my desktop,
> >>>       > which may be why it's now failing at a different block.
> >>>       >
> >>>       > Thanks!
> >>>       >
> >>>