Re: Recovery of BTRFS critical (device md126): corrupt leaf, bad key order: block=10872141938688, root=1, slot=119

["alex.challis" <alex.challis@btinternet.com>]

Thank you for the advice Hugo, have now replaced the RAM.

Would one of the devs be able to help with custom patch to btrfs check 
to fix it please?

Many thanks!

Cheers
Alex.



------ Original Message ------
From: "Hugo Mills" <hugo@carfax.org.uk>
To: "alex.challis" <alex.challis@btinternet.com>
Cc: linux-btrfs@vger.kernel.org
Sent: Thursday, 28 Apr, 2022 At 15:22
Subject: Re: Recovery of BTRFS critical (device md126): corrupt  leaf, 
bad key order: block=10872141938688, root=1, slot=119

On Thu, Apr 28, 2022 at 02:54:09PM +0100, alex.challis wrote:

Dear BTRFS Team



Have a NetGear ReadyNas that uses brtfs for the data volume

(/dev/disk/by-label/33eaff11\:HDD1).



Was attempting to stop a running container (Docker CE) around the time 
the

failure happened. Had just docker pulled a new version of container. Not

100% sure they were related but NAS dropped data volume into RO mode 
around

the time of stopping the container. Subsequent attempts to docker rm the

container failed with read-only file system errors. Upon re-boot the 
data

volume would no longer mount.



  uname -a:

Linux fatterboy 4.4.218.x86_64.1 #1 SMP Sun Nov 7 15:20:05 UTC 2021 
x86_64

GNU/Linux



   btrfs --version:

btrfs-progs v4.16



   btrfs fi show:

Label: '33eaff11:root'  uuid: e360cd8a-7496-4714-a0b7-dadb4829e6f5

         Total devices 1 FS bytes used 993.29MiB

         devid    1 size 4.00GiB used 2.45GiB path /dev/md0



Label: '33eaff11:HDD1'  uuid: 9dbd11f2-da2f-4f68-a4e9-552cbc90d1e0

         Total devices 2 FS bytes used 4.25TiB

         devid    1 size 5.44TiB used 4.41TiB path /dev/md126

         devid    2 size 461.13GiB used 7.03GiB path /dev/md127



   btrfs fi df /HDD1 :

Data, single: total=2.04GiB, used=979.09MiB

System, DUP: total=8.00MiB, used=16.00KiB

Metadata, DUP: total=204.56MiB, used=14.19MiB

GlobalReserve, single: total=16.00MiB, used=0.00B



   dmesg > dmesg.log

Attached





Culprit seems to be:

  dmesg | grep -i btrfs

[    1.337264] Btrfs loaded, crc32c=crc32c-generic

[   23.296969] BTRFS: device label 33eaff11:root devid 1 transid 2341967

/dev/md0

[   23.297437] BTRFS info (device md0): has skinny extents

[   24.505292] BTRFS: device label 33eaff11:HDD1 devid 2 transid 1424350

/dev/md127

[   24.643613] BTRFS: device label 33eaff11:HDD1 devid 1 transid 1424350

/dev/md126

[   24.800256] BTRFS info (device md126): has skinny extents

[   24.894582] BTRFS critical (device md126): corrupt leaf, bad key 
order:

block=10872141938688, root=1, slot=119

[   24.894596] BTRFS error (device md126): failed to read block groups: 
-5

[   24.894811] BTRFS error (device md126): failed to read block groups: 
-17

[   24.898074] BTRFS error (device md126): failed to read block groups: 
-17

[   24.912298] BTRFS error (device md126): failed to read block groups: 
-17

[   24.912851] BTRFS error (device md126): parent transid verify failed 
on

10872188272640 wanted 1424347 found 1424349

[   24.912857] BTRFS warning (device md126): failed to read tree root

[   24.933058] BTRFS error (device md126): open_ctree failed



  btrfs-debug-tree -b 10872141938688 /dev/disk/by-label/33eaff11\:HDD1

<clip>

         item 117 key (1127493074944 METADATA_ITEM 0) itemoff 27954 
itemsize

33

                 refs 1 gen 23101 flags TREE_BLOCK

                 tree block skinny level 0

                 tree block backref root 7

         item 118 key (1127493107712 METADATA_ITEM 0) itemoff 27894 
itemsize

60

                 refs 4 gen 718838 flags TREE_BLOCK|FULL_BACKREF

                 tree block skinny level 0

                 shared block backref parent 4593432821760

                 shared block backref parent 4593432788992

                 shared block backref parent 4593432756224

                 shared block backref parent 4593432723456

         item 119 key (2211708928 UNKNOWN.0 0) itemoff 27834 itemsize 60

         item 120 key (1127493173248 METADATA_ITEM 0) itemoff 27801 
itemsize

33

                 refs 1 gen 29828 flags TREE_BLOCK

                 tree block skinny level 0

                 tree block backref root 7

<clip>



Key 119 is out of sequence and type UNKNOWN (!?)



The first elements of the key tuples for 118-120 are:



0x10683d38000

0x00083d40000

0x10683d48000



    This, along with the UNKNOWN.0, suggests that something has written

a very small number of zero bytes into the metadata page while it was

in RAM (probably 4 or 8 bytes, as nothing else seems to be damaged).



    It's definitely happened in RAM, as the checksum is correct. We'd

have had a csum failure if the corruption happened on disk.



    This is an indication either of a broken driver that's done some

bad pointer arithmetic and stomped on memory that it doesn't own, or

(more likely, in my opinion) some bad RAM that's flipped a bit on an

address held in kernel memory somewhere, and led something to zero the

wrong area of RAM.



Please advise on recovery please?



    I don't think there's anything in btrfs check that could fix this

(although I might be wrong). Your first task, though, should be to try

to identify and replace the broken RAM on this machine. Once that's

done, one of the devs may be able to help you with a custom patch to

btrfs check to fix it -- but don't do that until the hardware's

repaired.



    Hugo.





-- 

Hugo Mills             | I spent most of my money on drink, women and 
fast

hugo@... carfax.org.uk | cars. The rest I wasted.

http://carfax.org.uk/  |

PGP: E2AB1DE4          | 
James Hunt