btrfs recovery

[Oliver Freyermuth <o.freyermuth@googlemail.com>]

Hi, 

I have just encountered on mount of one of my filesystems (after a clean reboot...): 
[  495.303313] BTRFS critical (device sdb1): corrupt node, bad key order: block=35028992, root=1, slot=243
[  495.315642] BTRFS critical (device sdb1): corrupt node, bad key order: block=35028992, root=1, slot=243
[  495.315694] BTRFS error (device sdb1): failed to read block groups: -5
[  495.327865] BTRFS error (device sdb1): open_ctree failed

The system is using a 4.9.0 kernel, and I have btrfs-progs 4.9 installed. 

Since the last backup is a few weeks old (but the data is not so crucial), I'd like to attempt to recover at least some of the files. 

btrfs check tells me:
# btrfs check /dev/sdb1
Checking filesystem on /dev/sdb1
UUID: cfd16c65-7f3b-4f5e-9029-971f2433d7ab
checking extents
bad block 35028992
ERROR: errors found in extent allocation tree or chunk allocation

IIRC, the FS has DUP metadata (but single DATA). It's on a classic spinning disk. 
I use: "space_cache,noatime,compress=lzo,commit=120" as mount options. 

What is the best way to go? 

Should I:
- reinit extent tree
- or collect debug info
- or is there a better way to go?

Cheers and thanks for any suggestions, 
	Oliver

PS: Please put my mail in CC, I'm not subscribed to the list. Thanks!