Need help mounting laptop corrupted root btrfs. Kernel BUG at fs/btrfs/volumes.c:3707

[Marc MERLIN <marc@merlins.org>]

Howdy,

I can wait a day or maybe 2 before I have to wipe and restore from backup.
Please let me know if you have a patch against 3.6.3 you'd like me to try
to mount/recover this filesystem, or whether you'd like me to try btrfsck.


My laptop had a problem with its boot drive which prevented linux
from writing to it, and in turn caused btrfs to have incomplete writes 
to it.
After reboot, the boot drive was fine, but the btrfs filesystem has
a corruption that prevents it from being mounted.

Unfortunately the mount crash prevents writing of crash data to even another
drive since linux stops before the crash data can be written to syslog.

Picture #1 shows a dump when my laptop crashed (before reboot).
btrfs no csum found for inode X start Y
http://marc.merlins.org/tmp/crash.jpg

Mounting with 3.5.0 and 3.6.3 gives the same error:

gandalfthegreat:~# mount -o recovery,skip_balance,ro /dev/mapper/bootdsk                                  

shows
btrfs: bdev /dev/mapper/bootdsk errs: wr 0, rd 0, flush 0, corrupt 1, gen 0
btrfs: bdev /dev/mapper/bootdsk errs: wr 0, rd 0, flush 0, corrupt 2, gen 0
(there are 2 lines, not sure why)

kernel BUG at fs/btrfs/volumes.c:3707
int btrfs_num_copies(struct btrfs_mapping_tree *map_tree, u64 logical, u64 len)
{
	struct extent_map *em;
	struct map_lookup *map;
	struct extent_map_tree *em_tree = &map_tree->map_tree;
	int ret;

	read_lock(&em_tree->lock);
	em = lookup_extent_mapping(em_tree, logical, len);
	read_unlock(&em_tree->lock);
	BUG_ON(!em);  <---

If the snapshot helps (sorry, hard to read, but usable):
http://marc.merlins.org/tmp/btrfs_bug.jpg

Questions:
1) Any better way to get a proper dump without serial console?
(I hate to give you pictures)

2) Should I try btrfsck now, or are there other mount options than
mount -o recovery,skip_balance,ro /dev/mapper/bootdsk 
I should try?

3) Want me to try btrfsck although it may make it impossible for me to
reproduce the bug and test a fix, as well as potentially break the filesystem
more (last time I tried btrfsck, it outputted thousands of lines and never converged
to a state it was happy with)

Thanks,
Marc
-- 
"A mouse is a device used to point at the xterm you want to type in" - A.S.R.
Microsoft is to operating systems ....
                                      .... what McDonalds is to gourmet cooking
Home page: http://marc.merlins.org/