How to fix btrfs extent issue

Question

I think I have been hit by a known bug in btrfs:

https://www.spinics.net/lists/linux-btrfs/msg60984.html

Certainly, the error messages seem similar. If it is the same issue, it seems somewhat unfortunate that the fix for a 2 year old issue hasn't been ported into the V4.9 stable kernel (which is what Debian 9 uses)

I am now in the situation where one of the filesystem inodes has extent problems (as reported by btrfs check):

root 257 inode 2607184 errors 100, file extent discount
Found file extent holes:
       start: 0, len: 81920

It seems there isn't much advice or documentation on how to get out of such a situation (fortunately I do have backups of the filesystem concerned, so worst case could reformat and restore)

It seems btrfs check --repair just loops printing the same error over and over, without actually fixing it.

Is there any way to fix the existing filesystem, or am I best to recreate it and restore the backup?

Answer 1

I think root 257 refers to the subvolume ID, then inode 2607184 points to the problematic inode. I would try to remove (unlink) every path linked to the inode.

1. Mount the subvolume:

   mount /dev/sdXN -o subvolid=257 /mnt/mountpoint
   

2. Locate every entry with matching inode number:

   find /mnt/mountpoint -xdev -inum 2607184
   

3. Investigate the objects. Hopefully you can afford to delete them.

   • (I'm not sure if it may be a directory in your case). If it's a directory, I suspect its listing may be incomplete.
     1. Move its contents (if any) to another, new directory (create it with the same ownership, permissions maybe); remove the old directory; mv the new directory to the old name.
     2. Compare to your backup, restore missing objects.
   • If it's one or more files –
     1. Remove them all.
     2. Restore the files from backup.

4. Unmount:

   umount /mnt/mountpoint
   

5. Check the filesystem. The problematic inode should be no more.

---

Alternatively you can delete the entire subvolume. This seems like overkill, yet it should get rid of the problematic inode.

1. Mount the root of the filesystem:

   mount /dev/sdXN -o subvol=/ /mnt/mountpoint
   

2. List subvolumes:

   btrfs subvolume list /mnt/mountpoint
   

   and locate the one with ID 257.

3. Delete the subvolume:

   btrfs subvolume delete -c /mnt/mountpoint/path/to/the/subvolume/with/ID/257
   

4. Unmount:

   umount /mnt/mountpoint
   

5. Check the filesystem. The problematic inode should be no more.

6. Restore your data from backup.