Re: kernel BUG at fs/btrfs/extent-tree.c:5038 (linux 3.4.7)

[Stefan Behrens <sbehrens@giantdisaster.de>]

On Wed, 8 Aug 2012 16:45:57 +0200, David Sterba wrote:
> On Sun, Aug 05, 2012 at 04:11:47PM +0200, Olivier Bonvalet wrote:
>> Aug  5 16:10:12 backup2 kernel: [   58.674758] parent transid verify failed on 615015833600 wanted 110423 found 110424

1st mirror fails verify_parent_transid().

>> Aug  5 16:10:12 backup2 kernel: [   58.675090] parent transid verify failed on 615015833600 wanted 110423 found 110424

2nd mirror fails verify_parent_transid().

>> Aug  5 16:10:12 backup2 kernel: [   58.675523] btrfs read error corrected: ino 1 off 615015833600 (dev /dev/mapper/vg--backupplug-backup sector 1209083504)

That's a bug. It is wrong to ignore the previous results from
verify_parent_transid() and to call repair_eb_io_failure() which
rewrites one mirror and claims to have corrected an error. But it's not
a major issue, just a misleading message in the kernel log and a disk
write operation which does not repair anything.

> This looks strange, the the corrupted block belongs to metadata, I
> assume you have the DUP profile, so there is a good copy that can be
> used instead, the error message confirms that, but ...
> 
>> Aug  5 16:10:12 backup2 kernel: [   58.675536] Failed to read block groups: -5

That's correct, because the UPTODATE flag in the extent is not set
(verify_parent_transid() clears it when it detects an error).

> ... ? -5 means EIO, which is returned when a block cannot be read, so
> unless there's a different reason for it, this looks like a missed
> oportunity to fix an error and continue.
> 
> The same error messages are present in the logs from 3.4 version.
> 
>> Aug  5 16:10:12 backup2 kernel: [   58.704720] btrfs: open_ctree failed

The summary is that the block was not correctable, both mirrors had the
same old transid. The bug is that the call to repair_io_failure() should
not have been done because verify_parent_transid() indicated errors.

I'll prepare a patch for it. Changing btree_read_extent_buffer_pages()
to set ret to -EIO if verify_parent_transid() fails should fix the issue.