Re: [PATCH 4/5] jbd: fix error handling for checkpoint io

Hidehiro Kawai
2008-06-24T11:53:29Z

Jan Kara wrote:

> On Mon 23-06-08 20:14:54, Hidehiro Kawai wrote:
>
>>Hi,
>>
>>I noticed a problem of this patch. Please see below.
>>
>>Jan Kara wrote:
>>
>>
>>>On Tue 03-06-08 13:40:25, Hidehiro Kawai wrote:
>>>
>>>
>>>>Subject: [PATCH 4/5] jbd: fix error handling for checkpoint io
>>>>
>>>>When a checkpointing IO fails, current JBD code doesn't check the
>>>>error and continue journaling. This means latest metadata can be
>>>>lost from both the journal and filesystem.
>>>>
>>>>This patch leaves the failed metadata blocks in the journal space
>>>>and aborts journaling in the case of log_do_checkpoint().
>>>>To achieve this, we need to do:
>>>>
>>>>1. don't remove the failed buffer from the checkpoint list where in
>>>> the case of __try_to_free_cp_buf() because it may be released or
>>>> overwritten by a later transaction
>>>>2. log_do_checkpoint() is the last chance, remove the failed buffer
>>>> from the checkpoint list and abort the journal
>>>>3. when checkpointing fails, don't update the journal super block to
>>>> prevent the journaled contents from being cleaned. For safety,
>>>> don't update j_tail and j_tail_sequence either
>>
>>3. is implemented as described below.
>> (1) if log_do_checkpoint() detects an I/O error during
>> checkpointing, it calls journal_abort() to abort the journal
>> (2) if the journal has aborted, don't update s_start and s_sequence
>> in the on-disk journal superblock
>>
>>So, if the journal aborts, journaled data will be replayed on the
>>next mount.
>>
>>Now, please remember that some dirty metadata buffers are written
>>back to the filesystem without journaling if the journal aborted.
>>We are happy if all dirty metadata buffers are written to the disk,
>>the integrity of the filesystem will be kept. However, replaying
>>the journaled data can overwrite the latest on-disk metadata blocks
>>partly with old data. It would break the filesystem.
>
> Yes, it would. But how do you think it can happen that a metadata buffer
> will be written back to the filesystem when it is a part of running
> transaction? Note that checkpointing code specifically checks whether the
> buffer being written back is part of a running transaction and if so, it
> waits for commit before writing back the buffer. So I don't think this can
> happen but maybe I miss something...

Checkpointing code checks it and may call log_wait_commit(), but this
problem is caused by transactions which have not started checkpointing.

For example, the tail transaction has an old update for block_B and
the running transaction has a new update for block_B. Then, the
committing transaction fails to write the commit record, it aborts the
journal, and new block_B will be written back to the file system without
journaling. Because this patch doesn't separate between normal abort
and checkpointing related abort, the tail transaction is left in the
journal space. So by replaying the tail transaction, new block_B is
overwritten with old one.

It can happen in the case of the checkpointing related abort.
For example, assuming the tail transaction has an update for block_A,
the next transaction has an old update for block_B, and the running
transaction has a new update for block_B.
Now, the running transaction needs more log space, and it calls
log_do_checkpoint(). But it aborts the journal because it detected
write error on block_A. In this case, new block_B will be
overwritten when the old block_B in the second transaction to the tail
is replayed.

Does this answer your question?

>>My idea to resolve this problem is that we don't write out metadata
>>buffers which belong to uncommitted transactions if journal has
>>aborted. Although the latest filesystem updates will be lost,
>>we can ensure the integrity. It will also be effective for the
>>kernel panic in the middle of writing metadata buffers without
>>journaling (this would occur in the `mount -o errors=panic' case.)
>>
>>Which integrity or latest state should we choose?

Thanks,
--
Hidehiro Kawai
Hitachi, Systems Development Laboratory
Linux Technology Center