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

Hidehiro Kawai
2008-06-30T05:09:45Z

Hi,

Jan Kara wrote:

> On Fri 27-06-08 17:06:56, Hidehiro Kawai wrote:
>
>>Jan Kara wrote:
>>
>>
>>>On Tue 24-06-08 20:52:59, Hidehiro Kawai wrote:
>>
>>>>>>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.
>>>
>>> Yes, and this is expected an correct. When we cannot properly finish a
>>>transaction, we have to discard everything in it. A bug would be (and I
>>>think it could currently happen) if we already checkpointed the previous
>>>transaction and then written over block_B new data from the uncommitted
>>>transaction. I think we have to avoid that - i.e., in case we abort the
>>>journal we should not mark buffers dirty when processing the forget loop.
>>
>>Yes.
>>
>>
>>>But this is not too serious since fsck has to be run anyway and it will
>>>fix the problems.
>>
>>Yes. The filesystem should be marked with an error, so fsck will check
>>and recover the filesystem on boot. But this means the filesystem loses
>>some latest updates even if it was cleanly unmounted (although some file
>>data has been lost.) I'm a bit afraid that some people would think of
>>this as a regression due to this PATCH 4/5. At least, to avoid
>>undesirable replay, we had better keep journaled data only when the
>>journal has been aborted for checkpointing related reason.
>
> I don't think this makes any difference. Look: We have transaction A
> modifying block B fully committed to the journal. Now there is a running
> (or committing, it does not really matter) transaction R also modifying block
> B. Until R gets fully committed, no block modified by R is checkpointed
> to the device - checkpointing code takes care of that and it must be so
> to satisfy journaling guarantees.
> So if we abort journal (for whatever reason) before R is fully committed,
> no change in R will be seen on the filesystem regardless whether you
> cleanup the journal or not.

No, changes in R will be seen on the filesystem.
The metadata buffer for block B is marked as dirty when it is unfiled
whether the journal has aborted or not. Eventually the buffer will be
written-back to the filesystem by pdflush. Actually I have confirmed
this behavior by using SystemTap. So if both journal abort and
system crash happen at the same time, the filesystem would become
inconsistent state. As I stated, replaying the journaled block B in
transaction A may also corrupt the filesystem, because changes in
transaction R are reflected halfway. That is why I sent a patch to
prevent metadata buffers from being dirtied on abort.

> And you cannot do much differenly - the principle of journaling is that
> either all changes in the transaction get to disk or none of them. So until
> the transaction is properly committed, you have the only way to satisfy
> this condition - take the "none of them" choice.
> Now fsck could of course be clever and try to use updates in not fully
> committed transaction but personally I don't think it's worth the effort.
> Please correct me if I just misunderstood your point...

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