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

Jan Kara
2008-06-24T13:33:21Z

On Tue 24-06-08 20:52:59, 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.
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.
But this is not too serious since fsck has to be run anyway and it will
fix the problems.

> 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.
Well, the scenario has to be a bit different (if we need more space than
there is in the journal, we commit the running transaction, do checkpoint
and start a new transaction) but something like what you describe could
happen. But again I think that this is a correct behavior - i.e., discard
all the data in the running transaction when the journal is aborted before
the transaction is properly committed.

Honza
--
Jan Kara <jack@suse.cz>
SUSE Labs, CR