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

[Jan Kara]

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...

> 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?

								Honza

-- 
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR
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