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

[Jan Kara]

  Hi,

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.
  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...

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