Re: + jbd-fix-error-handling-for-checkpoint-io.patch added to -mm tree

[Jan Kara <jack@suse.cz>]

  Hello,

On Thu 21-08-08 19:09:27, Hidehiro Kawai wrote:
> > The patch titled
> >      jbd: fix error handling for checkpoint io
> > has been added to the -mm tree.  Its filename is
> >      jbd-fix-error-handling-for-checkpoint-io.patch
> 
> [snip]
> 
> > Subject: jbd: fix error handling for checkpoint io
> > From: Hidehiro Kawai <hidehiro.kawai.ez@hitachi.com>
> > 
> > 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
> > 4. when checkpointing fails, notify this error to the ext3 layer so
> >    that ext3 don't clear the needs_recovery flag, otherwise the
> >    journaled contents are ignored and cleaned in the recovery phase
> > 5. if the recovery fails, keep the needs_recovery flag
> 
> > 6. prevent cleanup_journal_tail() from being called between
> >    __journal_drop_transaction() and journal_abort() (a race issue
> >    between journal_flush() and __log_wait_for_space()
> 
> When I read the source code again, I noticed the race condition described
> in 6 doesn't happen.  I've thought journal_flush() can invoke
> log_do_checkpoint() while __log_wait_for_space() is invoking
> log_do_checkpoint(), but it would be wrong.
> 
> First journal_flush() invokes __log_start_commit() and log_wait_commit()
> pair.  After this, there is no running transaction and no starting handle.
> New handles are also not created because j_barrier_count blocks it.
> Thus, when journal_flush() invokes log_do_checkpoint(), there is
> no other process which invokes __log_wait_for_space() and
> log_do_checkpoint() to get free log space.  So invocations of
> log_do_checkpoint() are always isolated, the race condition doesn't
> happen.
  I'm not quite following you. j_barrier_count is increased only in
journal_lock_updates(). Noone is forced to first call
journal_lock_updates() and only after that journal_flush() (although
usually it is done that way). So I think taking the j_checkpoint_mutex in
journal_flush() is really a good thing to do.

> If my understanding is correct, adding mutex_lock() around
> log_do_checkpoint() (see bellow) is unneeded.
> 
> What do you think about this?
> 
> [snip]
> > @@ -1359,10 +1369,16 @@ int journal_flush(journal_t *journal)
> >  	spin_lock(&journal->j_list_lock);
> >  	while (!err && journal->j_checkpoint_transactions != NULL) {
> >  		spin_unlock(&journal->j_list_lock);
> > +		mutex_lock(&journal->j_checkpoint_mutex);
> >  		err = log_do_checkpoint(journal);
> > +		mutex_unlock(&journal->j_checkpoint_mutex);
> >  		spin_lock(&journal->j_list_lock);

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