Re: [PATCH] jbd: fix assertion failure in journal_next_log_block

[Josef Bacik]

On Tuesday 05 February 2008 12:27:31 pm Jan Kara wrote:
>   Hello,
>
>   Sorry for replying a bit late but I'm currently falling behind in
> maling-list reading...
>
> > The way jbd tries to determine if there is enough space left on the
> > journal in order to start a new transaction is looking at the space left
> > in the journal and the space needed for the committing transaction, which
> > is 1/4 of the journal + the number of t_outstanding_credits for that
> > transaction.  In this case its assumed that t_outstanding_credits
> > accurately represents the number of credits
>
>   Yes.
>
> > waiting to be written to the journal, but this sometimes isn't the case. 
> > The transaction has two counters for buffers, t_outstanding_credits which
> > is used in conjunction with handles that are added to the transaction,
> > and t_nr_buffers which is only incremented/decremented when buffers are
> > added/removed from the transaction and are actually destined to be
> > journaled.  Normally these two
>
>   t_nr_buffers actually represents number of buffers on BJ_Metadata list
> and nobody uses it (except for the assertion in
> __journal_temp_unlink_buffer()). t_outstanding_credits is supposed to be
> *the* counter making sure we don't write more than we have credits for.
>
> > counters are the same, however there are cases where the committing
> > transaction can have buffers moved to the next running transaction, for
> > example any buffers on the committing transactions t_reserved list would
> > be moved to the next (running) transaction, and if it had been dirtied in
> > the process it would immediately make it onto the t_updates list, which
> > would increment t_nr_buffers
>
>   You probably mean t_buffers list here...
>
> > but not t_outstanding_credits.  So you get into this situation where
>
>   But which moving and dirtying do you mean? The caller which dirties
> the buffer must make sure that he has acquired enough credits for the
> transaction where the buffer ends up... So if there were not enough
> buffers in the running transaction where we refiled the buffer it is a
> bug in the caller which dirties the buffer.
>

You know now that you say that I feel like an idiot, you are right the only way 
for something to actually end up on that list was if somebody dirtied it and if 
they did it would have had to been accounted for at some point on the running 
transaction.

> > t_nr_buffers (the actual number of buffers that are on the transaction)
> > is greater than the number of buffers accounted for via
> > t_outstanding_credits. This presents a problem since as we loop through
> > writting buffers to the journal, we decrement t_outstanding_credits, and
> > if t_nr_buffers is more than t_outstanding_credits then we end up with a
> > negative number for
> > t_outstanding_credits, which means we start saying we need less than 1/4
> > of the journal for our committing transaction and allow more transactions
> > than we can handle to start, and then bam we fail because
> > journal_next_log_block doesn't have enough free blocks in order to handle
> > the request.  This has been tested and fixes the issue (which could not
> > be reproduced by me but several other people could get it to reproduce
> > using postmark), and although I couldn't reproduce the assertion, I could
> > very easily reproduce the situation where t_outstanding_credits was <
> > than t_nr_buffers.
>
>   I suppose you see the assertion J_ASSERT(journal->j_free > 1); to
> fail, right? I don't see how your patch could help avoid that assertion.
> You've just removed accounting of t_outstanding_credits which has no
> impact on the real number of free blocks in the journal stored in
> j_free. Anyway, if you can reproduce t_outstanding_credits <
> t_nr_buffers, then there's something fishy. Are you able to reproduce it
> also with a current kernel?
>   Thanks for looking into the problem :)
>

Well my patch helped avoid the assertion because t_outstanding_credits was going 
negative therefore we were letting transactions start when we shouldn't be, and 
eventually we would end up with too much of the journal in use and we'd assert.  
Course I can't reproduce where t_outstanding_credits < t_nr_buffers upstream 
(again I feel like an idiot, should have tested that first).  Thanks for 
looking at this Jan.

Mingming, would you mind pulling this patch out of the patch queue please since 
its wrong?  Thanks much,

Josef
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