linux-ext4 - Re: [PATCH 2/2] jbd2: Avoid leaking transaction credits when unreserving handle

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Date:   Wed, 20 May 2020 15:27:37 +0200
From:   Jan Kara <jack@...e.cz>
To:     Andreas Dilger <adilger@...ger.ca>
Cc:     Jan Kara <jack@...e.cz>, Ted Tso <tytso@....edu>,
        linux-ext4@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH 2/2] jbd2: Avoid leaking transaction credits when
 unreserving handle

On Tue 19-05-20 18:44:25, Andreas Dilger wrote:
> On May 18, 2020, at 3:21 AM, Jan Kara <jack@...e.cz> wrote:
> > 
> > When reserved transaction handle is unused, we subtract its reserved
> > credits in __jbd2_journal_unreserve_handle() called from
> > jbd2_journal_stop(). However this function forgets to remove reserved
> > credits from transaction->t_outstanding_credits and thus the transaction
> > space that was reserved remains effectively leaked. The leaked
> > transaction space can be quite significant in some cases and leads to
> > unnecessarily small transactions and thus reducing throughput of the
> > journalling machinery. E.g. fsmark workload creating lots of 4k files
> > was observed to have about 20% lower throughput due to this when ext4 is
> > mounted with dioread_nolock mount option.
> > 
> > Subtract reserved credits from t_outstanding_credits as well.
> > 
> > CC: stable@...r.kernel.org
> > Fixes: 8f7d89f36829 ("jbd2: transaction reservation support")
> > Signed-off-by: Jan Kara <jack@...e.cz>
> 
> Patch looks reasonable, with one minor nit below.
> 
> Reviewed-by: Andreas Dilger <adilger@...ger.ca>

Thanks!


> > @@ -721,8 +728,10 @@ static void stop_this_handle(handle_t *handle)
> > 	}
> > 	atomic_sub(handle->h_total_credits,
> > 		   &transaction->t_outstanding_credits);
> > -	if (handle->h_rsv_handle)
> > -		__jbd2_journal_unreserve_handle(handle->h_rsv_handle);
> > +	if (handle->h_rsv_handle) {
> > +		__jbd2_journal_unreserve_handle(handle->h_rsv_handle,
> > +						transaction);
> > +	}
> 
> There isn't any need for braces {} around this one-line if-block.

Yeah, I'm always undecided about this. I agree that in this case the code
wouldn't be any less readable without the braces. So I'll remove them and
resend with your tag.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR