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

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Date:   Thu, 28 May 2020 22:44:01 -0400
From:   "Theodore Y. Ts'o" <tytso@....edu>
To:     Jan Kara <jack@...e.cz>
Cc:     linux-ext4@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH 2/2] jbd2: Avoid leaking transaction credits when
 unreserving handle

On Wed, May 20, 2020 at 03:31:19PM +0200, Jan Kara 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")
> Reviewed-by: Andreas Dilger <adilger@...ger.ca>
> Signed-off-by: Jan Kara <jack@...e.cz>

Thanks, applied.

					- Ted