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

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Date:   Tue, 19 May 2020 18:44:25 -0600
From:   Andreas Dilger <adilger@...ger.ca>
To:     Jan Kara <jack@...e.cz>
Cc:     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 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>

> ---
> fs/jbd2/transaction.c | 17 +++++++++++++----
> 1 file changed, 13 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/jbd2/transaction.c b/fs/jbd2/transaction.c
> index 3dccc23cf010..b49a103cff1f 100644
> --- a/fs/jbd2/transaction.c
> +++ b/fs/jbd2/transaction.c
> @@ -541,17 +541,24 @@ handle_t *jbd2_journal_start(journal_t *journal, int nblocks)
> }
> EXPORT_SYMBOL(jbd2_journal_start);
> 
> -static void __jbd2_journal_unreserve_handle(handle_t *handle)
> +static void __jbd2_journal_unreserve_handle(handle_t *handle, transaction_t *t)
> {
> 	journal_t *journal = handle->h_journal;
> 
> 	WARN_ON(!handle->h_reserved);
> 	sub_reserved_credits(journal, handle->h_total_credits);
> +	if (t)
> +		atomic_sub(handle->h_total_credits, &t->t_outstanding_credits);
> }
> 
> void jbd2_journal_free_reserved(handle_t *handle)
> {
> -	__jbd2_journal_unreserve_handle(handle);
> +	journal_t *journal = handle->h_journal;
> +
> +	/* Get j_state_lock to pin running transaction if it exists */
> +	read_lock(&journal->j_state_lock);
> +	__jbd2_journal_unreserve_handle(handle, journal->j_running_transaction);
> +	read_unlock(&journal->j_state_lock);
> 	jbd2_free_handle(handle);
> }
> EXPORT_SYMBOL(jbd2_journal_free_reserved);
> @@ -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.


Cheers, Andreas






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