Re: [PATCH 2/2] fix possible journal overflow issues

[Jan Kara]

On Wed 27-02-08 13:51:53, Josef Bacik wrote:
> Hello,
> 
> There are several cases where the running transaction can get buffers added to 
> its BJ_Metadata list which it never dirtied, which makes its t_nr_buffers 
> counter end up larger than its t_outstanding_credits counter.  This will cause 
> issues when starting new transactions as while we are logging buffers we 
> decrement t_outstanding_buffers, so when t_outstanding_buffers goes negative, 
> we will report that we need less space in the journal than we actually need, so 
> transactions will be started even though there may not be enough room for them.  
> In the worst case scenario (which admittedly is almost impossible to reproduce) 
> this will result in the journal running out of space.  The fix is to only 
> refile buffers from the committing transaction to the running transactions 
> BJ_Modified list when b_modified is set on that journal, which is the only way 
> to be sure if the running transaction has modified that buffer.  This patch 
> also fixes an accounting error in journal_forget, it is possible that we can 
> call journal_forget on a buffer without having modified it, only gotten write 
> access to it, so instead of freeing a credit, we only do so if the buffer was 
> modified.  The assert will help catch if this problem occurs.  Without these 
> two patches I could hit this assert within minutes of running postmark, with 
> them this issue no longer arises.  Thank you,
> 
> Signed-off-by: Josef Bacik <jbacik@xxxxxxxxxx>
  Acked-by: Jan Kara <jack@xxxxxxx>

									Honza
> 
> Index: linux-2.6/fs/jbd/transaction.c
> ===================================================================
> --- linux-2.6.orig/fs/jbd/transaction.c
> +++ linux-2.6/fs/jbd/transaction.c
> @@ -1234,6 +1234,7 @@ int journal_forget (handle_t *handle, st
>  	struct journal_head *jh;
>  	int drop_reserve = 0;
>  	int err = 0;
> +	int was_modified = 0;
>  
>  	BUFFER_TRACE(bh, "entry");
>  
> @@ -1252,6 +1253,9 @@ int journal_forget (handle_t *handle, st
>  		goto not_jbd;
>  	}
>  
> +	/* keep track of wether or not this transaction modified us */
> +	was_modified = jh->b_modified;
> +
>  	/*
>  	 * The buffer's going from the transaction, we must drop
>  	 * all references -bzzz
> @@ -1269,7 +1273,12 @@ int journal_forget (handle_t *handle, st
>  
>  		JBUFFER_TRACE(jh, "belongs to current transaction: unfile");
>  
> -		drop_reserve = 1;
> +		/*
> +		 * we only want to drop a reference if this transaction
> +		 * modified the buffer
> +		 */
> +		if (was_modified)
> +			drop_reserve = 1;
>  
>  		/*
>  		 * We are no longer going to journal this buffer.
> @@ -1309,7 +1318,13 @@ int journal_forget (handle_t *handle, st
>  		if (jh->b_next_transaction) {
>  			J_ASSERT(jh->b_next_transaction == transaction);
>  			jh->b_next_transaction = NULL;
> -			drop_reserve = 1;
> +
> +			/*
> +			 * only drop a reference if this transaction modified
> +			 * the buffer
> +			 */
> +			if (was_modified)
> +				drop_reserve = 1;
>  		}
>  	}
>  
> @@ -2081,7 +2096,7 @@ void __journal_refile_buffer(struct jour
>  	jh->b_transaction = jh->b_next_transaction;
>  	jh->b_next_transaction = NULL;
>  	__journal_file_buffer(jh, jh->b_transaction,
> -				was_dirty ? BJ_Metadata : BJ_Reserved);
> +				jh->b_modified ? BJ_Metadata : BJ_Reserved);
>  	J_ASSERT_JH(jh, jh->b_transaction->t_state == T_RUNNING);
>  
>  	if (was_dirty)
> Index: linux-2.6/fs/jbd2/transaction.c
> ===================================================================
> --- linux-2.6.orig/fs/jbd2/transaction.c
> +++ linux-2.6/fs/jbd2/transaction.c
> @@ -1243,6 +1243,7 @@ int jbd2_journal_forget (handle_t *handl
>  	struct journal_head *jh;
>  	int drop_reserve = 0;
>  	int err = 0;
> +	int was_modified = 0;
>  
>  	BUFFER_TRACE(bh, "entry");
>  
> @@ -1261,6 +1262,9 @@ int jbd2_journal_forget (handle_t *handl
>  		goto not_jbd;
>  	}
>  
> +	/* keep track of wether or not this transaction modified us */
> +	was_modified = jh->b_modified;
> +
>  	/*
>  	 * The buffer's going from the transaction, we must drop
>  	 * all references -bzzz
> @@ -1278,7 +1282,12 @@ int jbd2_journal_forget (handle_t *handl
>  
>  		JBUFFER_TRACE(jh, "belongs to current transaction: unfile");
>  
> -		drop_reserve = 1;
> +		/*
> +		 * we only want to drop a reference if this transaction
> +		 * modified the buffer
> +		 */
> +		if (was_modified)
> +			drop_reserve = 1;
>  
>  		/*
>  		 * We are no longer going to journal this buffer.
> @@ -1318,7 +1327,13 @@ int jbd2_journal_forget (handle_t *handl
>  		if (jh->b_next_transaction) {
>  			J_ASSERT(jh->b_next_transaction == transaction);
>  			jh->b_next_transaction = NULL;
> -			drop_reserve = 1;
> +
> +			/*
> +			 * only drop a reference if this transaction modified
> +			 * the buffer
> +			 */
> +			if (was_modified)
> +				drop_reserve = 1;
>  		}
>  	}
>  
> @@ -2090,7 +2105,7 @@ void __jbd2_journal_refile_buffer(struct
>  	jh->b_transaction = jh->b_next_transaction;
>  	jh->b_next_transaction = NULL;
>  	__jbd2_journal_file_buffer(jh, jh->b_transaction,
> -				was_dirty ? BJ_Metadata : BJ_Reserved);
> +				jh->b_modified ? BJ_Metadata : BJ_Reserved);
>  	J_ASSERT_JH(jh, jh->b_transaction->t_state == T_RUNNING);
>  
>  	if (was_dirty)
> Index: linux-2.6/fs/jbd/commit.c
> ===================================================================
> --- linux-2.6.orig/fs/jbd/commit.c
> +++ linux-2.6/fs/jbd/commit.c
> @@ -472,6 +472,9 @@ void journal_commit_transaction(journal_
>  	 */
>  	commit_transaction->t_state = T_COMMIT;
>  
> +	J_ASSERT(commit_transaction->t_nr_buffers <=
> +		 commit_transaction->t_outstanding_credits);
> +
>  	descriptor = NULL;
>  	bufs = 0;
>  	while (commit_transaction->t_buffers) {
> Index: linux-2.6/fs/jbd2/commit.c
> ===================================================================
> --- linux-2.6.orig/fs/jbd2/commit.c
> +++ linux-2.6/fs/jbd2/commit.c
> @@ -568,6 +568,9 @@ void jbd2_journal_commit_transaction(jou
>  	stats.u.run.rs_blocks = commit_transaction->t_outstanding_credits;
>  	stats.u.run.rs_blocks_logged = 0;
>  
> +	J_ASSERT(commit_transaction->t_nr_buffers <=
> +		 commit_transaction->t_outstanding_credits);
> +
>  	descriptor = NULL;
>  	bufs = 0;
>  	while (commit_transaction->t_buffers) {
-- 
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR
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