Re: [PATCH 3/4] ext4: fix incorrect tid assumption in jbd2_journal_shrink_checkpoint_list()

[Luis Henriques <luis.henriques@linux.dev>]

On Wed, Jul 24 2024, Jan Kara wrote:

> On Tue 23-07-24 16:44:01, Luis Henriques (SUSE) wrote:
>> Function jbd2_journal_shrink_checkpoint_list() assumes that '0' is not a
>> valid value for transaction IDs, which is incorrect.  Don't assume that and
>> use two extra boolean variables to control the loop iterations and keep
>> track of the first and last tid.
>> 
>> Signed-off-by: Luis Henriques (SUSE) <luis.henriques@linux.dev>
>> ---
>>  fs/jbd2/checkpoint.c | 8 ++++++--
>>  1 file changed, 6 insertions(+), 2 deletions(-)
>> 
>> diff --git a/fs/jbd2/checkpoint.c b/fs/jbd2/checkpoint.c
>> index 77bc522e6821..f5a594237b7a 100644
>> --- a/fs/jbd2/checkpoint.c
>> +++ b/fs/jbd2/checkpoint.c
>> @@ -410,6 +410,7 @@ unsigned long jbd2_journal_shrink_checkpoint_list(journal_t *journal,
>>  	tid_t tid = 0;
>>  	unsigned long nr_freed = 0;
>>  	unsigned long freed;
>> +	bool is_first = true, is_last = false;
>>  
>>  again:
>>  	spin_lock(&journal->j_list_lock);
>> @@ -429,8 +430,10 @@ unsigned long jbd2_journal_shrink_checkpoint_list(journal_t *journal,
>>  	else
>>  		transaction = journal->j_checkpoint_transactions;
>>  
>> -	if (!first_tid)
>> +	if (is_first) {
>>  		first_tid = transaction->t_tid;
>> +		is_first = false;
>> +	}
>>  	last_transaction = journal->j_checkpoint_transactions->t_cpprev;
>>  	next_transaction = transaction;
>>  	last_tid = last_transaction->t_tid;
>> @@ -455,12 +458,13 @@ unsigned long jbd2_journal_shrink_checkpoint_list(journal_t *journal,
>>  	} else {
>>  		journal->j_shrink_transaction = NULL;
>>  		next_tid = 0;
>> +		is_last = true;
>>  	}
>>  
>>  	spin_unlock(&journal->j_list_lock);
>>  	cond_resched();
>>  
>> -	if (*nr_to_scan && next_tid)
>> +	if (*nr_to_scan && !is_last)
>
> I'd make this:
>
> 	if (*nr_to_scan && journal->j_shrink_transaction)
> 		goto again;
>
> and just remove is_last. Also we might rename is_first to first_set? At
> least to me it would be more comprehensible. Thanks!

Sure, both suggestions make sense.  I'll update the patches for v2.

Cheers,
-- 
Luís

>
> 								Honza
>
>>  		goto again;
>>  out:
>>  	trace_jbd2_shrink_checkpoint_list(journal, first_tid, tid, last_tid,
>> 
> -- 
> Jan Kara <jack@suse.com>
> SUSE Labs, CR