Re: [PATCH 2/2] jbd2: flush filesystem device before updating tail sequence

[Zhang Yi]

On 2024/12/3 14:53, Kemeng Shi wrote:
> 
> 
> on 12/3/2024 9:44 AM, Zhang Yi wrote:
>> From: Zhang Yi <yi.zhang@xxxxxxxxxx>
>>
>> When committing transaction in jbd2_journal_commit_transaction(), the
>> disk caches for the filesystem device should be flushed before updating
>> the journal tail sequence. However, this step is missed if the journal
>> is not located on the filesystem device. As a result, the filesystem may
>> become inconsistent following a power failure or system crash. Fix it by
>> ensuring that the filesystem device is flushed appropriately.
>>
>> Fixes: 3339578f0578 ("jbd2: cleanup journal tail after transaction commit")
>> Signed-off-by: Zhang Yi <yi.zhang@xxxxxxxxxx>
>> ---
>>  fs/jbd2/commit.c | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/fs/jbd2/commit.c b/fs/jbd2/commit.c
>> index 4305a1ac808a..f95cf272a1b5 100644
>> --- a/fs/jbd2/commit.c
>> +++ b/fs/jbd2/commit.c
>> @@ -776,9 +776,9 @@ void jbd2_journal_commit_transaction(journal_t *journal)
>>  	/*
>>  	 * If the journal is not located on the file system device,
>>  	 * then we must flush the file system device before we issue
>> -	 * the commit record
>> +	 * the commit record and update the journal tail sequence.
>>  	 */
>> -	if (commit_transaction->t_need_data_flush &&
>> +	if ((commit_transaction->t_need_data_flush || update_tail) &&
>>  	    (journal->j_fs_dev != journal->j_dev) &&
>>  	    (journal->j_flags & JBD2_BARRIER))
>>  		blkdev_issue_flush(journal->j_fs_dev);
>>
> In journal_submit_commit_record(), we will submit commit block with REQ_PREFLUSH
> which is supposed to ensure disk cache is flushed before writing commit block.
> So I think the current code is fine.
> Please correct me if I miss anything.
> 

The commit I/O with REQ_PREFLUSH only flushes 'journal->j_dev', not
'journal->j_fs_dev'. We need to flush journal->j_fs_dev to ensure that all
written metadata has been persisted to the filesystem disk, Until then, we
cannot update the tail sequence.

Thanks,
Yi.