Re: [RFC PATCH] jbd2: make '0' an invalid transaction sequence

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

On Tue, Jul 16 2024, Jan Kara wrote:

> On Fri 12-07-24 10:53:02, Luis Henriques wrote:
>> Since there's code (in fast-commit) that already handles a '0' tid as a
>> special case, it's better to ensure that jbd2 never sets it to that value
>> when journal->j_transaction_sequence increment wraps.
>> 
>> Suggested-by: Andreas Dilger <adilger@dilger.ca>
>> Signed-off-by: Luis Henriques (SUSE) <luis.henriques@linux.dev>
>
> Well, sadly it isn't so simple. If nothing else, journal replay
> (do_one_pass()) will get broken by the skipped tid as we do check:
>
>                 if (sequence != next_commit_ID) {
>                         brelse(bh);
>                         break;
>                 }
>
> So we'd abort journal replay too early. Secondly, there's also code
> handling journal replay in libext2fs which would need to be checked and
> fixed up. Finally, I've found code in mballoc which alternates between two
> lists based on tid & 1, so this logic would get broken by skipping 0 tid
> as well.
>
> Overall, I think we might be better off to go and fix places that assume
> tid 0 is not valid. I can see those assumptions in:
>
> ext4_fc_mark_ineligible()
> ext4_wait_for_tail_page_commit()
> __jbd2_log_wait_for_space()
> jbd2_journal_shrink_checkpoint_list()
>
> Now I don't see it as urgent to fix all these right now. Just for this
> series let's not add another place making tid 0 special. Later we can fixup
> the other places...

Yikes!  Looks like I haven't done my homework -- I should have caught at
least one of the three breakages you point out.  Obviously, because I've
seen this assumption in different places, I thought it would be OK.

Anyway, thanks a lot for point this out, Jan.  I'll add a new TODO to my
list to start looking at other places that need to be fixed.

Cheers,
-- 
Luís