Re: [RFC PATCH 2/2] jbd2: reset fast commit offset only after fs cleanup is done

Luis Henriques
2024-05-22T13:36:24Z

On Wed 22 May 2024 12:45:00 PM +02, Jan Kara wrote;

> On Tue 21-05-24 16:45:35, Luis Henriques (SUSE) wrote:
>> When doing a journal commit, the fast journal offset (journal->j_fc_off) is
>> set to zero too early in the process. Since ext4 filesystem calls function
>> jbd2_fc_release_bufs() in its j_fc_cleanup_callback (ext4_fc_cleanup()),
>> that call will be a no-op exactly because the offset is zero.
>>
>> Move the fast commit offset further down in the journal commit code, until
>> it's mostly done, immediately before clearing the on-going commit flags.
>>
>> Signed-off-by: Luis Henriques (SUSE) <luis.henriques@linux.dev>
>
> Did you see any particular failure because of this? Because AFAICS the
> buffers cleaned up by jbd2_fc_release_bufs() are only allocated during fast
> commit (from ext4_fc_reserve_space()). And the code in
> jbd2_journal_commit_transaction() is making sure fast commit isn't running
> before we set journal->j_fc_off to 0.

No, I did not see any failure caused by this, this patch is simply based
on my understanding of the code after spending some time reviewing it.

The problem I saw was that jbd2_journal_commit_transaction() will run the
clean-up callbacks, which includes ext4_fc_cleanup(). One of the first
things that this callback will do is to call jbd2_fc_release_bufs().
Because journal->j_fc_off is zero, this call is useless:

j_fc_off = journal->j_fc_off;

for (i = j_fc_off - 1; i >= 0; i--) {
[...]
}

(It's even a bit odd to start the loop with 'i = -1'...)

So the question is whether this call is actually useful at all. Maybe the
thing to do is to simply remove the call to jbd2_fc_release_bufs()? (And
in that case, remove the function too, as this is the only call site.)

Cheers,
--
Luis

>
> Honza
>
>> ---
>> fs/jbd2/commit.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/fs/jbd2/commit.c b/fs/jbd2/commit.c
>> index 75ea4e9a5cab..88b834c7c9c9 100644
>> --- a/fs/jbd2/commit.c
>> +++ b/fs/jbd2/commit.c
>> @@ -435,7 +435,6 @@ void jbd2_journal_commit_transaction(journal_t *journal)
>> commit_transaction->t_tid);
>>
>> write_lock(&journal->j_state_lock);
>> - journal->j_fc_off = 0;
>> J_ASSERT(commit_transaction->t_state == T_RUNNING);
>> commit_transaction->t_state = T_LOCKED;
>>
>> @@ -1133,6 +1132,7 @@ void jbd2_journal_commit_transaction(journal_t *journal)
>> journal->j_commit_sequence, journal->j_tail_sequence);
>>
>> write_lock(&journal->j_state_lock);
>> + journal->j_fc_off = 0;
>> journal->j_flags &= ~JBD2_FULL_COMMIT_ONGOING;
>> journal->j_flags &= ~JBD2_FAST_COMMIT_ONGOING;
>> spin_lock(&journal->j_list_lock);
>>
> --
> Jan Kara <jack@suse.com>
> SUSE Labs, CR