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

[Jan Kara <jack@suse.cz>]

On Wed 22-05-24 14:36:20, Luis Henriques wrote:
> 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.)

What is I guess confusing for you (and somewhat for me as well) is that
journal->j_fc_cleanup_callback() gets called from __jbd2_fc_end_commit()
*and* from jbd2_journal_commit_transaction(). I agree the
jbd2_fc_release_bufs() is useless for the call from
jbd2_journal_commit_transaction(), it is however needed for the call from
__jbd2_fc_end_commit(). There are however other bits - namely the
s_fc_dentry_q and s_fc_q list handling that need to happen both for normal
and fast commit...

								Honza

-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR