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

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Message-ID: <87le42dl4b.fsf@brahms.olymp>
Date: Wed, 22 May 2024 14:36:20 +0100
From: Luis Henriques <luis.henriques@...ux.dev>
To: Jan Kara <jack@...e.cz>
Cc: "Luis Henriques (SUSE)" <luis.henriques@...ux.dev>,  Theodore Ts'o
 <tytso@....edu>,  Andreas Dilger <adilger@...ger.ca>,  Jan Kara
 <jack@...e.com>,  linux-ext4@...r.kernel.org,
  linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 2/2] jbd2: reset fast commit offset only after fs
 cleanup is done

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@...ux.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@...e.com>
> SUSE Labs, CR