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

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Date: Wed, 22 May 2024 12:45:00 +0200
From: Jan Kara <jack@...e.cz>
To: "Luis Henriques (SUSE)" <luis.henriques@...ux.dev>
Cc: 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 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.

								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