linux-kernel - Re: [PATCH v1 5/5] jbd2: Add TASK

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Message-ID: <uj22sykbnhfsbk7abj3rdul46uko5vvhq425kdbtkzsw5l5kqa@ixs245eozsfe>
Date: Wed, 30 Jul 2025 12:52:30 +0200
From: Jan Kara <jack@...e.cz>
To: Dai Junbing <daijunbing@...o.com>
Cc: Alexander Viro <viro@...iv.linux.org.uk>, 
	Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>, Miklos Szeredi <miklos@...redi.hu>, 
	Theodore Ts'o <tytso@....edu>, linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-ext4@...r.kernel.org, opensource.kernel@...o.com
Subject: Re: [PATCH v1 5/5] jbd2: Add TASK_FREEZABLE to kjournald2 thread

On Wed 30-07-25 09:47:06, Dai Junbing wrote:
> Set the TASK_FREEZABLE flag when the kjournald2 kernel thread sleeps
> during journal commit operations. This prevents premature wakeups
> during system suspend/resume cycles, avoiding unnecessary CPU wakeups
> and power consumption.
> 
> in this case, the original code:
> 
> 	prepare_to_wait(&journal->j_wait_commit, &wait,
>                	 TASK_INTERRUPTIBLE);
> 	if (journal->j_commit_sequence != journal->j_commit_request)
>         	should_sleep = 0;
> 
> 	transaction = journal->j_running_transaction;
> 	if (transaction && time_after_eq(jiffies, transaction->t_expires))
>         	should_sleep = 0;
> 	......
> 	......
> 	if (should_sleep) {
>         	write_unlock(&journal->j_state_lock);
>         	schedule();
>         	write_lock(&journal->j_state_lock);
> 	}
> 
> is functionally equivalent to the more concise:
> 
> 	write_unlock(&journal->j_state_lock);
> 	wait_event_freezable_exclusive(&journal->j_wait_commit,
>         	journal->j_commit_sequence == journal->j_commit_request ||
>         	(journal->j_running_transaction &&
>          	time_after_eq(jiffies, transaction->t_expires)) ||
>         	(journal->j_flags & JBD2_UNMOUNT));
> 	write_lock(&journal->j_state_lock);

This would be actually wrong because you cannot safely do some of the
dereferences without holding j_state_lock. Luckily you didn't modify the
existing code in the patch, just the changelog is bogus so please fix it.

> Signed-off-by: Dai Junbing <daijunbing@...o.com>
> ---
>  fs/jbd2/journal.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/jbd2/journal.c b/fs/jbd2/journal.c
> index d480b94117cd..9a1def9f730b 100644
> --- a/fs/jbd2/journal.c
> +++ b/fs/jbd2/journal.c
> @@ -222,7 +222,7 @@ static int kjournald2(void *arg)
>  		DEFINE_WAIT(wait);
>  
>  		prepare_to_wait(&journal->j_wait_commit, &wait,
> -				TASK_INTERRUPTIBLE);
> +				TASK_INTERRUPTIBLE | TASK_FREEZABLE);

So this looks fine but I have one question. There's code like:

        if (freezing(current)) {
                /*
                 * The simpler the better. Flushing journal isn't a
                 * good idea, because that depends on threads that may
                 * be already stopped.
                 */
                jbd2_debug(1, "Now suspending kjournald2\n");
                write_unlock(&journal->j_state_lock);
                try_to_freeze();
                write_lock(&journal->j_state_lock);

a few lines above. Is it still needed after your change? I guess that
probably yes (e.g. when the freeze attempt happens while kjournald still
performs some work then the later schedule in TASK_FREEZABLE state doesn't
necessarily freeze the kthread). But getting a confirmation would be nice.

								Honza

>  		transaction = journal->j_running_transaction;
>  		if (transaction == NULL ||
>  		    time_before(jiffies, transaction->t_expires)) {
> -- 
> 2.25.1
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR