linux-kernel - Re: [PATCH 3/4] ext4: fix incorrect tid assumption in jbd2_journal_shrink_checkpoint

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Message-ID: <20240724092954.gw3hwyax7u5kpn5o@quack3>
Date: Wed, 24 Jul 2024 11:29:54 +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.cz>,
	Harshad Shirwadkar <harshadshirwadkar@...il.com>,
	linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/4] ext4: fix incorrect tid assumption in
 jbd2_journal_shrink_checkpoint_list()

On Tue 23-07-24 16:44:01, Luis Henriques (SUSE) wrote:
> Function jbd2_journal_shrink_checkpoint_list() assumes that '0' is not a
> valid value for transaction IDs, which is incorrect.  Don't assume that and
> use two extra boolean variables to control the loop iterations and keep
> track of the first and last tid.
> 
> Signed-off-by: Luis Henriques (SUSE) <luis.henriques@...ux.dev>
> ---
>  fs/jbd2/checkpoint.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/jbd2/checkpoint.c b/fs/jbd2/checkpoint.c
> index 77bc522e6821..f5a594237b7a 100644
> --- a/fs/jbd2/checkpoint.c
> +++ b/fs/jbd2/checkpoint.c
> @@ -410,6 +410,7 @@ unsigned long jbd2_journal_shrink_checkpoint_list(journal_t *journal,
>  	tid_t tid = 0;
>  	unsigned long nr_freed = 0;
>  	unsigned long freed;
> +	bool is_first = true, is_last = false;
>  
>  again:
>  	spin_lock(&journal->j_list_lock);
> @@ -429,8 +430,10 @@ unsigned long jbd2_journal_shrink_checkpoint_list(journal_t *journal,
>  	else
>  		transaction = journal->j_checkpoint_transactions;
>  
> -	if (!first_tid)
> +	if (is_first) {
>  		first_tid = transaction->t_tid;
> +		is_first = false;
> +	}
>  	last_transaction = journal->j_checkpoint_transactions->t_cpprev;
>  	next_transaction = transaction;
>  	last_tid = last_transaction->t_tid;
> @@ -455,12 +458,13 @@ unsigned long jbd2_journal_shrink_checkpoint_list(journal_t *journal,
>  	} else {
>  		journal->j_shrink_transaction = NULL;
>  		next_tid = 0;
> +		is_last = true;
>  	}
>  
>  	spin_unlock(&journal->j_list_lock);
>  	cond_resched();
>  
> -	if (*nr_to_scan && next_tid)
> +	if (*nr_to_scan && !is_last)

I'd make this:

	if (*nr_to_scan && journal->j_shrink_transaction)
		goto again;

and just remove is_last. Also we might rename is_first to first_set? At
least to me it would be more comprehensible. Thanks!

								Honza

>  		goto again;
>  out:
>  	trace_jbd2_shrink_checkpoint_list(journal, first_tid, tid, last_tid,
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR