ext4: fix incorrect tid assumption in jbd2_journal_shrink_checkpoint_list()

BugLink: https://bugs.launchpad.net/bugs/2089884

commit 7a6443e1dad70281f99f0bd394d7fd342481a632 upstream.

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@linux.dev>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://patch.msgid.link/20240724161119.13448-4-luis.henriques@linux.dev
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Koichiro Den <koichiro.den@canonical.com>
Signed-off-by: Roxana Nicolescu <roxana.nicolescu@canonical.com>

fs/jbd2/checkpoint.c

@@ -412,6 +412,7 @@ unsigned long jbd2_journal_shrink_checkpoint_list(journal_t *journal,
 	tid_t tid = 0;
 	unsigned long nr_freed = 0;
 	unsigned long freed;
+	bool first_set = false;
 
 again:
 	spin_lock(&journal->j_list_lock);
@@ -431,8 +432,10 @@ again:
 	else
 		transaction = journal->j_checkpoint_transactions;
 
-	if (!first_tid)
+	if (!first_set) {
 		first_tid = transaction->t_tid;
+		first_set = true;
+	}
 	last_transaction = journal->j_checkpoint_transactions->t_cpprev;
 	next_transaction = transaction;
 	last_tid = last_transaction->t_tid;
@@ -462,7 +465,7 @@ again:
 	spin_unlock(&journal->j_list_lock);
 	cond_resched();
 
-	if (*nr_to_scan && next_tid)
+	if (*nr_to_scan && journal->j_shrink_transaction)
 		goto again;
 out:
 	trace_jbd2_shrink_checkpoint_list(journal, first_tid, tid, last_tid,