[PATCH 3/4] ext4: fix incorrect tid assumption in jbd2_journal_shrink_checkpoint_list()

["Luis Henriques (SUSE)" <luis.henriques@linux.dev>]

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>
---
 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)
 		goto again;
 out:
 	trace_jbd2_shrink_checkpoint_list(journal, first_tid, tid, last_tid,