[PATCH v2] bcachefs: fix last_seq and last_empty_seq in bch2_fs_journal_start()

[Camila Alvarez <cam.alvarez.i@gmail.com>]

Values were left as the next possible sequence number when there were no
entries.

The fix involves updating the last_seq initial value and
setting last_empty_seq to cur_seq - 1.

Reported-by: syzbot+10b936c5eaee2819b49b@syzkaller.appspotmail.com
Signed-off-by: Camila Alvarez <cam.alvarez.i@gmail.com>
---
Changes in v2:
- Introduce a new variable to distinguish the last sequence number, from
  the last written sequence number
- Abstract `cur_seq - 1` into a variable named `last_seq`
- Reference to the last sequence number are changed from `cur_seq - 1`
  to `last_seq`
---
 fs/bcachefs/journal.c | 22 +++++++++++-----------
 1 file changed, 11 insertions(+), 11 deletions(-)

diff --git a/fs/bcachefs/journal.c b/fs/bcachefs/journal.c
index adec8e1ea73e..99fc32f0382d 100644
--- a/fs/bcachefs/journal.c
+++ b/fs/bcachefs/journal.c
@@ -1195,8 +1195,8 @@ int bch2_fs_journal_start(struct journal *j, u64 cur_seq)
 	struct journal_entry_pin_list *p;
 	struct journal_replay *i, **_i;
 	struct genradix_iter iter;
-	bool had_entries = false;
-	u64 last_seq = cur_seq, nr, seq;
+	bool had_entries = false;	
+	u64 last_written_seq = cur_seq - 1, last_seq = cur_seq - 1, nr, seq;
 
 	genradix_for_each_reverse(&c->journal_entries, iter, _i) {
 		i = *_i;
@@ -1204,11 +1204,11 @@ int bch2_fs_journal_start(struct journal *j, u64 cur_seq)
 		if (journal_replay_ignore(i))
 			continue;
 
-		last_seq = le64_to_cpu(i->j.last_seq);
+		last_written_seq = le64_to_cpu(i->j.last_seq);
 		break;
 	}
 
-	nr = cur_seq - last_seq;
+	nr = cur_seq - last_written_seq;
 
 	if (nr + 1 > j->pin.size) {
 		free_fifo(&j->pin);
@@ -1219,14 +1219,14 @@ int bch2_fs_journal_start(struct journal *j, u64 cur_seq)
 		}
 	}
 
-	j->replay_journal_seq	= last_seq;
+	j->replay_journal_seq	= last_written_seq;
 	j->replay_journal_seq_end = cur_seq;
-	j->last_seq_ondisk	= last_seq;
-	j->flushed_seq_ondisk	= cur_seq - 1;
-	j->seq_ondisk		= cur_seq - 1;
-	j->pin.front		= last_seq;
+	j->last_seq_ondisk	= last_written_seq;
+	j->flushed_seq_ondisk	= last_seq;
+	j->seq_ondisk		= last_seq;
+	j->pin.front		= last_written_seq;
 	j->pin.back		= cur_seq;
-	atomic64_set(&j->seq, cur_seq - 1);
+	atomic64_set(&j->seq, last_seq);
 
 	fifo_for_each_entry_ptr(p, &j->pin, seq)
 		journal_pin_list_init(p, 1);
@@ -1256,7 +1256,7 @@ int bch2_fs_journal_start(struct journal *j, u64 cur_seq)
 	}
 
 	if (!had_entries)
-		j->last_empty_seq = cur_seq;
+		j->last_empty_seq = last_seq;
 
 	spin_lock(&j->lock);
 
-- 
2.34.1