ext4: fix incorrect tid assumption in ext4_fc_mark_ineligible()

Function jbd2_journal_shrink_checkpoint_list() assumes that '0' is not a
valid value for transaction IDs, which is incorrect.

Furthermore, the sbi->s_fc_ineligible_tid handling also makes the same
assumption by being initialised to '0'.  Fortunately, the sb flag
EXT4_MF_FC_INELIGIBLE can be used to check whether sbi->s_fc_ineligible_tid
has been previously set instead of comparing it with '0'.
Signed-off-by: Luis Henriques (SUSE) <luis.henriques@linux.dev>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://patch.msgid.link/20240724161119.13448-5-luis.henriques@linux.devSigned-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@kernel.org

fs/ext4/fast_commit.c

@@ -339,22 +339,29 @@ void ext4_fc_mark_ineligible(struct super_block *sb, int reason, handle_t *handl
 {
 	struct ext4_sb_info *sbi = EXT4_SB(sb);
 	tid_t tid;
+	bool has_transaction = true;
+	bool is_ineligible;
 
 	if (ext4_fc_disabled(sb))
 		return;
 
-	ext4_set_mount_flag(sb, EXT4_MF_FC_INELIGIBLE);
 	if (handle && !IS_ERR(handle))
 		tid = handle->h_transaction->t_tid;
 	else {
 		read_lock(&sbi->s_journal->j_state_lock);
-		tid = sbi->s_journal->j_running_transaction ?
-				sbi->s_journal->j_running_transaction->t_tid : 0;
+		if (sbi->s_journal->j_running_transaction)
+			tid = sbi->s_journal->j_running_transaction->t_tid;
+		else
+			has_transaction = false;
 		read_unlock(&sbi->s_journal->j_state_lock);
 	}
 	spin_lock(&sbi->s_fc_lock);
-	if (tid_gt(tid, sbi->s_fc_ineligible_tid))
+	is_ineligible = ext4_test_mount_flag(sb, EXT4_MF_FC_INELIGIBLE);
+	if (has_transaction &&
+	    (!is_ineligible ||
+	     (is_ineligible && tid_gt(tid, sbi->s_fc_ineligible_tid))))
 		sbi->s_fc_ineligible_tid = tid;
+	ext4_set_mount_flag(sb, EXT4_MF_FC_INELIGIBLE);
 	spin_unlock(&sbi->s_fc_lock);
 	WARN_ON(reason >= EXT4_FC_REASON_MAX);
 	sbi->s_fc_stats.fc_ineligible_reason_count[reason]++;