ext4: fix largest free orders lists corruption on mb_optimize_scan switch

The grp->bb_largest_free_order is updated regardless of whether
mb_optimize_scan is enabled. This can lead to inconsistencies between
grp->bb_largest_free_order and the actual s_mb_largest_free_orders list
index when mb_optimize_scan is repeatedly enabled and disabled via remount.

For example, if mb_optimize_scan is initially enabled, largest free
order is 3, and the group is in s_mb_largest_free_orders[3]. Then,
mb_optimize_scan is disabled via remount, block allocations occur,
updating largest free order to 2. Finally, mb_optimize_scan is re-enabled
via remount, more block allocations update largest free order to 1.

At this point, the group would be removed from s_mb_largest_free_orders[3]
under the protection of s_mb_largest_free_orders_locks[2]. This lock
mismatch can lead to list corruption.

To fix this, whenever grp->bb_largest_free_order changes, we now always
attempt to remove the group from its old order list. However, we only
insert the group into the new order list if `mb_optimize_scan` is enabled.
This approach helps prevent lock inconsistencies and ensures the data in
the order lists remains reliable.

Fixes: 196e402 ("ext4: improve cr 0 / cr 1 group scanning")
CC: [email protected]
Suggested-by: Jan Kara <[email protected]>
Signed-off-by: Baokun Li <[email protected]>
Reviewed-by: Zhang Yi <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Theodore Ts'o <[email protected]>

Author: LiBaokun96

fs/ext4/mballoc.c

@@ -1152,33 +1152,28 @@ static void
 mb_set_largest_free_order(struct super_block *sb, struct ext4_group_info *grp)
 {
 	struct ext4_sb_info *sbi = EXT4_SB(sb);
-	int i;
+	int new, old = grp->bb_largest_free_order;
 
-	for (i = MB_NUM_ORDERS(sb) - 1; i >= 0; i--)
-		if (grp->bb_counters[i] > 0)
+	for (new = MB_NUM_ORDERS(sb) - 1; new >= 0; new--)
+		if (grp->bb_counters[new] > 0)
 			break;
+
 	/* No need to move between order lists? */
-	if (!test_opt2(sb, MB_OPTIMIZE_SCAN) ||
-	    i == grp->bb_largest_free_order) {
-		grp->bb_largest_free_order = i;
+	if (new == old)
 		return;
-	}
 
-	if (grp->bb_largest_free_order >= 0) {
-		write_lock(&sbi->s_mb_largest_free_orders_locks[
-					      grp->bb_largest_free_order]);
+	if (old >= 0 && !list_empty(&grp->bb_largest_free_order_node)) {
+		write_lock(&sbi->s_mb_largest_free_orders_locks[old]);
 		list_del_init(&grp->bb_largest_free_order_node);
-		write_unlock(&sbi->s_mb_largest_free_orders_locks[
-					      grp->bb_largest_free_order]);
+		write_unlock(&sbi->s_mb_largest_free_orders_locks[old]);
 	}
-	grp->bb_largest_free_order = i;
-	if (grp->bb_largest_free_order >= 0 && grp->bb_free) {
-		write_lock(&sbi->s_mb_largest_free_orders_locks[
-					      grp->bb_largest_free_order]);
+
+	grp->bb_largest_free_order = new;
+	if (test_opt2(sb, MB_OPTIMIZE_SCAN) && new >= 0 && grp->bb_free) {
+		write_lock(&sbi->s_mb_largest_free_orders_locks[new]);
 		list_add_tail(&grp->bb_largest_free_order_node,
-		      &sbi->s_mb_largest_free_orders[grp->bb_largest_free_order]);
-		write_unlock(&sbi->s_mb_largest_free_orders_locks[
-					      grp->bb_largest_free_order]);
+			      &sbi->s_mb_largest_free_orders[new]);
+		write_unlock(&sbi->s_mb_largest_free_orders_locks[new]);
 	}
 }