ext4: fix zombie groups in average fragment size lists

Groups with no free blocks shouldn't be in any average fragment size list.
However, when all blocks in a group are allocated(i.e., bb_fragments or
bb_free is 0), we currently skip updating the average fragment size, which
means the group isn't removed from its previous s_mb_avg_fragment_size[old]
list.

This created "zombie" groups that were always skipped during traversal as
they couldn't satisfy any block allocation requests, negatively impacting
traversal efficiency.

Therefore, when a group becomes completely full, bb_avg_fragment_size_order
is now set to -1. If the old order was not -1, a removal operation is
performed; if the new order is not -1, an insertion is performed.

Fixes: 196e402 ("ext4: improve cr 0 / cr 1 group scanning")
CC: [email protected]
Signed-off-by: Baokun Li <[email protected]>
Reviewed-by: Jan Kara <[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

@@ -841,30 +841,30 @@ static void
 mb_update_avg_fragment_size(struct super_block *sb, struct ext4_group_info *grp)
 {
 	struct ext4_sb_info *sbi = EXT4_SB(sb);
-	int new_order;
+	int new, old;
 
-	if (!test_opt2(sb, MB_OPTIMIZE_SCAN) || grp->bb_fragments == 0)
+	if (!test_opt2(sb, MB_OPTIMIZE_SCAN))
 		return;
 
-	new_order = mb_avg_fragment_size_order(sb,
-					grp->bb_free / grp->bb_fragments);
-	if (new_order == grp->bb_avg_fragment_size_order)
+	old = grp->bb_avg_fragment_size_order;
+	new = grp->bb_fragments == 0 ? -1 :
+	      mb_avg_fragment_size_order(sb, grp->bb_free / grp->bb_fragments);
+	if (new == old)
 		return;
 
-	if (grp->bb_avg_fragment_size_order != -1) {
-		write_lock(&sbi->s_mb_avg_fragment_size_locks[
-					grp->bb_avg_fragment_size_order]);
+	if (old >= 0) {
+		write_lock(&sbi->s_mb_avg_fragment_size_locks[old]);
 		list_del(&grp->bb_avg_fragment_size_node);
-		write_unlock(&sbi->s_mb_avg_fragment_size_locks[
-					grp->bb_avg_fragment_size_order]);
-	}
-	grp->bb_avg_fragment_size_order = new_order;
-	write_lock(&sbi->s_mb_avg_fragment_size_locks[
-					grp->bb_avg_fragment_size_order]);
-	list_add_tail(&grp->bb_avg_fragment_size_node,
-		&sbi->s_mb_avg_fragment_size[grp->bb_avg_fragment_size_order]);
-	write_unlock(&sbi->s_mb_avg_fragment_size_locks[
-					grp->bb_avg_fragment_size_order]);
+		write_unlock(&sbi->s_mb_avg_fragment_size_locks[old]);
+	}
+
+	grp->bb_avg_fragment_size_order = new;
+	if (new >= 0) {
+		write_lock(&sbi->s_mb_avg_fragment_size_locks[new]);
+		list_add_tail(&grp->bb_avg_fragment_size_node,
+				&sbi->s_mb_avg_fragment_size[new]);
+		write_unlock(&sbi->s_mb_avg_fragment_size_locks[new]);
+	}
 }
 
 /*