ext4: reduce computation of overhead during resize

This patch avoids doing an O(n**2)-complexity walk through every flex group.
Instead, it uses the already computed overhead information for the newly
allocated space, and simply adds it to the previously calculated
overhead stored in the superblock.  This drastically reduces the time
taken to resize very large bigalloc filesystems (from 3+ hours for a
64TB fs down to milliseconds).
Signed-off-by: Oleg Kiselev <okiselev@amazon.com>
Link: https://lore.kernel.org/r/CE4F359F-4779-45E6-B6A9-8D67FDFF5AE2@amazon.comSigned-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/resize.c

@@ -1383,6 +1383,17 @@ static int ext4_setup_new_descs(handle_t *handle, struct super_block *sb,
 	return err;
 }
 
+static void ext4_add_overhead(struct super_block *sb,
+                              const ext4_fsblk_t overhead)
+{
+       struct ext4_sb_info *sbi = EXT4_SB(sb);
+       struct ext4_super_block *es = sbi->s_es;
+
+       sbi->s_overhead += overhead;
+       es->s_overhead_clusters = cpu_to_le32(sbi->s_overhead);
+       smp_wmb();
+}
+
 /*
  * ext4_update_super() updates the super block so that the newly added
  * groups can be seen by the filesystem.
@@ -1484,9 +1495,17 @@ static void ext4_update_super(struct super_block *sb,
 	}
 
 	/*
-	 * Update the fs overhead information
+	 * Update the fs overhead information.
+	 *
+	 * For bigalloc, if the superblock already has a properly calculated
+	 * overhead, update it with a value based on numbers already computed
+	 * above for the newly allocated capacity.
 	 */
-	ext4_calculate_overhead(sb);
+	if (ext4_has_feature_bigalloc(sb) && (sbi->s_overhead != 0))
+		ext4_add_overhead(sb,
+			EXT4_NUM_B2C(sbi, blocks_count - free_blocks));
+	else
+		ext4_calculate_overhead(sb);
 	es->s_overhead_clusters = cpu_to_le32(sbi->s_overhead);
 
 	if (test_opt(sb, DEBUG))