[PATCH, RFC 10/12] ext4: teach ext4_statfs() to deal with clusters if bigalloc is enabled

[Theodore Ts'o <tytso@mit.edu>]

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
---
 fs/ext4/super.c |   35 +++++++++++++++++++++++------------
 1 files changed, 23 insertions(+), 12 deletions(-)

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index f9b25cd..24964da 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -4438,15 +4438,33 @@ restore_opts:
 	return err;
 }
 
+/*
+ * Note: calculating the overhead so we can be compatible with
+ * historical BSD practice is quite difficult in the face of
+ * clusters/bigalloc.  This is because multiple metadata blocks from
+ * different block group can end up in the same allocation cluster.
+ * Calculating the exact overhead in the face of clustered allocation
+ * requires either O(all block bitmaps) in memory or O(number of block
+ * groups**2) in time.  We will still calculate the superblock for
+ * older file systems --- and if we come across with a bigalloc file
+ * system with zero in s_overhead_blocks the estimate will be close to
+ * correct especially for very large cluster sizes --- but for newer
+ * file systems, it's better to calculate this figure once at mkfs
+ * time, and store it in the superblock.  If the superblock value is
+ * present (even for non-bigalloc file systems), we will use it.
+ */
 static int ext4_statfs(struct dentry *dentry, struct kstatfs *buf)
 {
 	struct super_block *sb = dentry->d_sb;
 	struct ext4_sb_info *sbi = EXT4_SB(sb);
 	struct ext4_super_block *es = sbi->s_es;
+	struct ext4_group_desc *gdp;
 	u64 fsid;
 
 	if (test_opt(sb, MINIX_DF)) {
 		sbi->s_overhead_last = 0;
+	} else if (es->s_overhead_blocks) {
+		sbi->s_overhead_last = le32_to_cpu(es->s_overhead_blocks);
 	} else if (sbi->s_blocks_last != ext4_blocks_count(es)) {
 		ext4_group_t i, ngroups = ext4_get_groups_count(sb);
 		ext4_fsblk_t overhead = 0;
@@ -4461,24 +4479,16 @@ static int ext4_statfs(struct dentry *dentry, struct kstatfs *buf)
 		 * All of the blocks before first_data_block are
 		 * overhead
 		 */
-		overhead = le32_to_cpu(es->s_first_data_block);
+		overhead = EXT4_B2C(sbi, le32_to_cpu(es->s_first_data_block));
 
 		/*
-		 * Add the overhead attributed to the superblock and
-		 * block group descriptors.  If the sparse superblocks
-		 * feature is turned on, then not all groups have this.
+		 * Add the overhead found in each block group
 		 */
 		for (i = 0; i < ngroups; i++) {
-			overhead += ext4_bg_has_super(sb, i) +
-				ext4_bg_num_gdb(sb, i);
+			gdp = ext4_get_group_desc(sb, i, NULL);
+			overhead += ext4_num_overhead_clusters(sb, i, gdp);
 			cond_resched();
 		}