1 files changed, 24 insertions, 13 deletions

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index d7e0e045b11b..6810957e0ac7 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -4580,16 +4580,34 @@ 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_clusters 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;
         s64 bfree;
 
         if (test_opt(sb, MINIX_DF)) {
                 sbi->s_overhead_last = 0;
+        } else if (es->s_overhead_clusters) {
+                sbi->s_overhead_last = le32_to_cpu(es->s_overhead_clusters);
         } 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;
@@ -4604,24 +4622,16 @@ 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();
                 }
-
-                /*
-                 * Every block group has an inode bitmap, a block
-                 * bitmap, and an inode table.
-                 */
-                overhead += ngroups * (2 + sbi->s_itb_per_group);
                 sbi->s_overhead_last = overhead;
                 smp_wmb();
                 sbi->s_blocks_last = ext4_blocks_count(es);
@@ -4629,7 +4639,8 @@ static int ext4_statfs(struct dentry *dentry, struct kstatfs *buf)
 
         buf->f_type = EXT4_SUPER_MAGIC;
         buf->f_bsize = sb->s_blocksize;
-        buf->f_blocks = ext4_blocks_count(es) - sbi->s_overhead_last;
+        buf->f_blocks = (ext4_blocks_count(es) -
+                         EXT4_C2B(sbi, sbi->s_overhead_last));
         bfree = percpu_counter_sum_positive(&sbi->s_freeclusters_counter) -
                 percpu_counter_sum_positive(&sbi->s_dirtyclusters_counter);
         /* prevent underflow in case that few free space is available */