ext4: update the cached overhead value in the superblock

If we (re-)calculate the file system overhead amount and it's
different from the on-disk s_overhead_clusters value, update the
on-disk version since this can take potentially quite a while on
bigalloc file systems.
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@kernel.org

fs/ext4/ext4.h

@@ -3068,6 +3068,7 @@ int ext4_fileattr_set(struct user_namespace *mnt_userns,
 		      struct dentry *dentry, struct fileattr *fa);
 int ext4_fileattr_get(struct dentry *dentry, struct fileattr *fa);
 extern void ext4_reset_inode_seed(struct inode *inode);
+int ext4_update_overhead(struct super_block *sb);
 
 /* migrate.c */
 extern int ext4_ext_migrate(struct inode *);

fs/ext4/ioctl.c

@@ -1652,3 +1652,19 @@ long ext4_compat_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 	return ext4_ioctl(file, cmd, (unsigned long) compat_ptr(arg));
 }
 #endif
+
+static void set_overhead(struct ext4_super_block *es, const void *arg)
+{
+	es->s_overhead_clusters = cpu_to_le32(*((unsigned long *) arg));
+}
+
+int ext4_update_overhead(struct super_block *sb)
+{
+	struct ext4_sb_info *sbi = EXT4_SB(sb);
+
+	if (sb_rdonly(sb) || sbi->s_overhead == 0 ||
+	    sbi->s_overhead == le32_to_cpu(sbi->s_es->s_overhead_clusters))
+		return 0;
+
+	return ext4_update_superblocks_fn(sb, set_overhead, &sbi->s_overhead);
+}

fs/ext4/super.c

@@ -5618,6 +5618,8 @@ static int ext4_fill_super(struct super_block *sb, struct fs_context *fc)
 		ext4_msg(sb, KERN_INFO, "mounted filesystem with%s. "
 			 "Quota mode: %s.", descr, ext4_quota_mode(sb));
 
+	/* Update the s_overhead_clusters if necessary */
+	ext4_update_overhead(sb);
 	return 0;
 
 free_sbi: