fat: introduce special inode for managing the FSINFO block

This is patchset makes fatfs stop using the VFS '->write_super()' method
for writing out the FSINFO block.

The final goal is to get rid of the 'sync_supers()' kernel thread. This
kernel thread wakes up every 5 seconds (by default) and calls
'->write_super()' for all mounted file-systems. And the bad thing is that
this is done even if all the superblocks are clean. Moreover, some
file-systems do not even need this end they do not register the
'->write_super()' method at all (e.g., btrfs).

So 'sync_supers()' most often just generates useless wake-ups and wastes
power. I am trying to make all file-systems independent of
'->write_super()' and plan to remove 'sync_supers()' and '->write_super'
completely once there are no more users.

The '->write_supers()' method is mostly used by baroque file-systems like
hfs, udf, etc. Modern file-systems like btrfs and xfs do not use it.
This justifies removing this stuff from VFS completely and make every FS
self-manage own superblock.

Tested with xfstests.

This patch:

Preparation for further changes. It introduces a special inode
('fsinfo_inode') in FAT file-system which we'll later use for managing the
FSINFO block. Note, this there is already one special inode ('fat_inode')
which is used for managing the FAT tables.

Introduce new 'MSDOS_FSINFO_INO' constant for this special inode. It is
safe to do because FAT file-system does not store inode numbers on the
media but generates them run-time.

I've also cleaned up the comment to existing 'MSDOS_ROOT_INO' constant,
while on it.

Signed-off-by: Artem Bityutskiy <[e-mail]>
Cc: OGAWA Hirofumi <[e-mail]>
Cc: Al Viro <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/fat/fat.h

···
 	int fatent_shift;
 	struct fatent_operations *fatent_ops;
 	struct inode *fat_inode;
+	struct inode *fsinfo_inode;
 
 	struct ratelimit_state ratelimit;
 

fs/fat/inode.c

···
 	if (sb->s_dirt)
 		fat_write_super(sb);
 
+	iput(sbi->fsinfo_inode);
 	iput(sbi->fat_inode);
 
 	unload_nls(sbi->nls_disk);
···
 		   void (*setup)(struct super_block *))
 {
 	struct inode *root_inode = NULL, *fat_inode = NULL;
+	struct inode *fsinfo_inode = NULL;
 	struct buffer_head *bh;
 	struct fat_boot_sector *b;
 	struct msdos_sb_info *sbi;
···
 		goto out_fail;
 	MSDOS_I(fat_inode)->i_pos = 0;
 	sbi->fat_inode = fat_inode;
+
+	fsinfo_inode = new_inode(sb);
+	if (!fsinfo_inode)
+		goto out_fail;
+	fsinfo_inode->i_ino = MSDOS_FSINFO_INO;
+	sbi->fsinfo_inode = fsinfo_inode;
+	insert_inode_hash(fsinfo_inode);
+
 	root_inode = new_inode(sb);
 	if (!root_inode)
 		goto out_fail;
···
 		fat_msg(sb, KERN_INFO, "Can't find a valid FAT filesystem");
 
 out_fail:
+	if (fsinfo_inode)
+		iput(fsinfo_inode);
 	if (fat_inode)
 		iput(fat_inode);
 	unload_nls(sbi->nls_io);

include/linux/msdos_fs.h

···
 #define CT_LE_W(v)	cpu_to_le16(v)
 #define CT_LE_L(v)	cpu_to_le32(v)
 
+#define MSDOS_ROOT_INO	 1	/* The root inode number */
+#define MSDOS_FSINFO_INO 2	/* Used for managing the FSINFO block */
 
-#define MSDOS_ROOT_INO	1	/* == MINIX_ROOT_INO */
 #define MSDOS_DIR_BITS	5	/* log2(sizeof(struct msdos_dir_entry)) */
 
 /* directory limit */