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 <artem.bityutskiy@linux.intel.com>
Cc: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 2 insertions, 1 deletions

diff --git a/include/linux/msdos_fs.h b/include/linux/msdos_fs.h
index 34066e65fdeb..11cc2ac67e75 100644
--- a/include/linux/msdos_fs.h
+++ b/include/linux/msdos_fs.h
@@ -21,8 +21,9 @@
 #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 */