[PATCH v1 0/3] do not use s_dirt in FAT FS

[Artem Bityutskiy <dedekind1@gmail.com>]

The FAT file-system uses the VFS '->write_super()' method for writing out
the FSINFO block, which contains useful but not critical information like
the amount of free clusters.

This patch-set makes FAT FS use its own mechanisms for writing-out the FSINFO
block and stop using the '->write_super()' VFS method. Namely, the FAT FS now
submits a delayed work for writing out the FSINFO block once it becomes dirty.

The reason of this exercises is to get rid of the 'sync_supers()' kernel thread.
This kernel thread wakes up every 5 (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.

Tested with xfstests.

Note: in the past I was trying to upstream patches which optimized 'sync_super()',
but Al Viro wanted me to kill it completely instead, which I am trying to do now.

======
Overall status:

1. ext4: patches submitted, waiting for reply from Ted Ts'o:
   https://lkml.org/lkml/2012/4/2/111
2. ext2: patches are in the ext2 tree maintained by Jan Kara:
   git://git.kernel.org/pub/scm/linux/kernel/git/jack/linux-fs.git for_next
   However, one patch is still not there:
   http://www.spinics.net/lists/linux-ext4/msg31492.html

TODO: affs, exofs, hfs, hfsplus, jffs2, reiserfs, sysv, udf, ufs
======

Thanks,
Artem.