[PATCH 0/12] make ->sync_fs mandatory

[Christoph Hellwig <hch@lst.de>]

Currently the superblock methods are a bit confusing.  There is the
newer ->sync_fs method which is unconditionally called from
__sync_filesystem for data integrity writebacks.  And there is the older
->write_super which is called from pdflush for the periodic superblock
writeback, but also from __sync_filesystem just before calling
->sync_fs. (And still from file_fsync, but that is about to go away).

This series makes sure all filesystems that need superblock writeouts
define a ->sync_fs so we can stop calling ->write_super for the data
integrity writeback.  This means the presence of ->write_super indicates
a filesystem does want periodic superblock writeback and can implement
these independently from the data integrity writeback. It also means
we don't need to bother with possible s_dirt races for the data
integrity syncs.

This patch series first adds ->sync_fs methods to all filesystems having
->write_super, often refactoring the code in that are (and also
preparing the ->write_super instances for moving MS_RDONLY checking into
the caller), then it makes sure the existing ->sync_fs instaces cover
the work previously done in ->write_super for the two cases where they
differed widely, and lastly removes the call to ->write_super from
__sync_filesystem.