RFC: allow building a kernel without buffer_heads

[Christoph Hellwig]

Hi all,

after all the talk about removing buffer_heads, here is a series that
shows how to build a kernel without buffer_heads.  And how unrealistic
it is to remove the entirely.

Most of the series refactors some common code to make implementing direct
I/O easier without use of the ->direct_IO method and the helpers based
around it.  It then switches buffered writes (but not writeback) for
block devices to use iomap unconditionally, but still using buffer_heads.

The final patch then adds a CONFIG_BUFFER_HEAD selected by all file
systems that need it (which is most block based file systems), makes the
buffer_head support in iomap optional, and adds an alternative
implementation of the block device address_operations using iomap.

With this you can build a kernel with block device support, but without
buffer_heads.  This kernel supports xfs and btrfs as full blown block
based filesystems, and a bunch of read-only ones like cramfs, erofs and
squashfs.  Note that the md software raid drivers is also disabled as it
has some (rather questionable) buffer_head usage in the unconditionally
built bitmap code.

The series is based on Linux 6.3 and will need some rebasing before it
can be fed to the maintainers incrementally.  All but the last patch
definitively seem useful for me.  The last one I think is just to avoid
introducing new buffer_head dependencies, even if I suspect the real
life usefulness of a !CONFIG_BUFFER_HEAD kernel might be rather limited.