Re: allow building a kernel without buffer_heads

[Bob Peterson <rpeterso@redhat.com>]

On 7/20/23 9:04 AM, Christoph Hellwig wrote:
> Hi all,
> 
> This series allows to build a kernel without buffer_heads, which I
> think is useful to show where the dependencies are, and maybe also
> for some very much limited environments, where people just needs
> xfs and/or btrfs and some of the read-only block based file systems.
> 
> It first switches buffered writes (but not writeback) for block devices
> to use iomap unconditionally, but still using buffer_heads, and 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.  This latter implementation
> will also be useful to support block size > PAGE_SIZE for block device
> nodes as buffer_heads won't work very well for that.
> 
> Note that for now the md software raid drivers is also disabled as it has
> some (rather questionable) buffer_head usage in the unconditionally built
> bitmap code.  I have a series pending to make the bitmap code conditional
> and deprecated it, but it hasn't been merged yet.
> 
> Changes since v1:
>   - drop the already merged prep patches
>   - depend on FS_IOMAP not IOMAP
>   - pick a better new name for block_page_mkwrite_return
> 
Hi Christoph,

Gfs2 still uses buffer_heads to manage the metadata being pushed through 
its journals. We've been reducing our dependency on them but eliminating 
them altogether is a large and daunting task. We can still work toward 
that goal, but it will take time.

Bob Peterson