linux-kernel - allow building a kernel without buffer

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Message-Id: <20230720140452.63817-1-hch@lst.de>
Date:   Thu, 20 Jul 2023 16:04:46 +0200
From:   Christoph Hellwig <hch@....de>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     "Darrick J. Wong" <djwong@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        Christian Brauner <christian@...uner.io>,
        linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-xfs@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: allow building a kernel without buffer_heads

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