From mboxrd@z Thu Jan  1 00:00:00 1970
From: Christoph Hellwig <hch@lst.de>
Date: Thu, 24 May 2018 20:13:33 +0200
Subject: [Cluster-devel] gfs2 iomap: BUG_ON(buffer_unmapped) in
	submit_bh_wbc
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Message-ID: <20180524181333.GA24254@lst.de>
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On Thu, May 24, 2018 at 08:05:03PM +0200, Andreas Gruenbacher wrote:
> Approach (1) is quite trivial:
> 
> --- a/fs/buffer.c
> +++ b/fs/buffer.c
> @@ -1908,7 +1908,7 @@ iomap_to_bh(struct inode *inode, sector_t,
> struct buffer_head *bh,
>                 set_buffer_unwritten(bh);
>                 /* FALLTHRU */
>         case IOMAP_MAPPED:
> -               if (offset >= i_size_read(inode))
> +               if ((iomap->flags & IOMAP_F_NEW) || offset >=
> i_size_read(inode))
>                         set_buffer_new(bh);
>                 bh->b_blocknr = (iomap->addr + offset - iomap->offset) >>
>                                 inode->i_blkbits;
> 

This is the right thing to do.  We just didn't need to do it for XFS
as we'll never allocate normal blocks in write_begin.

You'll probably need similar tweaks in my new iomap write code, which
you're hopefully lookin into as well.