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Date:   Thu, 26 May 2022 22:41:31 -0700
From:   Christoph Hellwig <hch@infradead.org>
To:     "Matthew Wilcox (Oracle)" <willy@infradead.org>
Cc:     jfs-discussion@lists.sourceforge.net,
        linux-fsdevel@vger.kernel.org,
        Christoph Hellwig <hch@infradead.org>,
        "Darrick J . Wong" <djwong@kernel.org>
Subject: Re: [RFC PATCH 2/9] jfs: Add jfs_iomap_begin()
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I suspect this might be where your problems lies:

blockdev_direct_IO calls __blockdev_direct_IO with DIO_SKIP_HOLES set.
DIO_SKIP_HOLES causes get_more_blocks to never set the create bit
to get_block except for writes beyond i_size.  If we want to replicate
that behavior with iomap, ->iomap_begin needs to return -ENOTBLK
when it encounters a hole for writing.  To properly supporting writing
to holes we'd need unwritten extents, which jfs does not support.
gfs2 might be a place to look for how to implement this.