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Date:   Fri, 27 May 2022 14:45:12 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Christoph Hellwig <hch@infradead.org>
Cc:     jfs-discussion@lists.sourceforge.net,
        linux-fsdevel@vger.kernel.org,
        "Darrick J . Wong" <djwong@kernel.org>
Subject: Re: [RFC PATCH 2/9] jfs: Add jfs_iomap_begin()
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On Thu, May 26, 2022 at 10:41:31PM -0700, Christoph Hellwig wrote:
> 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.

I think JFS does support unwritten extents,
fs/jfs/jfs_xtree.h:#define XAD_NOTRECORDED 0x08 /* allocated but not recorded */

However, we always pass 'false' to extAlloc() today, so I think it
hasn't been tested in a while?  I'm not sure I want to be the one to
start using new features on JFS for something that's supposed to be
a relatively quick cleanup.