linux-kernel - Re: [PATCH v7 7/8] fsdax: Introduce dax_iomap

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Date:   Mon, 23 Aug 2021 15:02:48 +0200
From:   Christoph Hellwig <hch@....de>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     "ruansy.fnst" <ruansy.fnst@...itsu.com>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Christoph Hellwig <hch@....de>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        david <david@...morbit.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux NVDIMM <nvdimm@...ts.linux.dev>,
        Goldwyn Rodrigues <rgoldwyn@...e.de>,
        Al Viro <viro@...iv.linux.org.uk>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH v7 7/8] fsdax: Introduce dax_iomap_ops for end of
 reflink

On Fri, Aug 20, 2021 at 08:18:44AM -0700, Dan Williams wrote:
> I notice that the @iomap argument to ->iomap_end() is reliably coming
> from @iter. So you could do the following in your iomap_end()
> callback:
> 
>         struct iomap_iter *iter = container_of(iomap, typeof(*iter), iomap);
>         struct xfs_inode *ip = XFS_I(inode);
>         ssize_t written = iter->processed;
>         bool cow = xfs_is_cow_inode(ip);
> 
>         if (cow) {
>                 if (written <= 0)
>                         xfs_reflink_cancel_cow_range(ip, pos, length, true)
>         }

I think this might be ok for now (with a big comment).  Willy's original
iomap iter series replaced the iomap_begin and iomap_end with a single
next callback that takes the iomap_iter, which would solve this issue.
My plan is to look into a series that implements this and sees if this
is indeed a net win.