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Date: Fri, 10 Nov 2017 10:08:18 +0100
From: Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH 14/15] dax: associate mappings with inodes, and warn if
        dma collides with truncate
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To: Dan Williams <dan.j.williams@intel.com>
Cc: linux-nvdimm@lists.01.org, Jan Kara <jack@suse.cz>, Matthew Wilcox <mawilcox@microsoft.com>, linux-kernel@vger.kernel.org, linux-xfs@vger.kernel.org, linux-mm@kvack.org, Jeff Moyer <jmoyer@redhat.com>, Ross Zwisler <ross.zwisler@linux.intel.com>, linux-fsdevel@vger.kernel.org, akpm@linux-foundation.org, hch@lst.de

> +		struct {
> +			/*
> +			 * ZONE_DEVICE pages are never on an lru or handled by
> +			 * a slab allocator, this points to the hosting device
> +			 * page map.
> +			 */
> +			struct dev_pagemap *pgmap;
> +			/*
> +			 * inode association for MEMORY_DEVICE_FS_DAX page-idle
> +			 * callbacks. Note that we don't use ->mapping since
> +			 * that has hard coded page-cache assumptions in
> +			 * several paths.
> +			 */

What assumptions?  I'd much rather fix those up than having two fields
that have the same functionality.