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From: Dan Williams <dan.j.williams@intel.com>
Date: Thu, 24 Aug 2017 13:26:55 -0700
Message-ID: <CAPcyv4j06mdEek-aYfZCbTW0MaL6gy7OpUsgyv4hBB5yy_rW6A@mail.gmail.com>
Subject: Re: [PATCH v6 4/5] fs, xfs: introduce MAP_DIRECT for creating
 block-map-atomic file ranges
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To: Christoph Hellwig <hch@lst.de>
Cc: Andrew Morton <akpm@linux-foundation.org>, Jan Kara <jack@suse.cz>, "Darrick J. Wong" <darrick.wong@oracle.com>, Linux API <linux-api@vger.kernel.org>, "linux-nvdimm@lists.01.org" <linux-nvdimm@lists.01.org>, Dave Chinner <david@fromorbit.com>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, linux-xfs@vger.kernel.org, Linux MM <linux-mm@kvack.org>, Jeff Moyer <jmoyer@redhat.com>, Alexander Viro <viro@zeniv.linux.org.uk>, Andy Lutomirski <luto@kernel.org>, linux-fsdevel <linux-fsdevel@vger.kernel.org>, Ross Zwisler <ross.zwisler@linux.intel.com>, xen-devel@lists.xen.org

On Thu, Aug 24, 2017 at 9:39 AM, Christoph Hellwig <hch@lst.de> wrote:
> On Thu, Aug 24, 2017 at 09:31:17AM -0700, Dan Williams wrote:
>> External agent is a DMA device, or a hypervisor like Xen. In the DMA
>> case perhaps we can use the fcntl lease mechanism, I'll investigate.
>> In the Xen case it actually would need to use fiemap() to discover the
>> physical addresses that back the file to setup their M2P tables.
>> Here's the discussion where we discovered that physical address
>> dependency:
>>
>>     https://lists.xen.org/archives/html/xen-devel/2017-04/msg00419.html
>
> fiemap does not work to discover physical addresses.  If they want
> to do anything involving physical address they will need a kernel
> driver.

True, it's broken with respect to multi-device filesystems and these
patches do nothing to fix that problem. Ok, I'm fine to let that use
case depend on a kernel driver and just focus on fixing the DMA case.