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Date: Fri, 21 Oct 2016 02:57:14 -0700
From: Christoph Hellwig <hch@infradead.org>
Subject: Re: [PATCH 0/3] iopmem : A block device for PCIe memory
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To: Dave Chinner <david@fromorbit.com>
Cc: jgunthorpe@obsidianresearch.com, sbates@raithin.com, "Raj, Ashok  <ashok.raj@intel.com>, haggaie@mellanox.com, linux-rdma@vger.kernel.org, linux-nvdimm@lists.01.org" <linux-nvdimm@ml01.01.org>, Jonathan Corbet <corbet@lwn.net>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, jim.macdonald@everspin.com, Stephen Bates <sbates@raithlin.com>, linux-block@vger.kernel.org, Linux MM <linux-mm@kvack.org>, Jens Axboe <axboe@fb.com>, Christoph Hellwig <hch@infradead.org>, David Woodhouse <dwmw2@infradead.org>
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On Fri, Oct 21, 2016 at 10:22:39AM +1100, Dave Chinner wrote:
> You do realise that local filesystems can silently change the
> location of file data at any point in time, so there is no such
> thing as a "stable mapping" of file data to block device addresses
> in userspace?
> 
> If you want remote access to the blocks owned and controlled by a
> filesystem, then you need to use a filesystem with a remote locking
> mechanism to allow co-ordinated, coherent access to the data in
> those blocks. Anything else is just asking for ongoing, unfixable
> filesystem corruption or data leakage problems (i.e.  security
> issues).

And at least for XFS we have such a mechanism :)  E.g. I have a
prototype of a pNFS layout that uses XFS+DAX to allow clients to do
RDMA directly to XFS files, with the same locking mechanism we use
for the current block and scsi layout in xfs_pnfs.c.
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