Date: Fri, 21 Oct 2016 22:12:53 +1100
From: Dave Chinner <david@fromorbit.com>
To: Christoph Hellwig <hch@infradead.org>
Cc: Stephen Bates <sbates@raithlin.com>,
        Dan Williams <dan.j.williams@intel.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-nvdimm@lists.01.org" <linux-nvdimm@ml01.01.org>,
        linux-rdma@vger.kernel.org, linux-block@vger.kernel.org,
        Linux MM <linux-mm@kvack.org>,
        Ross Zwisler <ross.zwisler@linux.intel.com>,
        Matthew Wilcox <willy@linux.intel.com>,
        jgunthorpe@obsidianresearch.com, haggaie@mellanox.com,
        Jens Axboe <axboe@fb.com>, Jonathan Corbet <corbet@lwn.net>,
        jim.macdonald@everspin.com, sbates@raithin.com,
        Logan Gunthorpe <logang@deltatee.com>,
        David Woodhouse <dwmw2@infradead.org>,
        "Raj, Ashok" <ashok.raj@intel.com>
Subject: Re: [PATCH 0/3] iopmem : A block device for PCIe memory
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On Fri, Oct 21, 2016 at 02:57:14AM -0700, Christoph Hellwig wrote:
> 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.

Oh, that's good to know - pNFS over XFS was exactly what I was
thinking of when I wrote my earlier reply. A few months ago someone
else was trying to use file mappings in userspace for direct remote
client access on fabric connected devices. I told them "pNFS on XFS
and write an efficient transport for you hardware"....

Now that I know we've got RDMA support for pNFS on XFS in the
pipeline, I can just tell them "just write an rdma driver for your
hardware" instead. :P

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com