Re: [PATCH 0/3] iopmem : A block device for PCIe memory

[Christoph Hellwig]

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.