Hi Dave and Christoph On Fri, Oct 21, 2016 at 10:12:53PM +1100, Dave Chinner wrote: > 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). > > Dave are you saying that even for local mappings of files on a DAX capable system it is possible for the mappings to move on you unless the FS supports locking? Does that not mean DAX on such FS is inherently broken? > > 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. > Thanks for fixing this issue on XFS Christoph! I assume this problem continues to exist on the other DAX capable FS? One more reason to consider a move to /dev/dax I guess ;-)... Stephen > 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@xxxxxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-block" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
