Re: [LSF/MM TOPIC] Discuss least bad options for resolving longterm-GUP usage by RDMA

[Dan Williams]

On Fri, Feb 8, 2019 at 3:11 AM Jan Kara <jack@xxxxxxx> wrote:
>
> On Fri 08-02-19 15:43:02, Dave Chinner wrote:
> > On Thu, Feb 07, 2019 at 04:55:37PM +0000, Christopher Lameter wrote:
> > > One approach that may be a clean way to solve this:
> > > 3. Filesystems that allow bypass of the page cache (like XFS / DAX) will
> > >    provide the virtual mapping when the PIN is done and DO NO OPERATIONS
> > >    on the longterm pinned range until the long term pin is removed.
> >
> > So, ummm, how do we do block allocation then, which is done on
> > demand during writes?
> >
> > IOWs, this requires the application to set up the file in the
> > correct state for the filesystem to lock it down so somebody else
> > can write to it.  That means the file can't be sparse, it can't be
> > preallocated (i.e. can't contain unwritten extents), it must have zeroes
> > written to it's full size before being shared because otherwise it
> > exposes stale data to the remote client (secure sites are going to
> > love that!), they can't be extended, etc.
> >
> > IOWs, once the file is prepped and leased out for RDMA, it becomes
> > an immutable for the purposes of local access.
> >
> > Which, essentially we can already do. Prep the file, map it
> > read/write, mark it immutable, then pin it via the longterm gup
> > interface which can do the necessary checks.
>
> Hum, and what will you do if the immutable file that is target for RDMA
> will be a source of reflink? That seems to be currently allowed for
> immutable files but RDMA store would be effectively corrupting the data of
> the target inode. But we could treat it similarly as swapfiles - those also
> have to deal with writes to blocks beyond filesystem control. In fact the
> similarity seems to be quite large there. What do you think?

This sounds so familiar...

    https://lwn.net/Articles/726481/

I'm not opposed to trying again, but leases was what crawled out
smoking crater when this last proposal was nuked.