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    SubjectRe: [LSF/MM TOPIC] Discuss least bad options for resolving longterm-GUP usage by RDMA
    On Fri, Feb 8, 2019 at 3:11 AM Jan Kara <jack@suse.cz> 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.

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