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

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Date:   Thu, 7 Feb 2019 10:26:22 -0700
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Doug Ledford <dledford@...hat.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Chinner <david@...morbit.com>,
        Christopher Lameter <cl@...ux.com>, Jan Kara <jack@...e.cz>,
        Ira Weiny <ira.weiny@...el.com>,
        lsf-pc@...ts.linux-foundation.org,
        linux-rdma <linux-rdma@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        John Hubbard <jhubbard@...dia.com>,
        Jerome Glisse <jglisse@...hat.com>,
        Michal Hocko <mhocko@...nel.org>
Subject: Re: [LSF/MM TOPIC] Discuss least bad options for resolving
 longterm-GUP usage by RDMA

On Thu, Feb 07, 2019 at 09:24:05AM -0800, Matthew Wilcox wrote:
> On Thu, Feb 07, 2019 at 11:25:35AM -0500, Doug Ledford wrote:
> > * Really though, as I said in my email to Tom Talpey, this entire
> > situation is simply screaming that we are doing DAX networking wrong. 
> > We shouldn't be writing the networking code once in every single
> > application that wants to do this.  If we had a memory segment that we
> > shared from server to client(s), and in that memory segment we
> > implemented a clustered filesystem, then applications would simply mmap
> > local files and be done with it.  If the file needed to move, the kernel
> > would update the mmap in the application, done.  If you ask me, it is
> > the attempt to do this the wrong way that is resulting in all this
> > heartache.  That said, for today, my recommendation would be to require
> > ODP hardware for XFS filesystem with the DAX option, but allow ext2
> > filesystems to mount DAX filesystems on non-ODP hardware, and go in and
> > modify the ext2 filesystem so that on DAX mounts, it disables hole punch
> > and ftrunctate any time they would result in the forced removal of an
> > established mmap.
> 
> I agree that something's wrong, but I think the fundamental problem is
> that there's no concept in RDMA of having an STag for storage rather
> than for memory.
> 
> Imagine if we could associate an STag with a file descriptor on the
> server.  The client could then perform an RDMA to that STag.  On the
> server, we'd need lots of smarts in the card and in the OS to know how
> to treat that packet on arrival -- depending on what the file descriptor
> referred to, it might only have to write into the page cache, or it
> might set up an NVMe DMA, or it might resolve the underlying physical
> address and DMA directly to an NV-DIMM.

I think you just described ODP MRs.

Jason