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Date:   Wed, 13 Mar 2019 19:21:37 +0000
From:   Christopher Lameter <cl@linux.com>
To:     Christoph Hellwig <hch@infradead.org>
cc:     Dave Chinner <david@fromorbit.com>,
        Ira Weiny <ira.weiny@intel.com>, john.hubbard@gmail.com,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        Al Viro <viro@zeniv.linux.org.uk>,
        Christian Benvenuti <benve@cisco.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Dennis Dalessandro <dennis.dalessandro@intel.com>,
        Doug Ledford <dledford@redhat.com>, Jan Kara <jack@suse.cz>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        Jerome Glisse <jglisse@redhat.com>,
        Matthew Wilcox <willy@infradead.org>,
        Michal Hocko <mhocko@kernel.org>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Mike Marciniszyn <mike.marciniszyn@intel.com>,
        Ralph Campbell <rcampbell@nvidia.com>,
        Tom Talpey <tom@talpey.com>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-fsdevel@vger.kernel.org, John Hubbard <jhubbard@nvidia.com>
Subject: Re: [PATCH v3 0/1] mm: introduce put_user_page*(), placeholder
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On Wed, 13 Mar 2019, Christoph Hellwig wrote:

> On Wed, Mar 13, 2019 at 09:11:13AM +1100, Dave Chinner wrote:
> > On Tue, Mar 12, 2019 at 03:39:33AM -0700, Ira Weiny wrote:
> > > IMHO I don't think that the copy_file_range() is going to carry us through the
> > > next wave of user performance requirements.  RDMA, while the first, is not the
> > > only technology which is looking to have direct access to files.  XDP is
> > > another.[1]
> >
> > Sure, all I doing here was demonstrating that people have been
> > trying to get local direct access to file mappings to DMA directly
> > into them for a long time. Direct Io games like these are now
> > largely unnecessary because we now have much better APIs to do
> > zero-copy data transfer between files (which can do hardware offload
> > if it is available!).
>
> And that is just the file to file case.  There are tons of other
> users of get_user_pages, including various drivers that do large
> amounts of I/O like video capture.  For them it makes tons of sense
> to transfer directly to/from a mmap()ed file.

That is very similar to the RDMA case and DAX etc. We need to have a way
to tell a filesystem that this is going to happen and that things need to
be setup for this to work properly.

But if that has not been done then I think its proper to fail a long term
pin operation on page cache pages. Meaning the regular filesystems
maintain control of whats happening with their pages.