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Date:   Thu, 14 Mar 2019 09:57:18 -0300
From:   Jason Gunthorpe <jgg@ziepe.ca>
To:     Jan Kara <jack@suse.cz>
Cc:     Christopher Lameter <cl@linux.com>,
        Jerome Glisse <jglisse@redhat.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>,
        Christoph Hellwig <hch@infradead.org>,
        Dan Williams <dan.j.williams@intel.com>,
        Dave Chinner <david@fromorbit.com>,
        Dennis Dalessandro <dennis.dalessandro@intel.com>,
        Doug Ledford <dledford@redhat.com>,
        Ira Weiny <ira.weiny@intel.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 Thu, Mar 14, 2019 at 10:03:45AM +0100, Jan Kara wrote:
> On Wed 13-03-19 19:16:51, Christopher Lameter wrote:
> > On Tue, 12 Mar 2019, Jerome Glisse wrote:
> > 
> > > > > This has been discuss extensively already. GUP usage is now widespread in
> > > > > multiple drivers, removing that would regress userspace ie break existing
> > > > > application. We all know what the rules for that is.
> > 
> > You are still misstating the issue. In RDMA land GUP is widely used for
> > anonyous memory and memory based filesystems. *Not* for real filesystems.
> 
> Maybe in your RDMA land. But there are apparently other users which do use
> mmap of a file on normal filesystem (e.g. ext4) as a buffer for DMA
> (Infiniband does not prohibit this if nothing else, video capture devices
> also use very similar pattern of gup-ing pages and using them as video
> buffers). And these users are reporting occasional kernel crashes. That's
> how this whole effort started. Sadly the DMA to file mmap is working good
> enough that people started using it so at this point we cannot just tell:
> Sorry it was a mistake to allow this, just rewrite your applications.

This is where we are in RDMA too.. People are trying it and the ones
that do enough load testing find their kernel OOPs

So it is not clear at all if this has graduated to a real use, or just
an experiment. Perhaps there are some system configurations that don't
trigger crashes..

Jason