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Date:   Tue, 12 Mar 2019 05:23:21 +0000
From:   Christopher Lameter <cl@linux.com>
To:     Dave Chinner <david@fromorbit.com>
cc:     john.hubbard@gmail.com, Andrew Morton <akpm@linux-foundation.org>,
        linux-mm@kvack.org, Al Viro <viro@zeniv.linux.org.uk>,
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        Christoph Hellwig <hch@infradead.org>,
        Dan Williams <dan.j.williams@intel.com>,
        Dennis Dalessandro <dennis.dalessandro@intel.com>,
        Doug Ledford <dledford@redhat.com>,
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        Jason Gunthorpe <jgg@ziepe.ca>,
        Jerome Glisse <jglisse@redhat.com>,
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        Michal Hocko <mhocko@kernel.org>,
        Mike Rapoport <rppt@linux.ibm.com>,
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        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 Mon, 11 Mar 2019, Dave Chinner wrote:

> > Direct IO on a mmapped file backed page doesnt make any sense.
>
> People have used it for many, many years as zero-copy data movement
> pattern. i.e. mmap the destination file, use direct IO to DMA direct
> into the destination file page cache pages, fdatasync() to force
> writeback of the destination file.

Well we could make that more safe through a special API that designates a
range of pages in a file in the same way as for RDMA. This is inherently
not reliable as we found out.

> Now we have copy_file_range() to optimise this sort of data
> movement, the need for games with mmap+direct IO largely goes away.
> However, we still can't just remove that functionality as it will
> break lots of random userspace stuff...

It is already broken and unreliable. Are there really "lots" of these
things around? Can we test this by adding a warning in the kernel and see
where it actually crops up?