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Date:   Tue, 12 Mar 2019 05:23:21 +0000
From:   Christopher Lameter <>
To:     Dave Chinner <>
cc:, Andrew Morton <>,, Al Viro <>,
        Christian Benvenuti <>,
        Christoph Hellwig <>,
        Dan Williams <>,
        Dennis Dalessandro <>,
        Doug Ledford <>,
        Ira Weiny <>, Jan Kara <>,
        Jason Gunthorpe <>,
        Jerome Glisse <>,
        Matthew Wilcox <>,
        Michal Hocko <>,
        Mike Rapoport <>,
        Mike Marciniszyn <>,
        Ralph Campbell <>,
        Tom Talpey <>,
        LKML <>,, John Hubbard <>
Subject: Re: [PATCH v3 0/1] mm: introduce put_user_page*(), placeholder

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?

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