Re: [PATCH v3 0/1] mm: introduce put_user_page*(), placeholder versions

[Christopher Lameter]

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?