Re: [Resend] Cross Memory Attach v3 [PATCH]

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 17 Mar 2011 15:40:26 +1030
Christopher Yeoh <cyeoh@au1.ibm.com> wrote:

> > Thinking out loud: if we had a way in which a process can add and
> > remove a local anonymous page into pagecache then other processes
> > could access that page via mmap.  If both processes map the file with
> > a nonlinear vma they they can happily sit there flipping pages into
> > and out of the shared mmap at arbitrary file offsets.  The details
> > might get hairy ;) We wouldn't want all the regular mmap semantics of
> 
> Yea, its the complexity of trying to do it that way that eventually lead me
> to implementing it via a syscall and get_user_pages instead, trying to 
> keep things as simple as possible.

The pagecache trick potentially gives zero-copy access, whereas the
proposed code is single-copy.  Although the expected benefits of that
may not be so great due to TLB manipulation overheads.

I worry that one day someone will come along and implement the
pagecache trick, then we're stuck with obsolete code which we have to
maintain for ever.

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