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

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

On Tue, 15 Mar 2011 14:35:47 +1030
Christopher Yeoh <cyeoh@au1.ibm.com> wrote:

> Hi Andrew,
> 
> I was wondering if you thought the cross memory attach patch is in
> suitable shape to go into your tree with view of getting it into
> mainline sometime in the not too distant future.

It looks reasonable to me, but I might have missed something and would
ask that some of the other guys take a close look, please.

It's regrettable that vmsplice() won't serve the purpose but I can see
that the blocking problems are there.

Minor thing: mm/memory.c is huge, and I think this new code would live
happily in a new mm/process_vm_access.c.

> There are some cases of MPI collectives where even a single copy
> interface does not get us the performance gain we could.  For example
> in an MPI_Reduce rather than copy the data from the source we would
> like to instead use it directly in a mathops (say the reduce is doing a
> sum) as this would save us doing a copy.  We don't need to keep a copy
> of the data from the source.  I haven't implemented this, but I think
> this interface could in the future do all this through the use of the
> flags - eg could specify the math operation and type and the kernel
> rather than just copying the data would apply the specified operation
> between the source and destination and store it in the destination.  

Well yes.  This smells like MAP_SHARED.

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 making
pages dirty, writing them back, etc so make that mmap be backed by a
new special device rather than by a regular file, perhaps.

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