On Tue, 15 Mar 2011 14:35:47 +1030
Christopher Yeoh [off-list ref] 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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