linux-kernel - Re: [ofa-general] Re: Demand paging for memory regions

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Date:	Wed, 13 Feb 2008 17:01:03 -0700
From:	Jason Gunthorpe <jgunthorpe@...idianresearch.com>
To:	Pete Wyckoff <pw@....edu>
Cc:	Christoph Lameter <clameter@....com>,
	Rik van Riel <riel@...hat.com>,
	Andrea Arcangeli <andrea@...ranet.com>, a.p.zijlstra@...llo.nl,
	izike@...ranet.com, Roland Dreier <rdreier@...co.com>,
	steiner@....com, linux-kernel@...r.kernel.org, avi@...ranet.com,
	linux-mm@...ck.org, daniel.blueman@...drics.com,
	Robin Holt <holt@....com>, general@...ts.openfabrics.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	kvm-devel@...ts.sourceforge.net
Subject: Re: [ofa-general] Re: Demand paging for memory regions

On Wed, Feb 13, 2008 at 06:23:08PM -0500, Pete Wyckoff wrote:
> christian.bell@...gic.com wrote on Tue, 12 Feb 2008 20:09 -0800:
> > One other area that has not been brought up yet (I think) is the
> > applicability of notifiers in letting users know when pinned memory
> > is reclaimed by the kernel.  This is useful when a lower-level
> > library employs lazy deregistration strategies on memory regions that
> > are subsequently released to the kernel via the application's use of
> > munmap or sbrk.  Ohio Supercomputing Center has work in this area but
> > a generalized approach in the kernel would certainly be welcome.
> 
> The whole need for memory registration is a giant pain.  There is no
> motivating application need for it---it is simply a hack around
> virtual memory and the lack of full VM support in current hardware.
> There are real hardware issues that interact poorly with virtual
> memory, as discussed previously in this thread.

Well, the registrations also exist to provide protection against
rouge/faulty remotes, but for the purposes of MPI that is probably not
important.

Here is a thought.. Some RDMA hardware can change the page tables on
the fly. What if the kernel had a mechanism to dynamically maintain a
full registration of the processes entire address space ('mlocked' but
able to be migrated)? MPI would never need to register a buffer, and
all the messy cases with munmap/sbrk/etc go away - the risk is that
other MPI nodes can randomly scribble all over the process :)

Christoph: It seemed to me you were first talking about
freeing/swapping/faulting RDMA'able pages - but would pure migration
as a special hardware supported case be useful like Catilan suggested?

Regards,
Jason
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