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

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Message-ID: <20080213012638.GD31435@obsidianresearch.com>
Date:	Tue, 12 Feb 2008 18:26:38 -0700
From:	Jason Gunthorpe <jgunthorpe@...idianresearch.com>
To:	Christoph Lameter <clameter@....com>
Cc:	Roland Dreier <rdreier@...co.com>, Rik van Riel <riel@...hat.com>,
	steiner@....com, Andrea Arcangeli <andrea@...ranet.com>,
	a.p.zijlstra@...llo.nl, izike@...ranet.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 Tue, Feb 12, 2008 at 05:01:17PM -0800, Christoph Lameter wrote:
> On Tue, 12 Feb 2008, Jason Gunthorpe wrote:
> 
> > Well, certainly today the memfree IB devices store the page tables in
> > host memory so they are already designed to hang onto packets during
> > the page lookup over PCIE, adding in faulting makes this time
> > larger.
> 
> You really do not need a page table to use it. What needs to be maintained 
> is knowledge on both side about what pages are currently shared across 
> RDMA. If the VM decides to reclaim a page then the notification is used to 
> remove the remote entry. If the remote side then tries to access the page 
> again then the page fault on the remote side will stall until the local 
> page has been brought back. RDMA can proceed after both sides again agree 
> on that page now being sharable.

The problem is that the existing wire protocols do not have a
provision for doing an 'are you ready' or 'I am not ready' exchange
and they are not designed to store page tables on both sides as you
propose. The remote side can send RDMA WRITE traffic at any time after
the RDMA region is established. The local side must be able to handle
it. There is no way to signal that a page is not ready and the remote
should not send.

This means the only possible implementation is to stall/discard at the
local adaptor when a RDMA WRITE is recieved for a page that has been
reclaimed. This is what leads to deadlock/poor performance..

Jason
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