linux-kernel - Re: [PATCH] mmu notifiers #v5

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Date:	Tue, 5 Feb 2008 16:12:22 -0600
From:	Robin Holt <holt@....com>
To:	Christoph Lameter <clameter@....com>
Cc:	Andrea Arcangeli <andrea@...ranet.com>, Robin Holt <holt@....com>,
	Avi Kivity <avi@...ranet.com>, Izik Eidus <izike@...ranet.com>,
	kvm-devel@...ts.sourceforge.net,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>, steiner@....com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	daniel.blueman@...drics.com
Subject: Re: [PATCH] mmu notifiers #v5

On Tue, Feb 05, 2008 at 02:06:23PM -0800, Christoph Lameter wrote:
> On Tue, 5 Feb 2008, Andrea Arcangeli wrote:
> 
> > On Tue, Feb 05, 2008 at 10:17:41AM -0800, Christoph Lameter wrote:
> > > The other approach will not have any remote ptes at that point. Why would 
> > > there be a coherency issue?
> > 
> > It never happens that two threads writes to two different physical
> > pages by working on the same process virtual address. This is an issue
> > only for KVM which is probably ok with it but certainly you can't
> > consider the dependency on the page-pin less fragile or less complex
> > than my PT lock approach.
> 
> You can avoid the page-pin and the pt lock completely by zapping the 
> mappings at _start and then holding off new references until _end.

XPMEM is doing this by putting our equivalent structure of the mm into a
recalling state which will cause all future faulters to back off, it then
marks any currently active faults in the range as invalid (we have a very
small number of possible concurrent faulters for a different reason),
proceeds to start remote shoot-downs, waits for those shoot-downs to
complete, then returns from the _begin callout with the mm-equiv still in
the recalling state.  Additional recalls may occur, but no new faults can.
The _end callout reduces the number of active recalls until there are
none left at which point the faulters are allowed to proceed again.

Thanks,
Robin
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