Message-ID: <20080516115005.GC4287@sgi.com>
Date:	Fri, 16 May 2008 06:50:05 -0500
From:	Robin Holt <holt@....com>
To:	Nick Piggin <npiggin@...e.de>
Cc:	Christoph Lameter <clameter@....com>, Robin Holt <holt@....com>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrea Arcangeli <andrea@...ranet.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jack Steiner <steiner@....com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	kvm-devel@...ts.sourceforge.net,
	Kanoj Sarcar <kanojsarcar@...oo.com>,
	Roland Dreier <rdreier@...co.com>,
	Steve Wise <swise@...ngridcomputing.com>,
	linux-kernel@...r.kernel.org, Avi Kivity <avi@...ranet.com>,
	linux-mm@...ck.org, general@...ts.openfabrics.org,
	Hugh Dickins <hugh@...itas.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Anthony Liguori <aliguori@...ibm.com>,
	Chris Wright <chrisw@...hat.com>,
	Marcelo Tosatti <marcelo@...ck.org>,
	Eric Dumazet <dada1@...mosbay.com>,
	"Paul E. McKenney" <paulmck@...ibm.com>
Subject: Re: [PATCH 08 of 11] anon-vma-rwsem

On Fri, May 16, 2008 at 06:23:06AM -0500, Robin Holt wrote:
> On Fri, May 16, 2008 at 01:52:03AM +0200, Nick Piggin wrote:
> > On Thu, May 15, 2008 at 10:33:57AM -0700, Christoph Lameter wrote:
> > > On Thu, 15 May 2008, Nick Piggin wrote:
> > > 
> > > > Oh, I get that confused because of the mixed up naming conventions
> > > > there: unmap_page_range should actually be called zap_page_range. But
> > > > at any rate, yes we can easily zap pagetables without holding mmap_sem.
> > > 
> > > How is that synchronized with code that walks the same pagetable. These 
> > > walks may not hold mmap_sem either. I would expect that one could only 
> > > remove a portion of the pagetable where we have some sort of guarantee 
> > > that no accesses occur. So the removal of the vma prior ensures that?
> >  
> > I don't really understand the question. If you remove the pte and invalidate
> > the TLBS on the remote image's process (importing the page), then it can
> > of course try to refault the page in because it's vma is still there. But
> > you catch that refault in your driver , which can prevent the page from
> > being faulted back in.
> 
> I think Christoph's question has more to do with faults that are
> in flight.  A recently requested fault could have just released the
> last lock that was holding up the invalidate callout.  It would then
> begin messaging back the response PFN which could still be in flight.
> The invalidate callout would then fire and do the interrupt shoot-down
> while that response was still active (essentially beating the inflight
> response).  The invalidate would clear up nothing and then the response
> would insert the PFN after it is no longer the correct PFN.

I just looked over XPMEM.  I think we could make this work.  We already
have a list of active faults which is protected by a simple spinlock.
I would need to nest this lock within another lock protected our PFN
table (currently it is a mutex) and then the invalidate interrupt handler
would need to mark the fault as invalid (which is also currently there).

I think my sticking points with the interrupt method remain at fault
containment and timeout.  The inability of the ia64 processor to handle
provide predictive failures for the read/write of memory on other
partitions prevents us from being able to contain the failure.  I don't
think we can get the information we would need to do the invalidate
without introducing fault containment issues which has been a continous
area of concern for our customers.


Thanks,
Robin
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