linux-kernel - Re: [PATCH 08 of 11] anon-vma-rwsem

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Date:	Fri, 16 May 2008 06:23:06 -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 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.

Thanks,
Robin
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