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

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Date:	Tue, 13 May 2008 10:32:38 -0500
From:	Robin Holt <holt@....com>
To:	Nick Piggin <nickpiggin@...oo.com.au>
Cc:	Robin Holt <holt@....com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrea Arcangeli <andrea@...ranet.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Lameter <clameter@....com>,
	Jack Steiner <steiner@....com>, Nick Piggin <npiggin@...e.de>,
	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 Tue, May 13, 2008 at 10:06:44PM +1000, Nick Piggin wrote:
> On Thursday 08 May 2008 10:38, Robin Holt wrote:
> > In order to invalidate the remote page table entries, we need to message
> > (uses XPC) to the remote side.  The remote side needs to acquire the
> > importing process's mmap_sem and call zap_page_range().  Between the
> > messaging and the acquiring a sleeping lock, I would argue this will
> > require sleeping locks in the path prior to the mmu_notifier invalidate_*
> > callouts().
> 
> Why do you need to take mmap_sem in order to shoot down pagetables of
> the process? It would be nice if this can just be done without
> sleeping.

We are trying to shoot down page tables of a different process running
on a different instance of Linux running on Numa-link connected portions
of the same machine.

The messaging is clearly going to require sleeping.  Are you suggesting
we need to rework XPC communications to not require sleeping?  I think
that is going to be impossible since the transfer engine requires a
sleeping context.

Additionally, the call to zap_page_range expects to have the mmap_sem
held.  I suppose we could use something other than zap_page_range and
atomically clear the process page tables.  Doing that will not alleviate
the need to sleep for the messaging to the other partitions.

Thanks,
Robin
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