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

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Message-Id: <200805132206.47655.nickpiggin@yahoo.com.au>
Date:	Tue, 13 May 2008 22:06:44 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Robin Holt <holt@....com>
Cc:	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 Thursday 08 May 2008 10:38, Robin Holt wrote:
> On Wed, May 07, 2008 at 02:36:57PM -0700, Linus Torvalds wrote:
> > On Wed, 7 May 2008, Andrea Arcangeli wrote:
> > > I think the spinlock->rwsem conversion is ok under config option, as
> > > you can see I complained myself to various of those patches and I'll
> > > take care they're in a mergeable state the moment I submit them. What
> > > XPMEM requires are different semantics for the methods, and we never
> > > had to do any blocking I/O during vmtruncate before, now we have to.
> >
> > I really suspect we don't really have to, and that it would be better to
> > just fix the code that does that.
>
> That fix is going to be fairly difficult.  I will argue impossible.
>
> First, a little background.  SGI allows one large numa-link connected
> machine to be broken into seperate single-system images which we call
> partitions.
>
> XPMEM allows, at its most extreme, one process on one partition to
> grant access to a portion of its virtual address range to processes on
> another partition.  Those processes can then fault pages and directly
> share the memory.
>
> 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.
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