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

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Date:	Wed, 7 May 2008 17:56:17 -0700 (PDT)
From:	Christoph Lameter <clameter@....com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <andrea@...ranet.com>, steiner@....com,
	holt@....com, npiggin@...e.de, a.p.zijlstra@...llo.nl,
	kvm-devel@...ts.sourceforge.net, kanojsarcar@...oo.com,
	rdreier@...co.com, swise@...ngridcomputing.com,
	linux-kernel@...r.kernel.org, avi@...ranet.com, linux-mm@...ck.org,
	general@...ts.openfabrics.org, hugh@...itas.com,
	rusty@...tcorp.com.au, aliguori@...ibm.com, chrisw@...hat.com,
	marcelo@...ck.org, dada1@...mosbay.com, paulmck@...ibm.com
Subject: Re: [PATCH 08 of 11] anon-vma-rwsem

On Wed, 7 May 2008, Linus Torvalds wrote:

> On Wed, 7 May 2008, Christoph Lameter wrote:
> > 
> > Multiple vmas may share the same mapping or refer to the same anonymous 
> > vma. The above code will deadlock since we may take some locks multiple 
> > times.
> 
> Ok, so that actually _is_ a problem. It would be easy enough to also add 
> just a flag to the vma (VM_MULTILOCKED), which is still cleaner than doing 
> a vmalloc and a whole sort thing, but if this is really rare, maybe Ben's 
> suggestion of just using stop-machine is actually the right one just 
> because it's _so_ simple.

Set the vma flag when we locked it and then skip when we find it locked 
right? This would be in addition to the global lock?

stop-machine would work for KVM since its a once in a Guest OS time of 
thing. But GRU, KVM and eventually Infiniband need the ability to attach 
in a reasonable timeframe without causing major hiccups for other 
processes.

> (That said, we're not running out of vm flags yet, and if we were, we 
> could just add another word. We're already wasting that space right now on 
> 64-bit by calling it "unsigned long").

We sure have enough flags.

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