linux-kernel - Re: [PATCH] mmu notifiers #v6

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Message-ID: <20080220123235.GX7128@v2.random>
Date:	Wed, 20 Feb 2008 13:32:36 +0100
From:	Andrea Arcangeli <andrea@...ranet.com>
To:	Robin Holt <holt@....com>
Cc:	Nick Piggin <npiggin@...e.de>, akpm@...ux-foundation.org,
	Avi Kivity <avi@...ranet.com>, Izik Eidus <izike@...ranet.com>,
	kvm-devel@...ts.sourceforge.net,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	general@...ts.openfabrics.org,
	Steve Wise <swise@...ngridcomputing.com>,
	Roland Dreier <rdreier@...co.com>,
	Kanoj Sarcar <kanojsarcar@...oo.com>, steiner@....com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	daniel.blueman@...drics.com, Christoph Lameter <clameter@....com>
Subject: Re: [PATCH] mmu notifiers #v6

On Wed, Feb 20, 2008 at 06:24:24AM -0600, Robin Holt wrote:
> We do not need to do any allocation in the messaging layer, all
> structures used for messaging are allocated at module load time.
> The allocation discussions we had early on were about trying to
> rearrange you notifiers to allow a seperate worker thread to do the
> invalidate and then the main thread would spin waiting for the worker to
> complete.  That was canned by the moving your notifier to before the
> lock was grabbed which led us to the point of needing a _begin and _end.

I thought you called some net/* function inside the mmu notifier
methods. Those always require several ram allocations internally.

> So, fundamentally, how would they be different?  Would we be required to
> add another notifier list to the mm and have two seperate callout
> points?  Reduction would end up with the same half-registered
> half-not-registered situation you point out above.  Then further
> reduction would lead to the elimination of the callouts you have just
> proposed and using the _begin/_end callouts and we are back to
> Christoph's current patch.

Did you miss Nick's argument that we'd need to change some VM lock to
mutex and solve lock issues first? Are you implying mutex are more
efficient for the VM? (you may seek support from preempt-rt folks at
least) or are you implying the VM would better run slower with mutex
in order to have a single config option?
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