linux-kernel - Re: [PATCH] mmu notifiers #v5

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Message-ID: <Pine.LNX.4.64.0802042206200.6739@schroedinger.engr.sgi.com>
Date:	Mon, 4 Feb 2008 22:11:24 -0800 (PST)
From:	Christoph Lameter <clameter@....com>
To:	Andrea Arcangeli <andrea@...ranet.com>
cc:	Robin Holt <holt@....com>, Avi Kivity <avi@...ranet.com>,
	Izik Eidus <izike@...ranet.com>,
	kvm-devel@...ts.sourceforge.net,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>, steiner@....com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	daniel.blueman@...drics.com
Subject: Re: [PATCH] mmu notifiers #v5

On Tue, 5 Feb 2008, Andrea Arcangeli wrote:

> On Mon, Feb 04, 2008 at 11:09:01AM -0800, Christoph Lameter wrote:
> > On Sun, 3 Feb 2008, Andrea Arcangeli wrote:
> > 
> > > > Right but that pin requires taking a refcount which we cannot do.
> > > 
> > > GRU can use my patch without the pin. XPMEM obviously can't use my
> > > patch as my invalidate_page[s] are under the PT lock (a feature to fit
> > > GRU/KVM in the simplest way), this is why an incremental patch adding
> > > invalidate_range_start/end would be required to support XPMEM too.
> > 
> > Doesnt the kernel in some situations release the page before releasing the 
> > pte lock? Then there will be an external pte pointing to a page that may 
> > now have a different use. Its really bad if that pte does allow writes.
> 
> Sure the kernel does that most of the time, which is for example why I
> had to use invalidate_page instead of invalidate_pages inside
> zap_pte_range. Zero problems with that (this is also the exact reason
> why I mentioned the tlb flushing code would need changes to convert
> some page in pages).

Zero problems only if you find having a single callout for every page 
acceptable. So the invalidate_range in your patch is only working 
sometimes. And even if it works then it has to be used on 2M range. Seems 
to be a bit fragile and needlessly complex.

"conversion of some page in pages"? A proposal to defer the freeing of the 
pages until after the pte_unlock?

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