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

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Date:	Fri, 1 Feb 2008 11:23:57 -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 Fri, 1 Feb 2008, Andrea Arcangeli wrote:

> Note that my #v5 doesn't require to increase the page count all the
> time, so GRU will work fine with #v5.

But that comes with the cost of firing invalidate_page for every page 
being evicted. In order to make your single invalidate_range work without 
it you need to hold a refcount on the page.

> invalidate_page[s] is always called before the page is freed. This
> will require modifications to the tlb flushing code logic to take
> advantage of _pages in certain places. For now it's just safe.

Yes so your invalidate_range is still some sort of dysfunctional 
optimization? Gazillions of invalidate_page's will have to be executed 
when tearing down large memory areas.

> > How does KVM insure the consistency of the shadow page tables? Atomic ops?
> 
> A per-VM mmu_lock spinlock is taken to serialize the access, plus
> atomic ops for the cpu.

And that would not be enough to hold of new references? With small tweaks 
this should work with a common scheme. We could also redefine the role 
of _start and _end slightly to just require that the refs are removed when 
_end completes. That would allow the KVM page count ref to work as is now 
and would avoid the individual invalidate_page() callouts.

> > The GRU has no page table on its own. It populates TLB entries on demand 
> > using the linux page table. There is no way it can figure out when to 
> > drop page counts again. The invalidate calls are turned directly into tlb 
> > flushes.
> 
> Yes, this is why it can't serialize follow_page with only the PT lock
> with your patch. KVM may do it once you add start,end to range_end
> only thanks to the additional pin on the page.

Right but that pin requires taking a refcount which we cannot do.

Frankly this looks as if this is a solution that would work only for KVM.

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