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

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Date:	Tue, 5 Feb 2008 23:26:58 +0100
From:	Andrea Arcangeli <andrea@...ranet.com>
To:	Christoph Lameter <clameter@....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, Feb 05, 2008 at 02:06:23PM -0800, Christoph Lameter wrote:
> On Tue, 5 Feb 2008, Andrea Arcangeli wrote:
> 
> > On Tue, Feb 05, 2008 at 10:17:41AM -0800, Christoph Lameter wrote:
> > > The other approach will not have any remote ptes at that point. Why would 
> > > there be a coherency issue?
> > 
> > It never happens that two threads writes to two different physical
> > pages by working on the same process virtual address. This is an issue
> > only for KVM which is probably ok with it but certainly you can't
> > consider the dependency on the page-pin less fragile or less complex
> > than my PT lock approach.
> 
> You can avoid the page-pin and the pt lock completely by zapping the 
> mappings at _start and then holding off new references until _end.

Avoid the PT lock? The PT lock has to be taken anyway by the linux
VM.

"holding off new references until _end" = per-range mutex less scalar
and more expensive than the PT lock that has to be taken anyway.

> As I said the implementation is up to the caller. Not sure what 
> XPmem is using there but then XPmem is not using follow_page. The GRU 
> would be using a lightway way of locking not rbtrees.

"lightway way of locking" = mm-wide-mutex (not necessary at all if we
take advantage of the per-pte-scalar PT lock that has to be taken
anyway like in my patch)

> Maybe that is true for KVM but certainly not true for the GRU. The GRU is 
> designed to manage several petabytes of memory that may be mapped by a 
> series of Linux instances. If a process only maps a small chunk of 4 
> Gigabytes then we already have to deal with 1 mio callbacks.

KVM is also going to map a lot of stuff, but mapping involves mmap,
munmap/mremap/mprotect not. The size of mmap is irrelevant in both
approaches. optimizing do_exit by making the tlb-miss runtime slower
doesn't sound great to me and that's your patch does if you force GRU
to use it.
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