linux-kernel - Re: [PATCH] mmu notifiers #v8

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Date:	Mon, 3 Mar 2008 14:24:59 +0100
From:	Andrea Arcangeli <andrea@...ranet.com>
To:	Nick Piggin <npiggin@...e.de>
Cc:	Jack Steiner <steiner@....com>, akpm@...ux-foundation.org,
	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>,
	general@...ts.openfabrics.org,
	Steve Wise <swise@...ngridcomputing.com>,
	Roland Dreier <rdreier@...co.com>,
	Kanoj Sarcar <kanojsarcar@...oo.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	daniel.blueman@...drics.com, Christoph Lameter <clameter@....com>
Subject: Re: [PATCH] mmu notifiers #v8

On Mon, Mar 03, 2008 at 02:10:17PM +0100, Nick Piggin wrote:
> Is this just a GRU problem? Can't we just require them to take a ref
> on the page (IIRC Jack said GRU could be changed to more like a TLB
> model).

Yes, it's just a GRU problem, it tries to optimize performance by
calling follow_page only in the fast path, and fallbacks to
get_user_pages; put_page in the slow path. xpmem could also send the
message in _begin and wait the message in _end, to reduce the wait
time. But if you forge GRU to call get_user_pages only (like KVM
does), the _begin can be removed. In theory we could also optimize KVM
to use follow_page only if the pte is already established. I'm not
sure how much that is a worthwhile optimization though.

However note that Quadrics also had a callback before and one after,
so they may be using the callback before for similar
optimizations. But functionality-wise _end is the only required bit if
everyone takes refcounts like KVM and XPMEM do.
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