Date: Mon, 3 Mar 2008 14:24:59 +0100
From: Andrea Arcangeli <andrea@qumranet.com>
To: Nick Piggin <npiggin@suse.de>
Cc: Jack Steiner <steiner@sgi.com>, akpm@linux-foundation.org,
       Robin Holt <holt@sgi.com>, Avi Kivity <avi@qumranet.com>,
       Izik Eidus <izike@qumranet.com>, kvm-devel@lists.sourceforge.net,
       Peter Zijlstra <a.p.zijlstra@chello.nl>, general@lists.openfabrics.org,
       Steve Wise <swise@opengridcomputing.com>,
       Roland Dreier <rdreier@cisco.com>, Kanoj Sarcar <kanojsarcar@yahoo.com>,
       linux-kernel@vger.kernel.org, linux-mm@kvack.org,
       daniel.blueman@quadrics.com, Christoph Lameter <clameter@sgi.com>
Subject: Re: [PATCH] mmu notifiers #v8
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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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