From mboxrd@z Thu Jan  1 00:00:00 1970
From: Jerome Glisse <j.glisse@gmail.com>
Subject: Re: [RFC] Heterogeneous memory management (mirror process address
 space on a device mmu).
Date: Tue, 6 May 2014 14:13:12 -0400
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Cc: Peter Zijlstra <peterz@infradead.org>, linux-mm <linux-mm@kvack.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Mel Gorman <mgorman@suse.de>, "H. Peter Anvin" <hpa@zytor.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Linda Wang <lwang@redhat.com>, Kevin E Martin <kem@redhat.com>,
	Jerome Glisse <jglisse@redhat.com>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Johannes Weiner <jweiner@redhat.com>,
	Larry Woodman <lwoodman@redhat.com>, Rik van Riel <riel@redhat.com>,
	Dave Airlie <airlied@redhat.com>, Jeff Law <law@redhat.com>,
	Brendan Conoboy <blc@redhat.com>, Joe Donohue <jdonohue@redhat.com>,
	Duncan Poole <dpoole@nvidia.com>,
	Sherry Cheung <SCheung@nvidia.com>,
	Subhash Gutti <sgutti@nvidia.com>,
	John Hubbard <jhubbard@nvidia.com>,
	Mark Hairgrove <mhairgrove@nvidia.com>,
	Lucien Dunning <ldunning@nvidia.com>,
To: Linus Torvalds <torvalds@linux-foundation.org>
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On Tue, May 06, 2014 at 10:43:22AM -0700, Linus Torvalds wrote:
> On Tue, May 6, 2014 at 10:28 AM, Jerome Glisse <j.glisse@gmail.com> wro=
te:
> >
> > Also, just to be sure, are my changes to the radix tree otherwise
> > acceptable at least in principle.
>=20
> It looks like it adds several "loop over each page" cases just to
> check whether each page might be on remote memory.
>=20
> Which seems a complete waste of time 99.99% of the time.
>=20
> But maybe I'm looking at the wrong patch.
>=20
>           Linus

It is patch 8 for core changes and patch 9 to demonstrate per fs changes.

So yes each place that does radix tree lookup need to check that the entr=
ies
it got out of the radix tree are not special one and because many place i=
n
the code gang lookup with pagevec_lookup there is need to go over entries
that were looked up to take appropriate action.

Other design is to do migration inside the various radix tree lookup func=
tions
but that means going out of rcu section and possibly sleeping waiting for=
 the
GPU to copy back things into system memory.

I could grow the radix function to return some bool to avoid looping over=
 for
case where there is no special entry.

Cheers,
J=E9r=F4me

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