linux-kernel - Re: [patch 1/6] mmu

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Message-Id: <20080216025803.40d8ccbc.akpm@linux-foundation.org>
Date:	Sat, 16 Feb 2008 02:58:03 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Brice Goglin <Brice.Goglin@...ia.fr>
Cc:	Christoph Lameter <clameter@....com>,
	Andrea Arcangeli <andrea@...ranet.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [patch 1/6] mmu_notifier: Core code

On Sat, 16 Feb 2008 11:41:35 +0100 Brice Goglin <Brice.Goglin@...ia.fr> wrote:

> Andrew Morton wrote:
> > What is the status of getting infiniband to use this facility?
> >
> > How important is this feature to KVM?
> >
> > To xpmem?
> >
> > Which other potential clients have been identified and how important it it
> > to those?
> >   
> 
> As I said when Andrea posted the first patch series, I used something
> very similar for non-RDMA-based HPC about 4 years ago. I haven't had
> time yet to look in depth and try the latest proposed API but my feeling
> is that it looks good.
> 

"looks good" maybe.  But it's in the details where I fear this will come
unstuck.  The likelihood that some callbacks really will want to be able to
block in places where this interface doesn't permit that - either to wait
for IO to complete or to wait for other threads to clear critical regions.

>From that POV it doesn't look like a sufficiently general and useful
design.  Looks like it was grafted onto the current VM implementation in a
way which just about suits two particular clients if they try hard enough.

Which is all perfectly understandable - it would be hard to rework core MM
to be able to make this interface more general.  But I do think it's
half-baked and there is a decent risk that future (or present) code which
_could_ use something like this won't be able to use this one, and will
continue to futz with mlock, page-pinning, etc.

Not that I know what the fix to that is..
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