linux-kernel - Re: [PATCH 1/1] mm: Prevent dereferencing non-allocated per

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Message-Id: <20071127154213.11970e63.akpm@linux-foundation.org>
Date:	Tue, 27 Nov 2007 15:42:13 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Christoph Lameter <clameter@....com>
Cc:	andi@...stfloor.org, travis@....com, ak@...e.de,
	pageexec@...email.hu, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] mm: Prevent dereferencing non-allocated per_cpu
 variables

On Tue, 27 Nov 2007 15:22:56 -0800 (PST)
Christoph Lameter <clameter@....com> wrote:

> On Tue, 27 Nov 2007, Andrew Morton wrote:
> 
> > The prefetch however might still need some work - we can indeed do
> > prefetch() against a not-possible CPU's memory here.  And I do recall that
> > 4-5 years ago we did have a CPU (one of mine, iirc) which would oops when
> > prefetching from a bad address.  I forget what the conclusion was on that
> > matter.
> > 
> > If we do want to fix the prefetch-from-outer-space then we should be using
> > cpu_isset(cpu, *cpumask) here rather than cpu_possible().
> 
> Generally the prefetch things have turned out to be not that useful. How 
> about dropping the prefetch? I kept it because it was there.

I don't recall anyone ever demonstrating that prefetch is useful in-kernel.

I think I've heard of situations where benefits have been seen in userspace
- if a loop does a lot of calculation on each datum which it fetches then
there's a good opportunity to pipeline the fetch with the on-core
crunching.  But kernel doesn't do that sort of thing..

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