linux-kernel - Re: [patch] mm: sparsemem memory_present() memory corruption fix

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Date:	Wed, 16 Apr 2008 02:36:24 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Christoph Lameter <clameter@....com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Pekka Enberg <penberg@...helsinki.fi>,
	linux-kernel@...r.kernel.org, Mel Gorman <mel@....ul.ie>,
	Nick Piggin <npiggin@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>, Yinghai.Lu@....com,
	apw@...dowen.org,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [patch] mm: sparsemem memory_present() memory corruption fix


* Christoph Lameter <clameter@....com> wrote:

> On Wed, 16 Apr 2008, Ingo Molnar wrote:
> 
> > if a !PAE x86 kernel is booted on a 32-bit system with more than 4GB 
> > of RAM, then we call memory_present() with a start/end that goes 
> > outside the scope of MAX_PHYSMEM_BITS.
> 
> Well okay this fixes it but is this the right fix? The arch should not 
> call memory_present() with an invalid pfn.

it is the right fix. The architecture memory setup code doesnt even 
_know_ the limits at this place in an open-coded way (and shouldnt know 
them) - and even later on we use pfn_valid() to determine whether to 
attempt to get to a struct page and free it into the buddy.

[ Of course the architecture code in general 'knows' about the limits - 
  but still it's cleaner to have a dumb enumeration interface here 
  combined with a resilient core code - that's always going to be less 
  fragile. ]

btw., i just did some bug history analysis, the calls were originally 
added when sparsemem support was added:

| commit 215c3409eed16c89b6d11ea1126bd9d4f36b9afd
| Author: Andy Whitcroft <apw@...dowen.org>
| Date:   Fri Jan 6 00:12:06 2006 -0800
|
|    [PATCH] i386 sparsemem for single node systems

in v2.6.15-1003-g215c340. (so this is appears to be an unfixed bug in 
v2.6.16 as well)

	Ingo
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