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Subject: Re: [PATCH v3] mm: make expand_downwards symmetrical to
 expand_upwards
From: James Bottomley <James.Bottomley@HansenPartnership.com>
In-Reply-To: <1303411537.9048.3583.camel@nimitz>
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Date: Fri, 22 Apr 2011 16:33:05 -0500
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To: Dave Hansen <dave@linux.vnet.ibm.com>
Cc: Christoph Lameter <cl@linux.com>, KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>, David Rientjes <rientjes@google.com>, Pekka Enberg <penberg@kernel.org>, Michal Hocko <mhocko@suse.cz>, Andrew Morton <akpm@linux-foundation.org>, Hugh Dickins <hughd@google.com>, linux-mm@kvack.org, LKML <linux-kernel@vger.kernel.org>, linux-parisc@vger.kernel.org, Ingo Molnar <mingo@elte.hu>, x86 maintainers <x86@kernel.org>, Tejun Heo <tj@kernel.org>, Mel Gorman <mel@csn.ul.ie>

On Thu, 2011-04-21 at 11:45 -0700, Dave Hansen wrote:
> On Thu, 2011-04-21 at 13:33 -0500, Christoph Lameter wrote:
> > http://www.linux-mips.org/archives/linux-mips/2008-08/msg00154.html
> > 

By the way, this reference is actively wrong for parisc (having just
debugged the problem).  The basic issue is that until we start paging,
we have the kernel and some memory beyond it barely covered with the pg0
page table set up in head.S  On our systems, that extends out to 16MB.
SPARSEMEM is much more bootmem resource greedy than DISCONTIGMEM, so if
we actually call sparse_init() before we have the page tables set up, we
fall off the end of our 16MB mapping and go boom.  For us, therefore, we
can't call sparse_init() until we have our proper page tables in place.

James


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