Message-ID: <48ECDD37.8050506@linux-foundation.org>
Date: Wed, 08 Oct 2008 11:17:59 -0500
From: Christoph Lameter <cl@linux-foundation.org>
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To: Andy Whitcroft <apw@shadowen.org>
CC: Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
       linux-kernel@vger.kernel.org, Jon Tollefson <kniht@linux.vnet.ibm.com>,
       Mel Gorman <mel@csn.ul.ie>, Nick Piggin <nickpiggin@yahoo.com.au>
Subject: Re: [PATCH 1/1] hugetlbfs: handle pages higher order than MAX_ORDER
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Andy Whitcroft wrote:
> When working with hugepages, hugetlbfs assumes that those hugepages
> are smaller than MAX_ORDER.  Specifically it assumes that the mem_map
> is contigious and uses that to optimise access to the elements of the
> mem_map that represent the hugepage.  Gigantic pages (such as 16GB pages
> on powerpc) by definition are of greater order than MAX_ORDER (larger
> than MAX_ORDER_NR_PAGES in size).  This means that we can no longer make
> use of the buddy alloctor guarentees for the contiguity of the mem_map,
> which ensures that the mem_map is at least contigious for maximmally
> aligned areas of MAX_ORDER_NR_PAGES pages.

But the memmap is contiguous in most cases. FLATMEM, VMEMMAP etc. Its only
some special sparsemem configurations that couldhave the issue because they
break up the vmemmap. x86_64 uses VMEMMAP by default. Is this for i386?

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