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Re: sparsemem support for mips with highmem

To: Dave Hansen <dave@linux.vnet.ibm.com>
Subject: Re: sparsemem support for mips with highmem
From: Christoph Lameter <cl@linux-foundation.org>
Date: Mon, 18 Aug 2008 16:33:29 -0500
Cc: Randy Dunlap <rdunlap@xenotime.net>, C Michael Sundius <Michael.sundius@sciatl.com>, Thomas Bogendoerfer <tsbogend@alpha.franken.de>, linux-mm@kvack.org, linux-mips@linux-mips.org, jfraser@broadcom.com, Andy Whitcroft <apw@shadowen.org>
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Dave Hansen wrote:
> On Mon, 2008-08-18 at 16:24 -0500, Christoph Lameter wrote:
>> This overhead can be avoided by configuring sparsemem to use a virtual 
>> vmemmap
>> (CONFIG_SPARSEMEM_VMEMMAP). In that case it can be used for non NUMA since 
>> the
>> overhead is less than even FLATMEM.
> 
> Is that all it takes these days, or do you need some other arch-specific
> code to help out?

Some information is in mm/sparse-vmemmap.c. Simplest configuration is to use
vmalloc for the populate function. Otherwise the arch can do what it wants to
reduce the overhead of virtual mappings (in the x86 case we use a 2M TLB
entry, and since 2M TLBs are also used for the 1-1 physical mapping the
overhead is the same as for 1-1 mappings).



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