linux-kernel - Re: x86_64 virtual memory map

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Date:	Mon, 25 Jul 2011 18:13:16 +0800
From:	Bob Zhang <zhanglinbao@...il.com>
To:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Cc:	hayfeng Lee <omycle@...il.com>, ak@...ux.intel.com,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Bob Zhang <2004.zhang@...il.com>, zhanglinbao@...il.com
Subject: Re: x86_64 virtual memory map

thanks in advanced!

I searched this question , so I directly reply to this mail.

1, for x86_64 machine ,for example ,HP's proliant machine.
from the kernel point of view , what is the max physical ram size ?
if I install 2TB physical memory(I mean physical RAM) , can linux
kernel recognize 2TB physical memory ?  from mm.txt , it seems that
physical ram size should be 64TB.

2,
from mm.txt :(http://lxr.linux.no/linux+v2.6.39/Documentation/x86/x86_64/mm.txt
from latest 2.6.39.

I am a little confused .
above said that :
ffff880000000000 - ffffc7ffffffffff (=64 TB) direct mapping of all phys. memory
10ffffc80000000000 - ffffc8ffffffffff (=40 bits) hole
11ffffc90000000000 - ffffe8ffffffffff (=45 bits) vmalloc/ioremap space  :32TB
ffffea0000000000 - ffffeaffffffffff (=40 bits) virtual memory map (1TB)
from this , we can know ,we can used max 64TB physical memory(physical
RAM).  vmalloc/ioremap space :32TB . but why tell us 1TB virtual
memory map ?  if the address that  vmalloc allocated are the address
that we can use , why say like this ? I am puzzled.
for example ,for 32bit kernel .vmalloc() range

and in the end of mm.txt , it says :
>>Current X86-64 implementations only support 40 bits of address space,
 >> 27but we support up to 46 bits. This expands into MBZ space in the
>>page tables.
so this 40bit(1TB) is responding to virtual memory map(1TB) ?

3,  if I installed 64TB physical memory (RAM) , but linux kernel only
support 1TB address space , what should I do next ?

4 , it seems that MBZ can support up to 256TB ,but who can tell how to
expand address space by MBZ ?
thanks very much!

-Bob
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