linux-kernel - Re: Does Linux have plan to support memory hole remapping?

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Message-ID: <86802c440804091102w410bc6f4v7a848b1328c2bbbd@mail.gmail.com>
Date:	Wed, 9 Apr 2008 11:02:22 -0700
From:	"Yinghai Lu" <yhlu.kernel@...il.com>
To:	"Andi Kleen" <andi@...stfloor.org>
Cc:	"Arne Georg Gleditsch" <arne.gleditsch@...phinics.no>,
	"Zhao Forrest" <forrest.zhao@...il.com>, discuss@...-64.org,
	linux-kernel@...r.kernel.org, mingo@...e.hu, ak@...e.de
Subject: Re: Does Linux have plan to support memory hole remapping?

On Wed, Apr 9, 2008 at 3:03 AM, Andi Kleen <andi@...stfloor.org> wrote:
>
> On Wed, Apr 09, 2008 at 11:50:00AM +0200, Arne Georg Gleditsch wrote:
>  > Andi Kleen <andi@...stfloor.org> writes:
>  >
>  > > "Zhao Forrest" <forrest.zhao@...il.com> writes:
>  > >>
>  > >> As we can see from above information that the physical memory in system
>  > >> is 32768MB(32GB). However OS is only using about
>  > >> (32768-512)MB(MemTotal:     33010240 kB). Does this mean that this
>  > >> linux kernel can't use the physical memory remapped
>  > >> from (4G-512M, 4G) to (32G, 32G+512M)?
>  > >
>  > > The linux kernel can only use the memory passed to it by the BIOS.
>  > > Sometimes they need special BIOS setup options to enable remapping. If
>  > > there are no such options and you can't upgrade it you're out of luck
>  >
>  > Hm, wouldn't the given e820 map and mtrr listing indicate that 512M were
>  > actually remapped to 32G+ in this case?
>
>  The way it usually works (if it is implemented correctly in the BIOS)
>  is that all memory starting at the hole moves up together
>  (often subject to DIMM boundaries etc.),
>  not that the area below the hole is remapped individually.
>
>  BTW it is not actually 512MB that is lost. MemTotal does not
>  include mem_map and that alone is ~512MB (64 bytes for each 4K page)
>
>  So as far as I can see there is no missing memory remapping in Zhao's case,
>  he's just confused by the MemTotal semantics.

agreed. the HW/BIOS already enable HW memory hole.

YH
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