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SubjectRe: Does Linux have plan to support memory hole remapping?
On Wed, Apr 9, 2008 at 3:03 AM, Andi Kleen <andi@firstfloor.org> wrote:
>
> On Wed, Apr 09, 2008 at 11:50:00AM +0200, Arne Georg Gleditsch wrote:
> > Andi Kleen <andi@firstfloor.org> writes:
> >
> > > "Zhao Forrest" <forrest.zhao@gmail.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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