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SubjectRe: questions about init_memory_mapping_high()
Hey,

(sorry about the earlier empty reply, fat finger on my phone)

On Mon, Feb 28, 2011 at 10:14:44AM -0800, H. Peter Anvin wrote:
> > 1. The only rationale given in the commit description is that a
> > RED-PEN is killed, which was the following.
> >
> > /*
> > * RED-PEN putting page tables only on node 0 could
> > * cause a hotspot and fill up ZONE_DMA. The page tables
> > * need roughly 0.5KB per GB.
> > */
> >
> > This already wasn't true with top-down memblock allocation.
> >
> > The 0.5KB per GiB comment is for 32bit w/ 3 level mapping. On
> > 64bit, it's ~4KiB per GiB when using 2MiB mappings and, well, very
> > small per GiB if 1GiB mapping is used. Even with 2MiB mapping,
> > 1TiB mapping would only be 4MiB. Under ZONE_DMA, this could be
> > problematic but with top-down this can't be a problem in any
> > realistic way in foreseeable future.
> >
>
> It's true on 64 bits too when PAE is not available (e.g. with Xen.)

Hmm... I don't follow. Can you elaborate? If PAE is not available
for whatever reason, the physical memory is limited to 4GiB but I
don't follow what that has to do with the above.

Thanks.

--
tejun


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