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SubjectRe: questions about init_memory_mapping_high()
On 03/01/2011 12:29 AM, Tejun Heo wrote:
> 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.

Sorry, PSE, not PAE.


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