Re: questions about init_memory_mapping_high()

From: H. Peter Anvin
Date: Mon Feb 28 2011 - 13:15:09 EST

On 02/23/2011 09:19 AM, Tejun Heo wrote:
> Hello, guys.
> I've been looking at init_memory_mapping_high() added by commit
> 1411e0ec31 (x86-64, numa: Put pgtable to local node memory) and I got
> curious about several things.
> 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.)

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