Messages in this thread | | | Date | Thu, 10 Jul 2008 09:20:23 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCh] x86: overmapped fix when 4K pages on tail - 64bit |
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* Yinghai Lu <yhlu.kernel@gmail.com> wrote:
> > that the number of mapping ranges depends on our programming, not on > > any external factor. I.e. if anyone adds a new mapping range to the > > kernel for any purpose, it must be extended - but otherwise it > > cannot run out due to new hardware. > > 4k, 2M, 1G, 2M, 4k > > some day will get 512g page?
i'd not be surprised to see that in ~10 years. Then we'll have to extend the array to 7 entries ;-)
btw., i have a weird system:
[ 0.000000] BIOS-provided physical RAM map: [ 0.000000] BIOS-e820: 0000000000000000 - 000000000009fc00 (usable) [ 0.000000] BIOS-e820: 000000000009fc00 - 00000000000a0000 (reserved) [ 0.000000] BIOS-e820: 00000000000e0000 - 0000000000100000 (reserved) [ 0.000000] BIOS-e820: 0000000000100000 - 000000003ed93000 (usable) [ 0.000000] BIOS-e820: 000000003ed93000 - 000000003ee4d000 (ACPI NVS) [ 0.000000] BIOS-e820: 000000003ee4d000 - 000000003fea2000 (usable) [ 0.000000] BIOS-e820: 000000003fea2000 - 000000003fee9000 (ACPI NVS) [ 0.000000] BIOS-e820: 000000003fee9000 - 000000003feed000 (usable) [ 0.000000] BIOS-e820: 000000003feed000 - 000000003feff000 (ACPI data) [ 0.000000] BIOS-e820: 000000003feff000 - 000000003ff00000 (usable)
look at the RAM splitup:
640K + BIOS-hole + ~1GB + acpi + 17MB + acpi + 16K + acpi + 4K
and the end of it is not 1024 MB but 1023 MB.
so the _best_ mapping strategy would probably be to do 2MB granular mapping up to 1GB, i.e. to 'overmap' into the end of RAM. But we also have to make sure that we have no PCI resources or weird chipset resources in the final 1MB that could hurt us with PAT, aliasing-wise.
Since i'm not sure we can really ensure sanity on that level, i guess your solution to precisely map everything without overmapping is our best choice. Thus sane hw with such end of RAM mappings:
BIOS-e820: 0000000100000000 - 0000000120000000 (usable)
and another one with:
BIOS-e820: 0000000100000000 - 0000000830000000 (usable)
... would be slightly faster (because it would use 2MB TLBs at the end of kernel RAM, instead of broken-up 4K TLBs)
perhaps we could also have a config and boot option that would sanitize the e820 map to just ignore all non-2MB granular RAM. Losing 1-2MB of RAM is not an issue on a 32GB system.
Ingo
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