Re: [PATCh] x86: overmapped fix when 4K pages on tail - 64bit

[Ingo Molnar <mingo@elte.hu>]

* 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