Re: [PATCH part1 v6 4/6] x86/mem-hotplug: Support initialize page tables in bottom-up

[Zhang Yanfei <zhangyanfei.yes@gmail.com>]

Hi tejun,

On 10/10/2013 03:20 AM, Tejun Heo wrote:
> Hello,
> 
> On Thu, Oct 10, 2013 at 01:14:23AM +0800, Zhang Yanfei wrote:
>>>> You meant that the memory size is about few megs. But here, page tables
>>>> seems to be large enough in big memory machines, so that page tables will
>>>
>>> Hmmm?  Even with 4k mappings and, say, 16Gigs of memory, it's still
>>> somewhere above 32MiB, right?  And, these physical mappings don't
>>> usually use 4k mappings to begin with.  Unless we're worrying about
>>> ISA DMA limit, I don't think it'd be problematic.
>>
>> I think Peter meant very huge memory machines, say 2T memory? In the worst
>> case, this may need 2G memory for page tables, seems huge....
> 
> Realistically tho, why would people be using 4k mappings on 2T
> machines?  For the sake of argument, let's say 4k mappings are
> required for some weird reason, even then, doing SRAT parsing early
> doesn't necessarily solve the problem in itself.  It'd still need
> heuristics to avoid occupying too much of 32bit memory because it
> isn't difficult to imagine specific NUMA settings which would drive
> page table allocation into low address.
> 
> No matter what we do, there's no way around the fact that this whole
> effort is mostly an incomplete solution in its nature and that's why I
> think we better keep things isolated and simple.  It isn't a good idea
> to make structural changes to accomodate something which isn't and
> doesn't have much chance of becoming a full solution.  In addition,
> the problem itself is niche to begin with.
> 
>> And I am not familiar with the ISA DMA limit, does this mean the memory 
>> below 4G? Just as we have the ZONE_DMA32 in x86_64. (16MB limit seems not
>> the case here)
> 
> Yeah, I was referring to the 16MB limit, which apparently ceased to
> exist.

Hmmmm...If we are talking 16MB limit hear, I don't think it a problem, either.
Currently, default loading & running address of kernel is 16MB, so the
kernel itself is above 16MB, memory allocated in bottom-up mode is obviously
above the 16MB. Just seeing from a RHEL6.3 server:

  01000000-01507ff4 : Kernel code
  01507ff5-01c07b2f : Kernel data
  01d4e000-02012023 : Kernel bss

IOW, even if kernel is loaded and running at 1MB, it self will occupy about
16MB from the above.

-- 
Thanks.
Zhang Yanfei

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