[2/2] powerpc: support for 256K pages on PPC 44x

[Milton Miller]

On Nov 13, 2008, at 10:32 PM, Yuri Tikhonov wrote:
> On Tuesday, November 11, 2008 Milton Miller wrote:
>>>>>  #ifdef CONFIG_PTE_64BIT
>>>>>  typedef unsigned long long pte_basic_t;
>>>>> +#ifdef CONFIG_PPC_256K_PAGES
>>>>> +#define PTE_SHIFT       (PAGE_SHIFT - 7)
>>>>
>>>> This seems to be missing the comment on how many ptes are actually 
>>>> in
>>>> the page that are in the other if and else cases.
>>>
>>> Ok. I'll fix this. Actually it's another hack: we don't use full page
>>> for PTE table because we need to reserve something for PGD
>
>> I don't understand "we need to reserve something for PGD".   Do you
>> mean that you would not require a second page for the PGD because the
>> full pagetable could fit in one page?
...
>> That does imply you want to allocate the pte page from a slab instead
>> of pgalloc.  Is that covered?
>
>  Well, in case of 256K PAGE_SIZE we do not need the PGD level indeed
> (18 bits are used for offset, and remaining 14 bits are for PTE index
> inside the PTE table). Even the full 256K PTE page isn't necessary to
> cover the full range: only half of it would be enough (with 14 bits we
> can address only 16K PTEs).
>
>  But the head_44x.S code is essentially based on the assumption of
> 2-level page addressing. Also, I may guess that eliminating of the
> PGD level won't be as easy as just a re-implementation of the TLB-miss
> handlers in head_44x.S. So, the current approach for 256K-pages
> support was just a compromise between the required for the project
> functionality, and the effort necessary to achieve it.

So are you allocating the < PAGE_SIZE levels from slabs (either kmalloc 
or dedicated) instead of allocating pages?   Or are you wasting the 
extra space?

At a very minimum you need to comment this in the code.  If I were 
maintiner I would say not wasting large fractions of pages when the 
page size is 256k would be my merge requirement.  As I said, I'm fine 
with keeping the page table two levels, but the tradeoff needs to be 
documented.

milton