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

[Yuri Tikhonov]

 Hello Milton,

On Friday, November 14, 2008 you wrote:

> 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?

 Wasting the extra space has a place here.

> 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.

 Agree, we'll document this fact, and re-submit the patch.

 Regards, Yuri

 --
 Yuri Tikhonov, Senior Software Engineer
 Emcraft Systems, www.emcraft.com