[PATCH 1/2] arm64: mem-model: add flatmem model for arm64

[Chen Feng]

Hi Will,

On 2016/4/11 18:40, Will Deacon wrote:
> On Mon, Apr 11, 2016 at 12:31:53PM +0200, Ard Biesheuvel wrote:
>> On 11 April 2016 at 11:59, Chen Feng <puck.chen at hisilicon.com> wrote:
>>> Please see the pg-tables below.
>>>
>>>
>>> With sparse and vmemmap enable.
>>>
>>> ---[ vmemmap start ]---
>>> 0xffffffbdc0200000-0xffffffbdc4800000          70M     RW NX SHD AF    UXN MEM/NORMAL
>>> ---[ vmemmap end ]---
>>>
>>
>> OK, I see what you mean now. Sorry for taking so long to catch up.
>>
>>> The board is 4GB, and the memap is 70MB
>>> 1G memory --- 14MB mem_map array.
>>
>> No, this is incorrect. 1 GB corresponds with 16 MB worth of struct
>> pages assuming sizeof(struct page) == 64
>>
>> So you are losing 6 MB to rounding here, which I agree is significant.
>> I wonder if it makes sense to use a lower value for SECTION_SIZE_BITS
>> on 4k pages kernels, but perhaps we're better off asking the opinion
>> of the other cc'ees.
> 
> You need to be really careful making SECTION_SIZE_BITS smaller because
> it has a direct correlation on the use of page->flags and you can end up
> running out of bits fairly easily.

Yes, making SECTION_SIZE_BITS smaller can solve the current situation.

But if the phys-addr is 64GB, but only 4GB ddr is the valid address. And the

holes are not always 512MB.

But, can you tell us why *smaller SIZE makes running out of bits fairly easily*?

And how about the flat-mem model?

> 
> Will
> 
> .
>