[PATCH 1/2] arm64: mm: Enable CONT_SIZE aligned sections for 64k page kernels.

[jeremy.linton@arm.com (Jeremy Linton)]

On 02/12/2016 10:28 AM, Ard Biesheuvel wrote:
> On 12 February 2016 at 17:21, Jeremy Linton <jeremy.linton@arm.com> wrote:
>> On 02/12/2016 10:11 AM, Ard Biesheuvel wrote:
>>>
>>> On 12 February 2016 at 17:06, Jeremy Linton <jeremy.linton@arm.com> wrote:
>>
>> (trimming)
>>>>
>>>>    #if defined(CONFIG_DEBUG_ALIGN_RODATA)
>>>> -#define ALIGN_DEBUG_RO                 . = ALIGN(1<<SECTION_SHIFT);
>>>> -#define ALIGN_DEBUG_RO_MIN(min)                ALIGN_DEBUG_RO
>>>> +#if defined(CONFIG_ARM64_64K_PAGES)
>>>> +#define ALIGN_DEBUG_RO_MIN(min)                . = ALIGN(CONT_SIZE);
>>>> +#else
>>>> +#define ALIGN_DEBUG_RO_MIN(min)                . = ALIGN(SECTION_SIZE);
>>>
>>>
>>> Doesn't this align to 32 MB on 16k pages kernels?
>>
>>
>> Yes, I considered whether it was more appropriate to use CONT_SIZE for 16k
>> as well.
>>
>> Opinions?
>>
>
> Looking at vmlinux.lds.S, I see that that would put _stext and
> __init_begin at 32 MB aligned boundaries. making the size of the
> kernel at least 64 MB. If I take your .rodata patch into account,
> which adds a third instance of ALIGN_DEBUG_RO_MIN, the Image footprint
> will rise to ~100 MB. Or am I missing something?
>

No, I think your correct. But, its an option, and it sort of depends on 
use case. In a system with 100+GB of RAM it might be useful. Not so much 
on a phone or small embedded system. I don't really see those people 
enabling ALIGN_RODATA anyway. Worse, I expect the loss of RAM 
efficiently going from 4k-16k pages in a RAM constrained system to be a 
pretty big hit too.

I don't have any hard data one way or the other, and I don't have a 
strong opinion. Although, I suspect at the moment the potential users of 
16k pages may tend toward the small side.