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

[Ard Biesheuvel]

On 12 February 2016 at 17:46, Ard Biesheuvel <ard.biesheuvel at linaro.org> wrote:
> On 12 February 2016 at 17:43, Jeremy Linton <jeremy.linton at arm.com> wrote:
>> On 02/12/2016 10:28 AM, Ard Biesheuvel wrote:
>>>
>>> On 12 February 2016 at 17:21, Jeremy Linton <jeremy.linton at arm.com> wrote:
>>>>
>>>> On 02/12/2016 10:11 AM, Ard Biesheuvel wrote:
>>>>>
>>>>>
>>>>> On 12 February 2016 at 17:06, Jeremy Linton <jeremy.linton at 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.
>>
>
> Well, the thing is, if you only use 2 MB of each 32 MB on average, you
> gain nothing by going from contiguous pages to proper sections, since
> the TLB footprint is the same. So my opinion would be to use CONT_SIZE
> for 16k pages as well.

Actually, none of this matters, since the physical alignment is not
guaranteed to be 32 MB. So anything beyond 2 MB is not really
meaningful imo