[PATCH V2] mm/ioremap: Probe platform for p4d huge map support

[Anshuman Khandual]

On 07/12/2019 12:37 PM, Michael Ellerman wrote:
> Anshuman Khandual <anshuman.khandual at arm.com> writes:
>> On 07/03/2019 04:36 AM, Andrew Morton wrote:
>>> On Fri, 28 Jun 2019 10:50:31 +0530 Anshuman Khandual <anshuman.khandual at arm.com> wrote:
>>>
>>>> Finishing up what the commit c2febafc67734a ("mm: convert generic code to
>>>> 5-level paging") started out while levelling up P4D huge mapping support
>>>> at par with PUD and PMD. A new arch call back arch_ioremap_p4d_supported()
>>>> is being added which just maintains status quo (P4D huge map not supported)
>>>> on x86, arm64 and powerpc.
>>>
>>> Does this have any runtime effects?  If so, what are they and why?  If
>>> not, what's the actual point?
>>
>> It just finishes up what the previous commit c2febafc67734a ("mm: convert
>> generic code to 5-level paging") left off with respect p4d based huge page
>> enablement for ioremap. When HAVE_ARCH_HUGE_VMAP is enabled its just a simple
>> check from the arch about the support, hence runtime effects are minimal.
> 
> The return value of arch_ioremap_p4d_supported() is stored in the
> variable ioremap_p4d_capable which is then returned by
> ioremap_p4d_enabled().
> 
> That is used by ioremap_try_huge_p4d() called from ioremap_p4d_range()
> from ioremap_page_range().

That is right.

> 
> The runtime effect is that it prevents ioremap_page_range() from trying
> to create huge mappings at the p4d level on arches that don't support
> it.

But now after first checking with an arch callback. Previously p4d huge
mappings were disabled on all platforms as ioremap_p4d_capable remained
clear through out being a static.