On 07/03/2019 04:36 AM, Andrew Morton wrote:

On Fri, 28 Jun 2019 10:50:31 +0530 Anshuman Khandual [off-list ref] wrote:

quoted

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.