[RFC] mm: Enable generic pfn_valid() to handle early sections with memmap holes

[Will Deacon]

On Thu, Mar 11, 2021 at 01:22:53PM +0530, Anshuman Khandual wrote:
> On 3/8/21 2:25 PM, Mike Rapoport wrote:
> > On Mon, Mar 08, 2021 at 08:57:53AM +0530, Anshuman Khandual wrote:
> >> Platforms like arm and arm64 have redefined pfn_valid() because their early
> >> memory sections might have contained memmap holes caused by memblock areas
> >> tagged with MEMBLOCK_NOMAP, which should be skipped while validating a pfn
> >> for struct page backing. This scenario could be captured with a new option
> >> CONFIG_HAVE_EARLY_SECTION_MEMMAP_HOLES and then generic pfn_valid() can be
> >> improved to accommodate such platforms. This reduces overall code footprint
> >> and also improves maintainability.
> > 
> > I wonder whether arm64 would still need to free parts of its memmap after
> 
> free_unused_memmap() is applicable when CONFIG_SPARSEMEM_VMEMMAP is not enabled.
> I am not sure whether there still might be some platforms or boards which would
> benefit from this. Hence lets just keep this unchanged for now.

In my opinion, unless there's a compelling reason for us to offer all of
these different implementations of the memmap on arm64 then we shouldn't
bother -- it's not like it's fun to maintain! Just use sparsemem vmemmap
and be done with it. Is there some reason we can't do that?

Will