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

[Mike Rapoport]

On Thu, Mar 11, 2021 at 09:33:02AM +0000, Will Deacon wrote:
> 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?

Regardless if the decision to stop supporting other memory models, I think
it's a long due for arm64 to stop using pfn_valid() for anything except to
check whether there is a valid struct page for a pfn.

Something like the completely untested patch below: