Re: [PATCH 2/2] mm/hugetlb: refactor subpage recording

[Jason Gunthorpe]

On Tue, Jan 26, 2021 at 05:58:53PM -0800, Mike Kravetz wrote:

> As pointed out by Joao, you can also see the differences in pfn_to_page
> for CONFIG_SPARSE_VMEMMAP and CONFIG_SPARSEMEM.  The only time we might
> have issues is with CONFIG_SPARSEMEM.  I would bet CONFIG_SPARSE_VMEMMAP
> is far more common.

I think it is fine to have a different pfn_to_page, it should just be
illegal to combine pages into a compound if their tail pages are not
linear in the map.

Matt's folio work might present an option to audit the whole mm for
this pattern and provide some folio_next_tail_page() accessor that
does the fast thing - but I question the value of such a project for a
2008 era PPC platform with 16GB pages (seriously?) that may be using
VMEMMAP today anyhow??

Maybe others know of more modern use cases

Jason