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
