On 04/12/2023 12:28, David Hildenbrand wrote: > On 04.12.23 13:26, Ryan Roberts wrote: >>>>> >>>>> Also, struct page (memmap) might not be always contiguous, using struct page >>>>> points to represent folio range might not give the result you want. >>>>> See nth_page() and folio_page_idx() in include/linux/mm.h. >>>> >>>> Is that true for pages within the same folio too? Or are all pages in a folio >>>> guarranteed contiguous? Perhaps I'm better off using pfn? >>> >>> folio_page_idx() says not all pages in a folio is guaranteed to be contiguous. >>> PFN might be a better choice. >> >> Hi Zi, Matthew, >> >> Zi made this comment a couple of months back that it is incorrect to assume that >> `struct page`s within a folio are (virtually) contiguous. I'm not sure if that's >> really the case though? I see other sites in the source that do page++ when >> iterating over a folio. e.g. smaps_account(), splice_folio_into_pipe(), >> __collapse_huge_page_copy(), etc. >> >> Any chance someone could explain the rules? > > With the vmemmap, they are contiguous. Without a vmemmap, but with sparsemem, we > might end up allocating one memmap chunk per memory section (e.g., 128 MiB). > > So, for example, a 1 GiB hugetlb page could cross multiple 128 MiB sections, and > therefore, the memmap might not be virtually consecutive. OK, is a "memory section" always 128M or is it variable? If fixed, does that mean that it's impossible for a THP to cross section boundaries? (because a THP is always smaller than a section?) Trying to figure out why my original usage in this series was wrong, but presumably the other places that I mentioned are safe. > >
