On 04.12.23 13:39, Ryan Roberts wrote:
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?)
Section size is variable (see SECTION_SIZE_BITS), but IIRC, buddy
allocations will never cross them.
Trying to figure out why my original usage in this series was wrong, but
presumably the other places that I mentioned are safe.
If only dealing with buddy allocations, *currently* it might always fall
into a single memory section.
--
Cheers,
David / dhildenb
