Re: [PATCH v1 4/4] mm/mmu_gather: Store and process pages in contig ranges

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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





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