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

[David Hildenbrand <david@redhat.com>]

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