Re: [PATCH v2 03/17] mm/mm_init: skip initializing shared vmemmap tail pages

[Mike Rapoport]

On Thu, Jul 30, 2026 at 09:09:00PM +0800, Muchun Song wrote:
> > On 2026-07-26 12:30:17+08:00, Muchun Song wrote:
> >>> 
> >>> #else
> >>> +struct mem_section;
> >>> +
> >>> #define sparse_vmemmap_init_nid_early(_nid) do {} while (0)
> >>> #define pfn_in_present_section pfn_valid
> >>> +static inline struct mem_section *__pfn_to_section(unsigned long pfn)
> >>> +{
> >>> +  	return NULL;
> >>> +}
> >> 
> >> I'd like to propose an alternative implementation that doesn't require
> >> exposing the mem_section. The idea is to add a new helper function,
> >> pfn_to_section_order(), so that for non-sparse-memory configurations,
> >> the mem_section concept stays hidden internally. I'd really appreciate
> >> any thoughts or concerns — if everyone is comfortable with it, I can go
> >> ahead and implement this in the next version.
> > 
> > A helper that keeps mem_section hidden from !SPARSMEM makes perfect
> > sense to me.
> > 
> > I'd even take it one step further and make it return how many pfns
> > should be skipped in pfn_vmemmap_optimizable case.
> 
> To make sure we're on the same page, let me walk you through the specific
> changes I have in mind. My initial plan is to introduce pfn_to_section_order,
> and the expected diff changes are as follow to keep mem_sectionhidden from
> !SPARSEMEM.
> 
> diff --git a/mm/mm_init.c b/mm/mm_init.c
> index dcb757b36902..0b0c2996d080 100644
> --- a/mm/mm_init.c
> +++ b/mm/mm_init.c
> @@ -884,7 +884,7 @@ void __meminit memmap_init_range(unsigned long size, int nid, unsigned long zone
>                 }
> 
>                 if (pfn_vmemmap_optimizable(pfn)) {
> -                       unsigned int order = section_order(__pfn_to_section(pfn));
> +                       unsigned int order = pfn_to_section_order(pfn);
> 
>                         pfn = min(ALIGN(pfn, 1UL << order), end_pfn);
>                         continue;
> diff --git a/mm/sparse.h b/mm/sparse.h
> index 030248030dc7..c5fbcdde3cee 100644
> --- a/mm/sparse.h
> +++ b/mm/sparse.h
> @@ -47,6 +47,11 @@ static inline void __section_mark_present(struct mem_section *ms,
> 
>         ms->section_mem_map |= SECTION_MARKED_PRESENT;
>  }
> +
> +static inline unsigned int pfn_to_section_order(unsigned long pfn)
> +{
> +       return section_order(__pfn_to_section(pfn));
> +}
>  #else
>  static inline void sparse_init(void) {}
>  #endif /* CONFIG_SPARSEMEM */
> 
> Since we also use __pfn_to_section in the patch 14 in this series for

I still didn't get to patch 14 :)

> !SPARSEMEM, we need to make corresponding adjustments—specifically, by using
> pfn_to_section_order to determine whether the vmemmap of a given section is
> optimizable. This new helper will be called from several places, so I'm afraid
> its introduction is unavoidable.

Do you mean that section_vmemmap_optimizable() will receive pfn as a
parameter and use pfn_to_section_order() internally?

If that's the case and pfn_to_section_order() will be used in several
places I think it's better than to have a dedicated helper just for
memmap_init_range().
 
> That said, I've also considered an alternative: introducing another helper that
> returns the exact number of PFNs to skip, and using it solely within
> memmap_init_range(). However, that approach doesn't seem to offer much in terms
> of code simplification. If I'm missing something or if my reasoning doesn't align
> with your expectations, I would really appreciate your guidance. Thank you for
> your patience!
> 
> Thanks,
> Muchun
> 
> 
> 

-- 
Sincerely yours,
Mike.