Re: [PATCH 02/17] mm/sparse-vmemmap: track compound page order in struct mem_section

[Mike Rapoport]

> HugeTLB and DAX both rely on vmemmap optimization, but sparsemem does
> not record what compound page order a section is populated with.
> 
> As a result, code that needs this information has to open-code
> separate handling across users of vmemmap optimization.  It also
> prevents other memory management code, such as struct page
> initialization, from skipping initialization of shared vmemmap pages
> when needed.
> 
> Track the compound page order in struct mem_section and provide small
> helpers to access it.  A compound page larger than a section naturally
> carries the same order across all covered sections.
> 
> This is a preparatory change for consolidating vmemmap optimization
> handling and for letting later code make initialization decisions
> based on the section's compound page order.
> 
> Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
>
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 1353bcf7b712..bacd89572c5c 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -2015,6 +2015,14 @@ struct mem_section {
>  	 */
>  	struct page_ext *page_ext;
>  #endif
> +#ifdef CONFIG_SPARSEMEM_VMEMMAP
> +	/*
> +	 * The order of compound pages in this section. Typically, the section
> +	 * holds compound pages of this order; a larger compound page will span
> +	 * multiple sections.
> +	 */

Typically a section holds order-0 pages ;-)

I think this comment should mention that this is for special sections,
like hugetlb and DAX.

> +	unsigned int order;
> +#endif
>  };
>  
>  #ifdef CONFIG_SPARSEMEM_EXTREME
> @@ -2361,10 +2369,34 @@ static inline unsigned long next_present_section_nr(unsigned long section_nr)
>  #endif
>  
>  #else
> +struct mem_section;
> +
>  #define sparse_vmemmap_init_nid_early(_nid) do {} while (0)
>  #define pfn_in_present_section pfn_valid
>  #endif /* CONFIG_SPARSEMEM */
>  
> +#ifdef CONFIG_SPARSEMEM_VMEMMAP
> +static inline void section_set_order(struct mem_section *section, unsigned int order)
> +{
> +	VM_WARN_ON(section->order && order && section->order != order);
> +	section->order = order;
> +}
> +
> +static inline unsigned int section_order(const struct mem_section *section)
> +{
> +	return section->order;
> +}

You mention open-coded handling, is there any open-coded case that can
be replaced with the new helpers right away?

We strive to have new APIs and at least some of their users in the same
patch.

-- 
Sincerely yours,
Mike.