Re: [PATCH v4 03/17] mm/mm_init: skip initializing shared vmemmap tail pages

[Muchun Song]

On 2026/8/24 16:59, Mike Rapoport wrote:
> > memmap_init_range() initializes every struct page in the target range.
> > For compound pages with vmemmap optimization, the tail struct pages are
> > backed by a shared vmemmap page.
> >
> > Initializing those tail struct pages would overwrite the shared
> > vmemmap page contents, requiring users such as HugeTLB to restore the
> > metadata afterwards.
> >
> > Track the compound order for HVO-backed sections and use that metadata
> > to detect struct pages that fall into the shared tail vmemmap range.
> > Skip those shared tail pages in memmap_init_range(), then initialize
> > pageblock migratetypes for the processed range with a helper after the
> > per-page initialization loop.
> >
> > Keep direct mem_section access inside sparse helpers by exposing
> > pfn_to_section_order() to users that only need the order associated with
> > a PFN. This lets memmap_init_range() skip shared tail vmemmap pages
> > without exposing __pfn_to_section() to !SPARSEMEM builds.
> >
> > This is a preparatory change for consolidating handling across users of
> > vmemmap optimization, and it also avoids redundant initialization of
> > shared tail vmemmap pages during early boot.
> >
> > Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> >
> > diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> > index 5fb9b37819d5..df31cac12311 100644
> > --- a/include/linux/mmzone.h
> > +++ b/include/linux/mmzone.h
> > @@ -2022,6 +2022,14 @@ struct mem_section {
> >   	unsigned long section_mem_map;
> >   
> >   	struct mem_section_usage *usage;
> > +#ifdef CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP
> > +	/*
> > +	 * Normally, sections hold regular (order-0) pages. However, for
> > +	 * sections with HVO enabled, this tracks the compound page order
> > +	 * to enable deduplication of redundant vmemmap pages.
> > +	 */
> > +	unsigned int order;
> > +#endif
> >   #ifdef CONFIG_PAGE_EXTENSION
> >   	/*
> >   	 * If SPARSEMEM, pgdat doesn't have page_ext pointer. We use
> > diff --git a/mm/mm_init.c b/mm/mm_init.c
> > index 1533aebafb68..05c09e755e0b 100644
> > --- a/mm/mm_init.c
> > +++ b/mm/mm_init.c
> > @@ -29,6 +29,7 @@
> >   #include <linux/cma.h>
> >   #include <linux/crash_dump.h>
> >   #include <linux/execmem.h>
> > +#include <linux/sizes.h>
> >   #include <linux/vmstat.h>
> >   #include <linux/kexec_handover.h>
> >   #include <linux/hugetlb.h>
> > @@ -677,21 +678,19 @@ static inline void fixup_hashdist(void)
> >   static inline void fixup_hashdist(void) {}
> >   #endif /* CONFIG_NUMA */
> >   
> > -#if defined(CONFIG_ZONE_DEVICE) || defined(CONFIG_DEFERRED_STRUCT_PAGE_INIT)
> >   static __meminit void pageblock_migratetype_init_range(unsigned long pfn,
> > -		unsigned long nr_pages, int migratetype, bool atomic)
> > +		unsigned long nr_pages, int migratetype, bool isolate, bool atomic)
> Growing boolean flags makes the callsites harder to read.
> One way to deal with it is to add comments to the callers saying what
> each true and false mean.

Make sense.

> >   {
> >   	const unsigned long end = pfn + nr_pages;
> >   
> >   	for (pfn = pageblock_align(pfn); pfn < end; pfn += pageblock_nr_pages) {
> >   		enum migratetype mt = kho_scratch_migratetype(pfn, migratetype);
> >   
> > -		init_pageblock_migratetype(pfn_to_page(pfn), mt, false);
> > -		if (!atomic && IS_ALIGNED(pfn, PAGES_PER_SECTION))
> > +		init_pageblock_migratetype(pfn_to_page(pfn), mt, isolate);
> > +		if (!atomic && IS_ALIGNED(pfn, PFN_DOWN(SZ_1G)))
> >   			cond_resched();
> >   	}
> >   }
> > -#endif
> >   
> >   #ifdef CONFIG_DEFERRED_STRUCT_PAGE_INIT
> >   static inline void pgdat_set_deferred_range(pg_data_t *pgdat)
> > @@ -886,6 +885,13 @@ void __meminit memmap_init_range(unsigned long size, int nid, unsigned long zone
> >   			}
> >   		}
> >   
> > +		if (vmemmap_optimizable_pfn(pfn)) {
> A short comment above would be nice :)

No problem. Thanks for your review.

Muchun,
Thanks.