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

[Muchun Song <muchun.song@linux.dev>]

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@bytedance.com>
>>
>> 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.