Re: [PATCH 04/17] mm/mm_init: skip initializing shared vmemmap tail pages

[Mike Rapoport <rppt@kernel.org>]

Hi Muchun,

Below are some preliminary comments, I'm planning to spend more time on
review next week.

On Thu, Jul 02, 2026 at 05:38:08PM +0800, Muchun Song 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, so users such as HugeTLB have to open-code
> follow-up handling to restore the metadata afterwards.
> 
> Use the section's compound page order to detect struct pages that fall
> into the shared tail vmemmap range and skip their initialization in
> memmap_init_range().  Still initialize the pageblock migratetypes for
> the skipped range so the surrounding setup remains intact.
> 
> 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>
> ---
>  include/linux/mmzone.h |  4 ++++
>  mm/internal.h          | 16 ++++++++++++++++
>  mm/mm_init.c           | 25 +++++++++++++++++++------
>  3 files changed, 39 insertions(+), 6 deletions(-)
> 
> @@ -673,19 +673,21 @@ 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)

What is isolate parameter for?

>  {
>  	const unsigned long end = pfn + nr_pages;
>  
>  	for (pfn = pageblock_align(pfn); pfn < end; pfn += pageblock_nr_pages) {
> -		init_pageblock_migratetype(pfn_to_page(pfn), migratetype, false);
> +		init_pageblock_migratetype(pfn_to_page(pfn), migratetype, isolate);
> +#ifdef CONFIG_SPARSEMEM
>  		if (!atomic && IS_ALIGNED(pfn, PAGES_PER_SECTION))
> +#else
> +		if (!atomic && IS_ALIGNED(pfn, MAX_FOLIO_NR_PAGES))
> +#endif

Let's trigger cond_resched() on some defined number of iterations or some
memory size chunk, e.g PAGES_PER_128M or even PAGES_PER_1G.

>  			cond_resched();
>  	}
>  }

-- 
Sincerely yours,
Mike.