Re: [PATCH v4 4/5] mm/mm_init: Fix pageblock migratetype for ZONE_DEVICE compound pages

["David Hildenbrand (Arm)" <david@kernel.org>]

On 4/22/26 10:14, Muchun Song wrote:
> The memmap_init_zone_device() function only initializes the migratetype
> of the first pageblock of a compound page. If the compound page size
> exceeds pageblock_nr_pages (e.g., 1GB hugepages with 2MB pageblocks),
> subsequent pageblocks in the compound page remain uninitialized.
> 
> Move the migratetype initialization out of __init_zone_device_page()
> and into a separate pageblock_migratetype_init_range() function. This
> iterates over the entire PFN range of the memory, ensuring that all
> pageblocks are correctly initialized.
> 
> Fixes: c4386bd8ee3a ("mm/memremap: add ZONE_DEVICE support for compound pages")
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
> Reviewed-by: Oscar Salvador <osalvador@suse.de>
> ---
>  mm/mm_init.c | 45 ++++++++++++++++++++++++++++++---------------
>  1 file changed, 30 insertions(+), 15 deletions(-)
> 
> diff --git a/mm/mm_init.c b/mm/mm_init.c
> index f9f8e1af921c..9d0fe79a94de 100644
> --- a/mm/mm_init.c
> +++ b/mm/mm_init.c
> @@ -674,6 +674,21 @@ static inline void fixup_hashdist(void)
>  static inline void fixup_hashdist(void) {}
>  #endif /* CONFIG_NUMA */
>  
> +#ifdef CONFIG_ZONE_DEVICE
> +static __meminit void pageblock_migratetype_init_range(unsigned long pfn,
> +						       unsigned long nr_pages,
> +						       int migratetype)

Two-tab indent says hi :)

> +{
> +	unsigned long end = pfn + nr_pages;

Can be const.

> +
> +	for (pfn = pageblock_align(pfn); pfn < end; pfn += pageblock_nr_pages) {
> +		init_pageblock_migratetype(pfn_to_page(pfn), migratetype, false);
> +		if (IS_ALIGNED(pfn, PAGES_PER_SECTION))
> +			cond_resched();
> +	}
> +}
> +#endif
> +
>  /*
>   * Initialize a reserved page unconditionally, finding its zone first.
>   */
> @@ -1011,21 +1026,6 @@ static void __ref __init_zone_device_page(struct page *page, unsigned long pfn,
>  	page_folio(page)->pgmap = pgmap;
>  	page->zone_device_data = NULL;
>  
> -	/*
> -	 * Mark the block movable so that blocks are reserved for
> -	 * movable at startup. This will force kernel allocations
> -	 * to reserve their blocks rather than leaking throughout
> -	 * the address space during boot when many long-lived
> -	 * kernel allocations are made.
> -	 *
> -	 * Please note that MEMINIT_HOTPLUG path doesn't clear memmap
> -	 * because this is done early in section_activate()
> -	 */
> -	if (pageblock_aligned(pfn)) {
> -		init_pageblock_migratetype(page, MIGRATE_MOVABLE, false);
> -		cond_resched();
> -	}
> -
>  	/*
>  	 * ZONE_DEVICE pages other than MEMORY_TYPE_GENERIC are released
>  	 * directly to the driver page allocator which will set the page count
> @@ -1122,6 +1122,9 @@ void __ref memmap_init_zone_device(struct zone *zone,
>  
>  		__init_zone_device_page(page, pfn, zone_idx, nid, pgmap);
>  
> +		if (IS_ALIGNED(pfn, PAGES_PER_SECTION))
> +			cond_resched();
> +
>  		if (pfns_per_compound == 1)
>  			continue;
>  
> @@ -1129,6 +1132,18 @@ void __ref memmap_init_zone_device(struct zone *zone,
>  				     compound_nr_pages(altmap, pgmap));
>  	}
>  
> +	/*
> +	 * Mark the block movable so that blocks are reserved for
> +	 * movable at startup. This will force kernel allocations
> +	 * to reserve their blocks rather than leaking throughout
> +	 * the address space during boot when many long-lived
> +	 * kernel allocations are made.
> +	 *
> +	 * Please note that MEMINIT_HOTPLUG path doesn't clear memmap
> +	 * because this is done early in section_activate()
> +	 */

I am deeply confused about the "memmap" comment here.

There is no MEMINIT_HOTPLUG special-casing. Nobody "clears memmap".


That comment was introduced in

commit 966cf44f637e6aeea7e3d01ba004bf8b5beac78f
Author: Alexander Duyck <alexander.h.duyck@linux.intel.com>
Date:   Fri Oct 26 15:07:52 2018 -0700

    mm: defer ZONE_DEVICE page initialization to the point where we init pgmap

Where we added memmap_init_zone_device().



Is that commit still valid, and this it actually belong here, above the
migratetype setting?


> +	pageblock_migratetype_init_range(start_pfn, nr_pages, MIGRATE_MOVABLE);
> +
>  	pr_debug("%s initialised %lu pages in %ums\n", __func__,
>  		nr_pages, jiffies_to_msecs(jiffies - start));
>  }


-- 
Cheers,

David