Subject: Re: [PATCH RFC 09/35] mm/mm_init: make memmap_init_compound() look more like prep_compound_page()

From: Mike Rapoport <[email-masked]>
Date: Fri, 22 Aug 2025 18:27:22 +0300

On Thu, Aug 21, 2025 at 10:06:35PM +0200, David Hildenbrand wrote:
> Grepping for "prep_compound_page" leaves on clueless how devdax gets its
> compound pages initialized.
> 
> Let's add a comment that might help finding this open-coded
> prep_compound_page() initialization more easily.
> 
> Further, let's be less smart about the ordering of initialization and just
> perform the prep_compound_head() call after all tail pages were
> initialized: just like prep_compound_page() does.
> 
> No need for a lengthy comment then: again, just like prep_compound_page().
> 
> Note that prep_compound_head() already does initialize stuff in page[2]
> through prep_compound_head() that successive tail page initialization
> will overwrite: _deferred_list, and on 32bit _entire_mapcount and
> _pincount. Very likely 32bit does not apply, and likely nobody ever ends
> up testing whether the _deferred_list is empty.
> 
> So it shouldn't be a fix at this point, but certainly something to clean
> up.
> 
> Signed-off-by: David Hildenbrand <[email-masked]>
> ---
>  mm/mm_init.c | 13 +++++--------
>  1 file changed, 5 insertions(+), 8 deletions(-)
> 
> diff --git a/mm/mm_init.c b/mm/mm_init.c
> index 5c21b3af216b2..708466c5b2cc9 100644
> --- a/mm/mm_init.c
> +++ b/mm/mm_init.c
> @@ -1091,6 +1091,10 @@ static void __ref memmap_init_compound(struct page *head,
>  	unsigned long pfn, end_pfn = head_pfn + nr_pages;
>  	unsigned int order = pgmap->vmemmap_shift;
>  
> +	/*
> +	 * This is an open-coded prep_compound_page() whereby we avoid
> +	 * walking pages twice by initializing them in the same go.
> +	 */

While on it, can you also mention that prep_compound_page() is not used to
properly set page zone link?

With this

Reviewed-by: Mike Rapoport (Microsoft) <[email-masked]>

>  	__SetPageHead(head);
>  	for (pfn = head_pfn + 1; pfn < end_pfn; pfn++) {
>  		struct page *page = pfn_to_page(pfn);
> @@ -1098,15 +1102,8 @@ static void __ref memmap_init_compound(struct page *head,
>  		__init_zone_device_page(page, pfn, zone_idx, nid, pgmap);
>  		prep_compound_tail(head, pfn - head_pfn);
>  		set_page_count(page, 0);
> -
> -		/*
> -		 * The first tail page stores important compound page info.
> -		 * Call prep_compound_head() after the first tail page has
> -		 * been initialized, to not have the data overwritten.
> -		 */
> -		if (pfn == head_pfn + 1)
> -			prep_compound_head(head, order);
>  	}
> +	prep_compound_head(head, order);
>  }
>  
>  void __ref memmap_init_zone_device(struct zone *zone,
> -- 
> 2.50.1
> 

-- 
Sincerely yours,
Mike.