linux-kernel - Re: [PATCH v1 09/36] mm/mm_init: make memmap_init_compound() look more like prep_compound

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Message-ID: <a1ea7621-6791-4075-91e1-7cff372ffdd0@lucifer.local>
Date: Thu, 28 Aug 2025 15:54:57 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org,
        "Mike Rapoport (Microsoft)" <rppt@...nel.org>,
        Alexander Potapenko <glider@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
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        Christoph Lameter <cl@...two.org>, Dennis Zhou <dennis@...nel.org>,
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Subject: Re: [PATCH v1 09/36] mm/mm_init: make memmap_init_compound() look
 more like prep_compound_page()

On Thu, Aug 28, 2025 at 12:01:13AM +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 comment to describe the initialization order: again,
> just like prep_compound_page().

Wow this is great, thank you for putting a quality comment for this and
thinking of this :)

We have too much 'special case you just have to know' stuff sitting around,
so this kind of thing is always great to see.

>
> Reviewed-by: Mike Rapoport (Microsoft) <rppt@...nel.org>
> Signed-off-by: David Hildenbrand <david@...hat.com>

LGTM, so:

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>

> ---
>  mm/mm_init.c | 15 +++++++--------
>  1 file changed, 7 insertions(+), 8 deletions(-)
>
> diff --git a/mm/mm_init.c b/mm/mm_init.c
> index 5c21b3af216b2..df614556741a4 100644
> --- a/mm/mm_init.c
> +++ b/mm/mm_init.c
> @@ -1091,6 +1091,12 @@ static void __ref memmap_init_compound(struct page *head,
>  	unsigned long pfn, end_pfn = head_pfn + nr_pages;
>  	unsigned int order = pgmap->vmemmap_shift;
>
> +	/*
> +	 * We have to initialize the pages, including setting up page links.
> +	 * prep_compound_page() does not take care of that, so instead we
> +	 * open-code prep_compound_page() so we can take care of initializing
> +	 * the pages in the same go.
> +	 */
>  	__SetPageHead(head);
>  	for (pfn = head_pfn + 1; pfn < end_pfn; pfn++) {
>  		struct page *page = pfn_to_page(pfn);
> @@ -1098,15 +1104,8 @@ 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
>