netdev - Re: [PATCH v1 13/36] mm/hugetlb: cleanup hugetlb_folio_init_tail

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Message-ID: <cebd5356-0fc6-40aa-9bc6-a3a5ffe918f8@lucifer.local>
Date: Thu, 28 Aug 2025 16:37:50 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, Alexander Potapenko <glider@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Brendan Jackman <jackmanb@...gle.com>,
        Christoph Lameter <cl@...two.org>, Dennis Zhou <dennis@...nel.org>,
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Subject: Re: [PATCH v1 13/36] mm/hugetlb: cleanup
 hugetlb_folio_init_tail_vmemmap()

On Thu, Aug 28, 2025 at 12:01:17AM +0200, David Hildenbrand wrote:
> We can now safely iterate over all pages in a folio, so no need for the
> pfn_to_page().
>
> Also, as we already force the refcount in __init_single_page() to 1,

Mega huge nit (ignore if you want), but maybe worth saying 'via
init_page_count()'.

> we can just set the refcount to 0 and avoid page_ref_freeze() +
> VM_BUG_ON. Likely, in the future, we would just want to tell
> __init_single_page() to which value to initialize the refcount.

Right yes :)

>
> Further, adjust the comments to highlight that we are dealing with an
> open-coded prep_compound_page() variant, and add another comment explaining
> why we really need the __init_single_page() only on the tail pages.

Ah nice another 'anchor' to grep for!

>
> Note that the current code was likely problematic, but we never ran into
> it: prep_compound_tail() would have been called with an offset that might
> exceed a memory section, and prep_compound_tail() would have simply
> added that offset to the page pointer -- which would not have done the
> right thing on sparsemem without vmemmap.
>
> Signed-off-by: David Hildenbrand <david@...hat.com>

LGTM, so:

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

> ---
>  mm/hugetlb.c | 20 ++++++++++++--------
>  1 file changed, 12 insertions(+), 8 deletions(-)
>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 4a97e4f14c0dc..1f42186a85ea4 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -3237,17 +3237,18 @@ static void __init hugetlb_folio_init_tail_vmemmap(struct folio *folio,
>  {
>  	enum zone_type zone = zone_idx(folio_zone(folio));
>  	int nid = folio_nid(folio);
> +	struct page *page = folio_page(folio, start_page_number);
>  	unsigned long head_pfn = folio_pfn(folio);
>  	unsigned long pfn, end_pfn = head_pfn + end_page_number;
> -	int ret;
> -
> -	for (pfn = head_pfn + start_page_number; pfn < end_pfn; pfn++) {
> -		struct page *page = pfn_to_page(pfn);
>
> +	/*
> +	 * We mark all tail pages with memblock_reserved_mark_noinit(),
> +	 * so these pages are completely uninitialized.
> +	 */
> +	for (pfn = head_pfn + start_page_number; pfn < end_pfn; page++, pfn++) {
>  		__init_single_page(page, pfn, zone, nid);
>  		prep_compound_tail((struct page *)folio, pfn - head_pfn);
> -		ret = page_ref_freeze(page, 1);
> -		VM_BUG_ON(!ret);
> +		set_page_count(page, 0);
>  	}
>  }
>
> @@ -3257,12 +3258,15 @@ static void __init hugetlb_folio_init_vmemmap(struct folio *folio,
>  {
>  	int ret;
>
> -	/* Prepare folio head */
> +	/*
> +	 * This is an open-coded prep_compound_page() whereby we avoid
> +	 * walking pages twice by initializing/preparing+freezing them in the
> +	 * same go.
> +	 */
>  	__folio_clear_reserved(folio);
>  	__folio_set_head(folio);
>  	ret = folio_ref_freeze(folio, 1);
>  	VM_BUG_ON(!ret);
> -	/* Initialize the necessary tail struct pages */
>  	hugetlb_folio_init_tail_vmemmap(folio, 1, nr_pages);
>  	prep_compound_head((struct page *)folio, huge_page_order(h));
>  }
> --
> 2.50.1
>