Re: [PATCH v4 4/6] mm: page_alloc: add alloc_contig_frozen_{range,pages}()

[Zi Yan <ziy@nvidia.com>]

On 16 Dec 2025, at 6:48, Kefeng Wang wrote:

> In order to allocate given range of pages or allocate compound
> pages without incrementing their refcount, adding two new helper
> alloc_contig_frozen_{range,pages}() which may be beneficial
> to some users (eg hugetlb).
>
> The new alloc_contig_{range,pages} only take !__GFP_COMP gfp now,
> and the free_contig_range() is refactored to only free non-compound
> pages, the only caller to free compound pages in cma_free_folio() is
> changed accordingly, and the free_contig_frozen_range() is provided
> to match the alloc_contig_frozen_range(), which is used to free
> frozen pages.
>
> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
> ---
>  include/linux/gfp.h |  52 +++++--------
>  mm/cma.c            |  15 ++--
>  mm/hugetlb.c        |   9 ++-
>  mm/internal.h       |  13 ++++
>  mm/page_alloc.c     | 183 ++++++++++++++++++++++++++++++++------------
>  5 files changed, 184 insertions(+), 88 deletions(-)
>

<snip>

> diff --git a/mm/internal.h b/mm/internal.h
> index e430da900430..75f624236ff8 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -513,6 +513,19 @@ static inline void set_page_refcounted(struct page *page)
>  	set_page_count(page, 1);
>  }
>
> +static inline void set_pages_refcounted(struct page *page, unsigned long nr_pages)
> +{
> +	unsigned long pfn = page_to_pfn(page);
> +
> +	if (PageHead(page)) {
> +		set_page_refcounted(page);
> +		return;
> +	}

This looks fragile, since if a tail page is passed, the refcount will be wrong.
But I see you remove this part in the next patch. It might be OK as a temporary
step.

> +
> +	for (; nr_pages--; pfn++)
> +		set_page_refcounted(pfn_to_page(pfn));
> +}
> +
>  /*
>   * Return true if a folio needs ->release_folio() calling upon it.
>   */
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index aa30d4436296..a7fc83bf806f 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c

<snip>

>
> +static void __free_contig_frozen_range(unsigned long pfn, unsigned long nr_pages)
> +{
> +	for (; nr_pages--; pfn++)
> +		free_frozen_pages(pfn_to_page(pfn), 0);
> +}
> +

Is it possible to use pageblock_order to speed this up?
And can it be moved before free_contig_frozen_range() for a easy read?

<snip>

> +
> +/**
> + * free_contig_frozen_range() -- free the contiguous range of frozen pages
> + * @pfn:	start PFN to free
> + * @nr_pages:	Number of contiguous frozen pages to free
> + *
> + * This can be used to free the allocated compound/non-compound frozen pages.
> + */
> +void free_contig_frozen_range(unsigned long pfn, unsigned long nr_pages)
> +{
> +	struct page *first_page = pfn_to_page(pfn);
> +	const unsigned int order = ilog2(nr_pages);

Maybe WARN_ON_ONCE(first_page != compound_head(first_page) and return
immediately here to catch a tail page.

> +
> +	if (PageHead(first_page)) {
> +		WARN_ON_ONCE(order != compound_order(first_page));
> +		free_frozen_pages(first_page, order);
>  		return;
>  	}
>
> -	for (; nr_pages--; pfn++) {
> -		struct page *page = pfn_to_page(pfn);
> +	__free_contig_frozen_range(pfn, nr_pages);
> +}
> +EXPORT_SYMBOL(free_contig_frozen_range);
> +

Best Regards,
Yan, Zi