linux-kernel - Re: [PATCH v1 06/36] mm/page_alloc: reject unreasonable folio/compound page sizes in alloc_contig_range

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Message-ID: <f195300e-42e2-4eaa-84c8-c37501c3339c@lucifer.local>
Date: Thu, 28 Aug 2025 15:37:35 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, Zi Yan <ziy@...dia.com>,
        SeongJae Park <sj@...nel.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>,
        Dmitry Vyukov <dvyukov@...gle.com>, dri-devel@...ts.freedesktop.org,
        intel-gfx@...ts.freedesktop.org, iommu@...ts.linux.dev,
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        John Hubbard <jhubbard@...dia.com>, kasan-dev@...glegroups.com,
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Subject: Re: [PATCH v1 06/36] mm/page_alloc: reject unreasonable
 folio/compound page sizes in alloc_contig_range_noprof()

On Thu, Aug 28, 2025 at 12:01:10AM +0200, David Hildenbrand wrote:
> Let's reject them early, which in turn makes folio_alloc_gigantic() reject
> them properly.
>
> To avoid converting from order to nr_pages, let's just add MAX_FOLIO_ORDER
> and calculate MAX_FOLIO_NR_PAGES based on that.
>
> Reviewed-by: Zi Yan <ziy@...dia.com>
> Acked-by: SeongJae Park <sj@...nel.org>
> Signed-off-by: David Hildenbrand <david@...hat.com>

Some nits, but overall LGTM so:

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

> ---
>  include/linux/mm.h | 6 ++++--
>  mm/page_alloc.c    | 5 ++++-
>  2 files changed, 8 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 00c8a54127d37..77737cbf2216a 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2055,11 +2055,13 @@ static inline long folio_nr_pages(const struct folio *folio)
>
>  /* Only hugetlbfs can allocate folios larger than MAX_ORDER */
>  #ifdef CONFIG_ARCH_HAS_GIGANTIC_PAGE
> -#define MAX_FOLIO_NR_PAGES	(1UL << PUD_ORDER)
> +#define MAX_FOLIO_ORDER		PUD_ORDER
>  #else
> -#define MAX_FOLIO_NR_PAGES	MAX_ORDER_NR_PAGES
> +#define MAX_FOLIO_ORDER		MAX_PAGE_ORDER
>  #endif
>
> +#define MAX_FOLIO_NR_PAGES	(1UL << MAX_FOLIO_ORDER)

BIT()?

> +
>  /*
>   * compound_nr() returns the number of pages in this potentially compound
>   * page.  compound_nr() can be called on a tail page, and is defined to
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index baead29b3e67b..426bc404b80cc 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -6833,6 +6833,7 @@ static int __alloc_contig_verify_gfp_mask(gfp_t gfp_mask, gfp_t *gfp_cc_mask)
>  int alloc_contig_range_noprof(unsigned long start, unsigned long end,
>  			      acr_flags_t alloc_flags, gfp_t gfp_mask)
>  {
> +	const unsigned int order = ilog2(end - start);
>  	unsigned long outer_start, outer_end;
>  	int ret = 0;
>
> @@ -6850,6 +6851,9 @@ int alloc_contig_range_noprof(unsigned long start, unsigned long end,
>  					    PB_ISOLATE_MODE_CMA_ALLOC :
>  					    PB_ISOLATE_MODE_OTHER;
>
> +	if (WARN_ON_ONCE((gfp_mask & __GFP_COMP) && order > MAX_FOLIO_ORDER))
> +		return -EINVAL;

Possibly not worth it for a one off, but be nice to have this as a helper function, like:

static bool is_valid_order(gfp_t gfp_mask, unsigned int order)
{
	return !(gfp_mask & __GFP_COMP) || order <= MAX_FOLIO_ORDER;
}

Then makes this:

	if (WARN_ON_ONCE(!is_valid_order(gfp_mask, order)))
		return -EINVAL;

Kinda self-documenting!

> +
>  	gfp_mask = current_gfp_context(gfp_mask);
>  	if (__alloc_contig_verify_gfp_mask(gfp_mask, (gfp_t *)&cc.gfp_mask))
>  		return -EINVAL;
> @@ -6947,7 +6951,6 @@ int alloc_contig_range_noprof(unsigned long start, unsigned long end,
>  			free_contig_range(end, outer_end - end);
>  	} else if (start == outer_start && end == outer_end && is_power_of_2(end - start)) {
>  		struct page *head = pfn_to_page(start);
> -		int order = ilog2(end - start);
>
>  		check_new_pages(head, order);
>  		prep_new_page(head, order, gfp_mask, 0);
> --
> 2.50.1
>